MySQL Reference Manual for version 3.23.42.


1 General Information About MySQL

This is the MySQL reference manual; it documents MySQL Version 3.23.42. As MySQL is work in progress, the manual gets updated frequently. There is a very good chance that this version is out of date, unless you are looking at it online. The most recent version of this manual is available at http://www.mysql.com/documentation/ in many different formats. If you have a hard time finding information in the manual, you can try the searchable PHP version at http://www.mysql.com/documentation/manual.php.

MySQL is a very fast, multi-threaded, multi-user, and robust SQL (Structured Query Language) database server.

MySQL is free software. It is licensed with the GNU GENERAL PUBLIC LICENSE http://www.gnu.org/. See section 1.3 MySQL Licensing and Support.

The MySQL home page provides the latest information about MySQL.

The following list describes some useful sections of the manual:

IMPORTANT:

Reports of errors (often called bugs), as well as questions and comments, should be sent to the mailing list at mysql@lists.mysql.com. See section 1.2.22.3 How to Report Bugs or Problems. The mysqlbug script should be used to generate bug reports. For source distributions, the mysqlbug script can be found in the `scripts' directory. For binary distributions, mysqlbug can be found in the `bin' directory. If you have found a sensitive security bug in MySQL, you should send an email to security@mysql.com.

If you have any suggestions concerning additions or corrections to this manual, please send them to the manual team at docs@mysql.com.

This is a reference manual; it does not provide general instruction on SQL or relational database concepts. If you want general information about SQL, see section 1.2.2 General SQL Information and Tutorials. For books that focus more specifically on MySQL, see section 1.2.1 Books About MySQL.

1.1 MySQL, MySQL AB, and Open Source

1.1.1 What Is MySQL

MySQL, the most popular Open Source SQL database, is provided by MySQL AB. MySQL AB is a commercial company that builds its business providing services around the MySQL database. See section 1.1.2 What Is MySQL AB.

MySQL is a database management system.
A database is a structured collection of data. It may be anything from a simple shopping list to a picture gallery or the vast amounts of information in a corporate network. To add, access, and process data stored in a computer database, you need a database management system such as MySQL. Since computers are very good at handling large amounts of data, database management plays a central role in computing, as stand-alone utilities, or as parts of other applications.
MySQL is a relational database management system.
A relational database stores data in separate tables rather than putting all the data in one big storeroom. This adds speed and flexibility. The tables are linked by defined relations making it possible to combine data from several tables on request. The SQL part of MySQL stands for "Structured Query Language" - the most common standardized language used to access databases.
MySQL is Open Source Software.
Open Source means that it is possible for anyone to use and modify. Anybody can download MySQL from the Internet and use it without paying anything. Anybody so inclined can study the source code and change it to fit their needs. MySQL uses the GPL (GNU General Public License) http://www.gnu.org, to define what you may and may not do with the software in different situations. If you feel uncomfortable with the GPL or need to embed MySQL into a commercial application you can buy a commercially licensed version from us.
Why use MySQL?
MySQL is very fast, reliable, and easy to use. If that is what you are looking for, you should give it a try. MySQL also has a very practical set of features developed in very close cooperation with our users. You can find a performance comparison of MySQL to some other database managers on our benchmark page. See section 5.1.4 The MySQL Benchmark Suite. MySQL was originally developed to handle very large databases much faster than existing solutions and has been successfully used in highly demanding production environments for several years. Though under constant development, MySQL today offers a rich and very useful set of functions. The connectivity, speed, and security make MySQL highly suited for accessing databases on the Internet.
The technical features of MySQL
For advanced technical information, see section 6 MySQL Language Reference. MySQL is a client/server system that consists of a multi-threaded SQL server that supports different backends, several different client programs and libraries, administrative tools, and several programming interfaces. We also provide MySQL as a multi-threaded library which you can link into your application to get a smaller, faster, easier to manage product.
MySQL has a lot of contributed software available.
It is very likely that you will find that your favorite application or language already supports MySQL.

The official way to pronounce MySQL is ``My Ess Que Ell'' (not MY-SEQUEL). But we try to avoid correcting people who say MY-SEQUEL.

1.1.2 What Is MySQL AB

MySQL AB is the Swedish company owned and run by the MySQL founders and main developers. We are dedicated to developing MySQL and spreading our database to new users. MySQL AB owns the copyright to the MySQL server source code and the MySQL trademark. A significant amount of revenues from our services goes to developing MySQL. See section 1.1.1 What Is MySQL.

MySQL AB has been profitable providing MySQL from the start. We don't get any outside funding, but have earned all our money ourselves.

We are searching after partners that would like to support our development of MySQL so that we could accelerate the development pace. If you are interested in doing this, you can email partner@mysql.com about this!

MySQL AB has currently 20+ people (http://www.mysql.com/development/team.html) on its payroll and is growing rapidly.

Our main sources of income are:

The MySQL core values show our dedication to MySQL and Open Source.

We want MySQL to be:

MySQL AB and the people of MySQL AB:

1.1.3 About This Manual

This manual is currently available in Texinfo, plain text, Info, HTML, PostScript, and PDF versions. The primary document is the Texinfo file. The HTML version is produced automatically using a modified version of texi2html. The plain text and Info versions are produced with makeinfo. The Postscript version is produced using texi2dvi and dvips. The PDF version is produced with pdftex.

This manual is written and maintained by David Axmark, Michael (Monty) Widenius, Jeremy Cole, and Paul DuBois. For other contributors, see section E Credits.

1.1.4 Conventions Used in This Manual

This manual uses certain typographical conventions:

constant
Constant-width font is used for command names and options; SQL statements; database, table and column names; C and Perl code; and environment variables. Example: ``To see how mysqladmin works, invoke it with the --help option.''
`filename'
Constant-width font with surrounding quotes is used for filenames and pathnames. Example: ``The distribution is installed under the `/usr/local/' directory.''
`c'
Constant-width font with surrounding quotes is also used to indicate character sequences. Example: ``To specify a wild card, use the `%' character.''
italic
Italic font is used for emphasis, like this.
boldface
Boldface font is used for access privilege names (for example, ``do not grant the process privilege lightly'') and occasionally to convey especially strong emphasis.

When commands are shown that are meant to be executed by a particular program, the program is indicated by a prompt shown before the command. For example, shell> indicates a command that you execute from your login shell, and mysql> indicates a command that you execute from the mysql client program:

shell> type a shell command here
mysql> type a mysql command here

Shell commands are shown using Bourne shell syntax. If you are using a csh-style shell, you may need to issue commands slightly differently. For example, the sequence to set an environment variable and run a command looks like this in Bourne shell syntax:

shell> VARNAME=value some_command

For csh, you would execute the sequence like this:

shell> setenv VARNAME value
shell> some_command

Often, database, table, and column names must be substituted into commands. To indicate that such substitution is necessary, this manual uses db_name, tbl_name and col_name. For example, you might see a statement like this:

mysql> SELECT col_name FROM db_name.tbl_name;

This means that if you were to enter a similar statement, you would supply your own database, table, and column names, perhaps like this:

mysql> SELECT author_name FROM biblio_db.author_list;

SQL statements may be written in uppercase or lowercase. When this manual shows a SQL statement, uppercase is used for particular keywords if those keywords are under discussion (to emphasize them) and lowercase is used for the rest of the statement. For example, you might see the following in a discussion of the SELECT statement:

mysql> SELECT count(*) FROM tbl_name;

On the other hand, in a discussion of the COUNT() function, the same statement would be written like this:

mysql> select COUNT(*) from tbl_name;

If no particular emphasis is intended, all keywords are written uniformly in uppercase.

In syntax descriptions, square brackets (`[' and `]') are used to indicate optional words or clauses:

DROP TABLE [IF EXISTS] tbl_name

When a syntax element consists of a number of alternatives, the alternatives are separated by vertical bars (`|'). When one member from a set of choices may be chosen, the alternatives are listed within square brackets (`[' and `]'):

TRIM([[BOTH | LEADING | TRAILING] [remstr] FROM] str)

When one member from a set of choices must be chosen, the alternatives are listed within braces (`{' and `}'):

{DESCRIBE | DESC} tbl_name {col_name | wild}

1.1.5 History of MySQL

We once started out with the intention of using mSQL to connect to our tables using our own fast low-level (ISAM) routines. However, after some testing we came to the conclusion that mSQL was not fast enough nor flexible enough for our needs. This resulted in a new SQL interface to our database but with almost the same API interface as mSQL. This API was chosen to ease porting of third-party code.

The derivation of the name MySQL is not perfectly clear. Our base directory and a large number of our libraries and tools have had the prefix ``my'' for well over 10 years. However, Monty's daughter (some years younger) is also named My. Which of the two gave its name to MySQL is still a mystery, even for us.

1.1.6 The Main Features of MySQL

The following list describes some of the important characteristics of MySQL:

1.1.7 How Stable Is MySQL?

This section addresses the questions ``How stable is MySQL?'' and ``Can I depend on MySQL in this project?'' We will try to clarify some issues and to answer some of the more important questions that seem to concern many people. This section has been put together from information gathered from the mailing list (which is very active in reporting bugs).

At TcX, MySQL has worked without any problems in our projects since mid-1996. When MySQL was released to a wider public, we noticed that there were some pieces of ``untested code'' that were quickly found by the new users who made queries in a manner different than our own. Each new release has had fewer portability problems than the previous one (even though each has had many new features).

Each release of MySQL has been usable, and there have been problems only when users start to use code from the ``gray zones.'' Naturally, outside users don't know what the gray zones are; this section attempts to indicate those that are currently known. The descriptions deal with Version 3.23 of MySQL. All known and reported bugs are fixed in the latest version, with the exception of the bugs listed in the bugs section, which are things that are design-related. See section 1.4.7 Known errors and design deficiencies in MySQL.

MySQL is written in multiple layers and different independent modules. These modules are listed below with an indication of how well-tested each of them is:

The ISAM table handler -- Stable
This manages storage and retrieval of all data in MySQL Version 3.22 and earlier. In all MySQL releases there hasn't been a single (reported) bug in this code. The only known way to get a corrupted table is to kill the server in the middle of an update. Even that is unlikely to destroy any data beyond rescue, because all data are flushed to disk between each query. There hasn't been a single bug report about lost data because of bugs in MySQL.
The MyISAM table handler -- Stable
This is new in MySQL Version 3.23. It's largely based on the ISAM table code but has a lot of new and very useful features.
The parser and lexical analyser -- Stable
There hasn't been a single reported bug in this system for a long time.
The C client code -- Stable
No known problems. In early Version 3.20 releases, there were some limitations in the send/receive buffer size. As of Version 3.21, the buffer size is now dynamic up to a default of 16M.
Standard client programs -- Stable
These include mysql, mysqladmin, mysqlshow, mysqldump, and mysqlimport.
Basic SQL -- Stable
The basic SQL function system and string classes and dynamic memory handling. Not a single reported bug in this system.
Query optimizer -- Stable
Range optimizer -- Stable
Join optimizer -- Stable
Locking -- Gamma
This is very system-dependent. On some systems there are big problems using standard OS locking (fcntl()). In these cases, you should run the MySQL daemon with the --skip-locking flag. Problems are known to occur on some Linux systems, and on SunOS when using NFS-mounted file systems.
Linux threads -- Stable
The major problem found has been with the fcntl() call, which is fixed by using the --skip-locking option to mysqld. Some people have reported lockup problems with Version 0.5. LinuxThreads will need to be recompiled if you plan to use 1000+ concurrent connections. Although it is possible to run that many connections with the default LinuxThreads (however, you will never go above 1021), the default stack spacing of 2 MB makes the application unstable, and we have been able to reproduce a coredump after creating 1021 idle connections. See section 2.6.1 Linux Notes (All Linux Versions).
Solaris 2.5+ pthreads -- Stable
We use this for all our production work.
MIT-pthreads (Other systems) -- Stable
There have been no reported bugs since Version 3.20.15 and no known bugs since Version 3.20.16. On some systems, there is a ``misfeature'' where some operations are quite slow (a 1/20 second sleep is done between each query). Of course, MIT-pthreads may slow down everything a bit, but index-based SELECT statements are usually done in one time frame so there shouldn't be a mutex locking/thread juggling.
Other thread implementions -- Beta - Gamma
The ports to other systems are still very new and may have bugs, possibly in MySQL, but most often in the thread implementation itself.
LOAD DATA ..., INSERT ... SELECT -- Stable
Some people thought they had found bugs here, but these usually have turned out to be misunderstandings. Please check the manual before reporting problems!
ALTER TABLE -- Stable
Small changes in Version 3.22.12.
DBD -- Stable
Now maintained by Jochen Wiedmann (wiedmann@neckar-alb.de). Thanks!
mysqlaccess -- Stable
Written and maintained by Yves Carlier (Yves.Carlier@rug.ac.be). Thanks!
GRANT -- Stable
Big changes made in MySQL Version 3.22.12.
MyODBC (uses ODBC SDK 2.5) -- Gamma
It seems to work well with some programs.
Replication -- Beta / Gamma
We are still working on replication, so don't expect this to be rock solid yet. On the other hand, some MySQL users are already using this with good results.
BDB Tables -- Beta
The Berkeley DB code is very stable, but we are still improving the interface between MySQL and BDB tables, so it will take some time before this is as tested as the other table types.
InnoDB Tables -- Beta
This is a recent addition to MySQL. They appear to work good and can be used after some initial testing.
Automatic recovery of MyISAM tables - Beta
This only affects the new code that checks if the table was closed properly on open and executes an automatic check/repair of the table if it wasn't.
MERGE tables -- Beta / Gamma
The usage of keys on MERGE tables is still not that tested. The other part of the MERGE code is quite well tested.
FULLTEXT -- Beta
Text search seems to work, but is still not widely used.

MySQL AB provides e-mail support for paying customers, but the MySQL mailing list usually provides answers to common questions. Bugs are usually fixed right away with a patch; for serious bugs, there is almost always a new release.

1.1.8 How Big Can MySQL Tables Be?

MySQL Version 3.22 has a 4G limit on table size. With the new MyISAM in MySQL Version 3.23 the maximum table size is pushed up to 8 million terabytes (2 ^ 63 bytes).

Note, however, that operating systems have their own file size limits. Here are some examples:

Operating System File Size Limit
Linux-Intel 32 bit 2G, 4G or more, depends on Linux version
Linux-Alpha 8T (?)
Solaris 2.5.1 2G (possible 4G with patch)
Solaris 2.6 4G
Solaris 2.7 Intel 4G
Solaris 2.7 ULTRA-SPARC 8T (?)

On Linux 2.2 you can get bigger tables than 2G by using the LFS patch for the ext2 file system. On Linux 2.4 there exists also patches for ReiserFS to get support for big files.

This means that the table size for MySQL is normally limited by the operating system.

By default, MySQL tables have a maximum size of about 4G. You can check the maximum table size for a table with the SHOW TABLE STATUS command or with the myisamchk -dv table_name. See section 4.5.5 SHOW Syntax.

If you need bigger tables than 4G (and your operating system supports this), you should set the AVG_ROW_LENGTH and MAX_ROWS parameter when you create your table. See section 6.5.3 CREATE TABLE Syntax. You can also set these later with ALTER TABLE. See section 6.5.4 ALTER TABLE Syntax.

If your big table is going to be read-only, you could use myisampack to merge and compress many tables to one. myisampack usually compresses a table by at least 50%, so you can have, in effect, much bigger tables. See section 4.7.4 myisampack, The MySQL Compressed Read-only Table Generator.

You can go around the operating system file limit for MyISAM data files by using the RAID option. See section 6.5.3 CREATE TABLE Syntax.

Another solution can be the included MERGE library, which allows you to handle a collection of identical tables as one. See section 7.2 MERGE Tables.

1.1.9 Year 2000 Compliance

MySQL itself has no problems with Year 2000 (Y2K) compliance:

You may run into problems with applications that use MySQL in a way that is not Y2K-safe. For example, many old applications store or manipulate years using 2-digit values (which are ambiguous) rather than 4-digit values. This problem may be compounded by applications that use values such as 00 or 99 as ``missing'' value indicators.

Unfortunately, these problems may be difficult to fix, because different applications may be written by different programmers, each of whom may use a different set of conventions and date-handling functions.

Here is a simple demonstration illustrating that MySQL doesn't have any problems with dates until the year 2030:

mysql> DROP TABLE IF EXISTS y2k;
Query OK, 0 rows affected (0.01 sec)

mysql> CREATE TABLE y2k (date date, date_time datetime, time_stamp timestamp);
Query OK, 0 rows affected (0.00 sec)

mysql> INSERT INTO y2k VALUES 
    -> ("1998-12-31","1998-12-31 23:59:59",19981231235959),
    -> ("1999-01-01","1999-01-01 00:00:00",19990101000000),
    -> ("1999-09-09","1999-09-09 23:59:59",19990909235959),
    -> ("2000-01-01","2000-01-01 00:00:00",20000101000000),
    -> ("2000-02-28","2000-02-28 00:00:00",20000228000000),
    -> ("2000-02-29","2000-02-29 00:00:00",20000229000000),
    -> ("2000-03-01","2000-03-01 00:00:00",20000301000000),
    -> ("2000-12-31","2000-12-31 23:59:59",20001231235959),
    -> ("2001-01-01","2001-01-01 00:00:00",20010101000000),
    -> ("2004-12-31","2004-12-31 23:59:59",20041231235959),
    -> ("2005-01-01","2005-01-01 00:00:00",20050101000000),
    -> ("2030-01-01","2030-01-01 00:00:00",20300101000000),
    -> ("2050-01-01","2050-01-01 00:00:00",20500101000000);
Query OK, 13 rows affected (0.01 sec)
Records: 13  Duplicates: 0  Warnings: 0

mysql> SELECT * FROM y2k;
+------------+---------------------+----------------+
| date       | date_time           | time_stamp     |
+------------+---------------------+----------------+
| 1998-12-31 | 1998-12-31 23:59:59 | 19981231235959 |
| 1999-01-01 | 1999-01-01 00:00:00 | 19990101000000 |
| 1999-09-09 | 1999-09-09 23:59:59 | 19990909235959 |
| 2000-01-01 | 2000-01-01 00:00:00 | 20000101000000 |
| 2000-02-28 | 2000-02-28 00:00:00 | 20000228000000 |
| 2000-02-29 | 2000-02-29 00:00:00 | 20000229000000 |
| 2000-03-01 | 2000-03-01 00:00:00 | 20000301000000 |
| 2000-12-31 | 2000-12-31 23:59:59 | 20001231235959 |
| 2001-01-01 | 2001-01-01 00:00:00 | 20010101000000 |
| 2004-12-31 | 2004-12-31 23:59:59 | 20041231235959 |
| 2005-01-01 | 2005-01-01 00:00:00 | 20050101000000 |
| 2030-01-01 | 2030-01-01 00:00:00 | 20300101000000 |
| 2050-01-01 | 2050-01-01 00:00:00 | 00000000000000 |
+------------+---------------------+----------------+
13 rows in set (0.00 sec)

This shows that the DATE and DATETIME types will not give any problems with future dates (they handle dates until the year 9999).

The TIMESTAMP type, which is used to store the current time, has a range up to only 2030-01-01. TIMESTAMP has a range of 1970 to 2030 on 32-bit machines (signed value). On 64-bit machines it handles times up to 2106 (unsigned value).

Even though MySQL is Y2K-compliant, it is your responsibility to provide unambiguous input. See section 6.2.2.1 Y2K Issues and Date Types for MySQL's rules for dealing with ambiguous date input data (data containing 2-digit year values).

1.2 MySQL Information Sources

1.2.1 Books About MySQL

For the latest book information, with user comments, please visit http://www.mysql.com/portal/books/html/index.html.

While this manual is still the right place for up to date technical information, its primary goal is to contain everything there is to know about MySQL. It is sometimes nice to have a bound book to read in bed or while you travel. Here is a list of books about MySQL and related subjects (in English).

By purchasing a book through these hyperlinks provided herein, you are contributing to the development of MySQL.

MySQL
Available Barnes and Noble
Publisher New Riders
Author Paul DuBois
Pub Date 1st Edition December 1999
ISBN 0735709211
Pages 800
Price $49.99 US
Downloadable examples samp_db distribution
Errata are available here

Foreword by Michael ``Monty'' Widenius, MySQL Moderator.

In MySQL, Paul DuBois provides you with a comprehensive guide to one of the most popular relational database systems. Paul has contributed to the online documentation for MySQL and is an active member of the MySQL community. The principal MySQL developer, Monty Widenius, and a network of his fellow developers reviewed the manuscript, and provided Paul with the kind of insight no one else could supply.

Instead of merely giving you a general overview of MySQL, Paul teaches you how to make the most of its capabilities. Through two sample database applications that run throughout the book, he gives you solutions to problems you're sure to face. He helps you integrate MySQL efficiently with third-party tools, such as PHP and Perl, enabling you to generate dynamic Web pages through database queries. He teaches you to write programs that access MySQL databases, and also provides a comprehensive set of references to column types, operators, functions, SQL syntax, MySQL programming, C API, Perl DBI, and PHP API. MySQL simply gives you the kind of information you won't find anywhere else.

If you use MySQL, this book provides you with:


MySQL & mSQL
Available Barnes and Noble
Publisher O'Reilly
Authors Randy Jay Yarger, George Reese & Tim King
Pub Date 1st Edition July 1999
ISBN 1-56592-434-7, Order Number: 4347
Pages 506
Price $34.95

This book teaches you how to use MySQL and mSQL, two popular and robust database products that support key subsets of SQL on both Linux and Unix systems. Anyone who knows basic C, Java, Perl, or Python can write a program to interact with a database, either as a stand-alone application or through a Web page. This book takes you through the whole process, from installation and configuration to programming interfaces and basic administration. Includes plenty of tutorial material.

Sams' Teach Yourself MySQL in 21 Days
Available Barnes and Noble
Publisher Sams
Authors Mark Maslakowski and Tony Butcher
Pub Date June 2000
ISBN 0672319144
Pages 650
Price $39.99

Sams' Teach Yourself MySQL in 21 Days is for intermediate Linux users who want to move into databases. A large share of the audience is Web developers who need a database to store large amounts of information that can be retrieved via the Web.

Sams' Teach Yourself MySQL in 21 Days is a practical, step-by-step tutorial. The reader will learn to design and employ this open source database technology into his or her Web site using practical, hands-on examples to follow.

E-Commerce Solutions with MySQL
Available Barnes and Noble
Publisher Prima Communications, Inc.
Authors N/A
Pub Date January 2000
ISBN 0761524452
Pages 500
Price $39.99

No description available.

MySQL and PHP from Scratch
Available Barnes and Noble
Publisher Que
Authors N/A
Pub Date September 2000
ISBN 0789724405
Pages 550
Price $34.99

This book puts together information on installing, setting up, and troubleshooting Apache, MySQL, PHP3, and IMP into one complete volume. You also learn how each piece is part of a whole by learning, step-by-step, how to create a web-based e-mail system. Learn to run the equivalent of Active Server Pages (ASP) using PHP3, set up an e-commerce site using a database and the Apache web server, and create a data entry system (such as sales, product quality tracking, customer preferences, etc) that no installation in the PC.

Professional MySQL Programming
Available Barnes and Noble
Publisher Wrox Press, Inc.
Authors N/A
Pub Date Late 2001
ISBN 1861005164
Pages 1000
Price $49.99

No description available.

Professional Linux Programming
Available Barnes and Noble
Publisher Wrox Press, Inc.
Authors N/A
Pub Date September 2000
ISBN 1861003013
Pages 1155
Price $47.99

In this follow-up to the best-selling Beginning Linux Programming, you will learn from the authors' real-world knowledge and experience of developing software for Linux; you'll be taken through the development of a sample 'DVD Store' application, with 'theme' chapters addressing different aspects of its implementation. Meanwhile, individual ``take-a-break'' chapters cover important topics that go beyond the bounds of the central theme. All focus on the practical aspects of programming, showing how crucial it is to choose the right tools for the job, use them as they should be used, and get things right first time.

PHP and MySQL Web Development
Available Barnes and Noble
Publisher Sams
Authors Luke Welling, Laura Thomson
Pub Date March 2001
ISBN 0672317842
Pages 700
Price $49.99

PHP and MySQL Web Development introduces you to the advantages of implementing both MySQL and PHP. These advantages are detailed through the provision of both statistics and several case studies. A practical web application is developed throughout the book, providing you with the tools necessary to implement a functional online database. Each function is developed separately, allowing you the choice to incorporate only those parts that you would like to implement. Programming concepts of the PHP language are highlighted, including functions which tie MySQL support into a PHP script and advanced topics regarding table manipulation.

Books recommended by the MySQL Developers

SQL-99 Complete, Really
Available Barnes and Noble
Publisher CMP Books
Authors Peter Gulutzan, Trudy Pelzer
Pub Date April 1999
ISBN 0879305681
Pages 1104
Price $55.96

This book contains complete descriptions of the new standards for syntax, data structures, and retrieval processes of SQL databases. As an example-based reference manual, it includes all of the CLI functions, information, schema tables, and status codes, as well as a working SQL database provided on the companion disk.

C, A reference manual
Available Barnes and Noble
Publisher Prentice Hall
Authors Samuel P. Harbison, Guy L. Steele
Pub Date September 1994
ISBN 0133262243
Pages 480
Price $35.99

A new and improved revision of the bestselling C language reference. This manual introduces the notion of "Clean C", writing C code that can be compiled as a C++ program, C programming style that emphasizes correctness, portability, maintainability, and incorporates the ISO C Amendment 1 (1994) which specifies new facilities for writing portable, international programs in C.

C++ for Real Programmers
Available Barnes and Noble
Publisher Academic Press, Incorporated
Authors Jeff Alger, Jim Keogh
Pub Date February 1998
ISBN 0120499428
Pages 388
Price $39.95

C++ For Real Programmers bridges the gap between C++ as described in beginner and intermediate-level books and C++ as it is practiced by experts. Numerous valuable techniques are described, organized into three simple themes: indirection, class hierarchies, and memory management. It also provides in-depth coverage of template creation, exception handling, pointers and optimization techniques. The focus of the book is on ANSI C++ and, as such, is compiler independent.

C++ For Real Programmers is a revision of Secrets of the C++ Masters and includes a new appendix comparing C++ with Java. The book comes with a 3.5" disk for Windows with source code.

Algorithms in C
Available Barnes and Noble
Publisher Addison Wesley Longman, Inc.
Authors Robert Sedgewick
Pub Date April 1990
ISBN 0201514257
Pages 648
Price $45.75

Algorithms in C describes a variety of algorithms in a number of areas of interest, including: sorting, searching, string-processing, and geometric, graph and mathematical algorithms. The book emphasizes fundamental techniques, providing readers with the tools to confidently implement, run, and debug useful algorithms.

Multithreaded Programming with Pthreads
Available Barnes and Noble
Publisher Prentice Hall
Authors Bil Lewis, Daniel J. Berg
Pub Date October 1997
ISBN 0136807291
Pages 432
Price $34.95

Based on the best-selling Threads Primer, Multithreaded Programming with Pthreads gives you a solid understanding of Posix threads: what they are, how they work, when to use them, and how to optimize them. It retains the clarity and humor of Threads Primer, but includes expanded comparisons to Win32 and OS/2 implementations. Code examples tested on all of the major UNIX platforms are featured along with detailed explanations of how and why they use threads.

Programming the PERL DBI: Database Programming with PERL
Available Barnes and Noble
Publisher O'Reilly & Associates, Incorporated
Authors Alligator Descartes, Tim Bunce
Pub Date February 2000
ISBN 1565926994
Pages 400
Price $27.96

Programming the Perl DBI is coauthored by Alligator Descartes, one of the most active members of the DBI community, and by Tim Bunce, the inventor of DBI. For the uninitiated, the book explains the architecture of DBI and shows you how to write DBI-based programs. For the experienced DBI dabbler, this book explains DBI's nuances and the peculiarities of each individual DBD.

The book includes:


1.2.2 General SQL Information and Tutorials

The following book has been recommended by several people on the MySQL mailing list:

Judith S. Bowman, Sandra L. Emerson and Marcy Darnovsky
The Practical SQL Handbook: Using Structured Query Language
Second Edition
Addison-Wesley
ISBN 0-201-62623-3
http://www.awl.com

The following book has also received some recommendations by MySQL users:

Martin Gruber
Understanding SQL
ISBN 0-89588-644-8
Publisher Sybex 510 523 8233
Alameda, CA USA

A SQL tutorial is available on the net at http://w3.one.net/~jhoffman/sqltut.htm

1.2.3 Useful MySQL-related Links

Apart from the following links, you can find and download a lot of MySQL programs, tools and APIs from the Contrib directory.

MySQL

1.2.4 Tutorials and Manuals

MySQL Myths Debunked
MySQL used in the real world.
http://www.4t2.com/mysql
Information about the German MySQL mailing list.
http://www2.rent-a-database.de/mysql/
MySQL handbook in German.
http://www.bitmover.com:8888//home/bk/mysql
Web access to the MySQL BitKeeper repository.
http://www.analysisandsolutions.com/code/mybasic.htm
Beginners MySQL Tutorial on how to install and set up MySQL on a Windows machine.
http://www.devshed.com/Server_Side/MySQL/
A lot of MySQL tutorials.
http://mysql.hitstar.com/
MySQL manual in Chinese.
http://www.linuxplanet.com/linuxplanet/tutorials/1046/1/
Setting Up a MySQL-based Web site.
http://www.hotwired.com/webmonkey/backend/tutorials/tutorial1.html
MySQL-Perl tutorial.
http://www.iserver.com/support/contrib/perl5/modules.html
Installing new Perl modules that require locally installed modules.
http://www.hotwired.com/webmonkey/databases/tutorials/tutorial4.html
PHP/MySQL Tutorial.
http://www.useractive.com/
Hands on tutorial for MySQL.

1.2.5 Porting MySQL/Using MySQL on Different Systems

http://xclave.macnn.com/MySQL/
The Mac OS Xclave. Running MySQL on Mac OS X.
http://www.prnet.de/RegEx/mysql.html
MySQL for Mac OS X Server.
http://www.latencyzero.com/macosx/mysql.html
Building MySQL for Mac OS X.
http://www.essencesw.com/Software/mysqllib.html
New Client libraries for the Mac OS Classic (Macintosh).
http://www.lilback.com/macsql/
Client libraries for Mac OS Classic (Macintosh).
http://sixk.maniasys.com/index_en.html
MySQL for Amiga

1.2.6 Perl-related Links

http://dbimysql.photoflux.com/
Perl DBI with MySQL FAQ.

1.2.7 MySQL Discussion Forums

http://www.weberdev.com/
Examples using MySQL; (check Top 20)
http://futurerealm.com/forum/futureforum.htm
FutureForum Web Discussion Software.

1.2.8 Commercial Applications that Support MySQL

http://www.supportwizard.com/
SupportWizard; Interactive helpdesk on the Web (This product includes a licensed copy of MySQL.)
http://www.sonork.com/
Sonork, Instant Messenger that is not only Internet oriented. It's focused on private networks and on small to medium companies. Client is free, server is free for up to 5 seats.
http://www.stweb.org/
StWeb - Stratos Web and Application server - An easy-to-use, cross platform, Internet/Intranet development and deployment system for development of web-enabled applications. The standard version of StWeb has a native interface to MySQL database.
http://www.rightnowtech.com/
Right Now Web; Web automation for customer service.
http://www.icaap.org/Bazaar/
Bazaar; Interactive Discussion Forums with Web interface.
http://www.phonesweep.com/
PhoneSweepT is the world's first commercial Telephone Scanner. Many break-ins in recent years have come not through the Internet, but through unauthorized dial-up modems. PhoneSweep lets you find these modems by repeatedly placing phone calls to every phone number that your organization controls. PhoneSweep has a built-in expert system that can recognize more than 250 different kinds of remote-access programs, including Carbon Copy(TM), pcANYWHERE(TM), and Windows NT RAS. All information is stored in the SQL database. It then generates a comprehensive report detailing which services were discovered on which dial-up numbers in your organization.

1.2.9 SQL Clients and Report Writers

urSQL
SQL Editor and Query Utility. Custom syntax highlighting, editable results grid, exportable result-sets, basic MySQL admin functions, Etc.. For Windows.
MySQL Data Manager
MySQL Data Manager * is platform independent web client (written in perl) for MySQL server over TCP/IP.
http://ksql.sourceforge.net/
KDE MySQL client.
http://www.ecker-software.de
A Windows GUI client by David Ecker.
http://www.icaap.org/software/kiosk/
Kiosk; a MySQL client for database management. Written in Perl. Will be a part of Bazaar.
http://www.casestudio.com/
Db design tool that supports MySQL 3.23.
http://home.skif.net/~voland/zeos/eng/index.html
Zeos - A client that supports MySQL, Interbase and PostgreSQL.
http://www.geocities.com/SiliconValley/Ridge/4280/GenericReportWriter/grwhome.html
A free report writer in Java
http://www.javaframework.de
MySQLExport - Export of MySQL create statements and data in a lot of different formats (SQL, HTML, CVS, text, ZIP, GZIP...)
http://dlabs.4t2.com
M2D, a MySQL Administration client for Windows. M2D supports administration of MySQL databases, creation of new databases and tables, editing, and more.
http://dlabs.4t2.com
Dexter, a small server written in Perl which can be used as a proxy server for MySQL or as a database extender.
http://www.scibit.com/Products/Software/Utils/Mascon.asp
Mascon is a powerful Win32 GUI for administering MySQL databases.
http://www.rtlabs.com/
MacSQL Monitor. GUI for MySQL, ODBC, and JDBC databases for the Mac OS.

1.2.10 Distributions that Include MySQL

http://www.suse.com/
SuSE Linux (6.1 and above)
http://www.redhat.com/
RedHat Linux (7.0 and above)
http://distro.conectiva.com.br
Conectiva Linux (4.0 and above)

1.2.11 Web Development Tools that Support MySQL

http://www.php.net/
PHP: A server-side HTML-embedded scripting language.
http://www.midgard-project.org
The Midgard Application Server; a powerful Web development environment based on MySQL and PHP.
http://www.smartworker.org
SmartWorker is a platform for Web application development.
http://xsp.lentus.se/
XSP: e(X)tendible (S)erver (P)ages and is a HTML embedded tag language written in Java (previously known as XTAGS.)
http://www.dbServ.de/
dbServ is an extension to a web server to integrate database output into your HTML code. You may use any HTML function in your output. Only the client will stop you. It works as standalone server or as Java servlet.
http://www.chilisoft.com/
Platform independent ASP from Chili!Soft
http://www.voicenet.com/~zellert/tjFM
A JDBC driver for MySQL.
http://www.wernhart.priv.at/php/
MySQL + PHP demos.
http://www.dbwww.com/
ForwardSQL: HTML interface to manipulate MySQL databases.
http://www.daa.com.au/~james/www-sql/
WWW-SQL: Display database information.
http://www.minivend.com/minivend/
Minivend: A Web shopping cart.
http://www.heitml.com/
HeiTML: A server-side extension of HTML and a 4GL language at the same time.
http://www.metahtml.com/
Metahtml: A Dynamic Programming Language for WWW Applications.
http://www.binevolve.com/
VelocityGen for Perl and Tcl.
http://hawkeye.net/
Hawkeye Internet Server Suite.
http://www.fastflow.com/
Network Database Connection For Linux
http://www.wdbi.net/
WDBI: Web browser as a universal front end to databases which supports MySQL well.
http://www.webgroove.com/
WebGroove Script: HTML compiler and server-side scripting language.
http://www.ihtml.com/
A server-side Web site scripting language.
ftp://ftp.igc.apc.org/pub/myodbc/README
How to use MySQL with ColdFusion on Solaris.
http://calistra.com/MySQL/
Calistra's ODBC MySQL Administrator.
http://www.webmerger.com
Webmerger - This CGI tool interprets files and generates dynamic output based on a set of simple tags. Ready-to-run drivers for MySQL and PostgreSQL through ODBC.
http://phpclub.net/
PHPclub - Tips and tricks for PHP.
http://www.penguinservices.com/scripts
MySQL and Perl Scripts.
http://www.widgetchuck.com
The Widgetchuck; Web Site Tools and Gadgets
http://www.adcycle.com/
AdCycle - advertising management software.
http://sourceforge.net/projects/pwpage/
pwPage - provides an extremely fast and simple approach to the creation of database forms. That is, if a database table exists and an HTML page has been constructed using a few simple guidelines, pwPage can be immediately used for table data selections, insertions, updates, deletions and selectable table content reviewing.
http://www.omnis-software.com/products/studio/studio.html
OMNIS Studio is a rapid application development (RAD) tool.
http://www.webplus.com
talentsoft Web+ 4.6 - a powerful and comprehensive development language for use in creating web-based client/server applications without writing complicated, low-level, and time-consuming CGI programs.

1.2.12 Database Design Tools with MySQL Support

http://www.mysql.com/documentation/dezign/
"DeZign for databases" is a database development tool that uses an entity relationship diagram (ERD).

1.2.13 Web Servers with MySQL Tools

ftp://ftp.kcilink.com/pub/
mod_auth_mysql, An Apache authentication module.
http://www.roxen.com/
The Roxen Challenger Web server.

1.2.14 Extensions for Other Programs

http://www.seawood.org/msql_bind/
MySQL support for BIND (The Internet Domain Name Server).
http://www.inet-interactive.com/sendmail/
MySQL support for Sendmail and Procmail.

1.2.15 Using MySQL with Other Programs

http://www.iserver.com/support/addonhelp/database/mysql/msaccess.html
Using MySQL with Access.
http://www.iserver.com/support/contrib/perl5/modules.html
Installing new Perl modules that require locally installed modules.

1.2.16 ODBC-related Links

http://www.iodbc.org/
Popular iODBC Driver Manager (libiodbc) now available as Open Source.
http://users.ids.net/~bjepson/freeODBC/
The FreeODBC Pages.
http://genix.net/unixODBC/
The unixODBC Project goals are to develop and promote unixODBC to be the definitive standard for ODBC on the Linux platform. This is to include GUI support for KDE.
http://www.sw-soft.com/products/BtrieveODBC/
A MySQL-based ODBC driver for Btrieve.

1.2.17 API-related Links

http://www.jppp.com/
Partially implemented TDataset-compatible components for MySQL.
http://www.riverstyx.net/qpopmysql/
qpopmysql - A patch to allow POP3 authentication from a MySQL database. There's also a link to Paul Khavkine's patch for Procmail to allow any MTA to deliver to users in a MySQL database.
http://www.pbc.ottawa.on.ca
Visual Basic class generator for Active X.
http://www.essencesw.com/Software/mysqllib.html
New Client libraries for the Mac OS Classic (Macintosh).
http://www.lilback.com/macsql/
Client libraries for the Macintosh.
http://www.essencesw.com/Plugins/mysqlplug.html
Plugin for REALbasic (for Macintosh)
http://www.iis.ee.ethz.ch/~neeri/macintosh/gusi-qa.html
A library that emulates BSD sockets and pthreads on Macintosh. This can be used if you want to compile the MySQL client library on Mac. It could probably even be sued to port MySQL to Macintosh, but we don't know of anyone that has tried that.
http://www.dedecker.net/jessie/scmdb/
SCMDB - an add-on for SCM that ports the MySQL C library to scheme (SCM). With this library scheme developers can make connections to a MySQL database and use embedded SQL in their programs.

1.2.18 Other MySQL-related Links

SAT
The Small Application Toolkit (SAT) is a collection of utilities intended to simplify the development of small, multi-user, GUI based applications in a (Microsoft -or- X) Windows Client / Unix Server environment.
http://www.wix.com/mysql-hosting/
Registry of Web providers who support MySQL.
http://www.softagency.co.jp/mysql/index.en.html
Links about using MySQL in Japan/Asia.
http://abattoir.cc.ndsu.nodak.edu/~nem/mysql/udf/
MySQL UDF Registry.
http://www.open.com.au/products.html
Commercial Web defect tracking system.
http://www.stonekeep.com/pts/
PTS: Project Tracking System.
http://tomato.nvgc.vt.edu/~hroberts/mot
Job and software tracking system.
http://www.cynergi.net/exportsql/
ExportSQL: A script to export data from Access95+.
http://SAL.KachinaTech.COM/H/1/MYSQL.html
SAL (Scientific Applications on Linux) MySQL entry.
http://www.infotech-nj.com/itech/index.shtml
A consulting company which mentions MySQL in the right company.
http://www.pmpcs.com/
PMP Computer Solutions. Database developers using MySQL and mSQL.
http://www.aewa.org/
Airborne Early Warning Association.
http://www.dedserius.com/y2kmatrix/
Y2K tester.

1.2.19 SQL and Database Interfaces

http://java.sun.com/products/jdbc/
The JDBC database access API.
http://www.gagme.com/mysql
Patch for mSQL Tcl.
http://www.amsoft.ru/easysql/
EasySQL: An ODBC-like driver manager.
http://www.lightlink.com/hessling/rexxsql.html
A REXX interface to SQL databases.
http://www.mytcl.cx/
Tcl interface based on tcl-sql with many bugfixes.
http://www.binevolve.com/~tdarugar/tcl-sql/
Tcl interface.
http://www.contrib.andrew.cmu.edu/~shadow/sql.html
SQL Reference Page with a lot of interesting links.

1.2.20 Examples of MySQL Use

http://www.little6.com/about/linux/
Little6 Inc., An online contract and job finding site that is powered by MySQL, PHP3, and Linux.
http://www.delec.com/is/products/prep/examples/BookShelf/index.html
DELECis - A tool that makes it very easy to create an automatically generated table documentation. They have used MySQL as an example.
http://www.worldrecords.com
World Records - A search engine for information about music that uses MySQL and PHP.
http://www.webtechniques.com/archives/1998/01/note/
A Contact Database using MySQL and PHP.
http://modems.rosenet.net/mysql/
Web based interface and Community Calendar with PHP.
http://www.odbsoft.com/cook/sources.htm
Perl package to generate html from a SQL table structure and for generating SQL statements from an html form.
http://www.gusnet.cx/proj/telsql/
Basic telephone database using DBI/DBD.
http://tecfa.unige.ch/guides/java/staf2x/ex/jdbc/coffee-break
JDBC examples by Daniel K. Schneider.
http://www.spade.com/linux/howto/PostgreSQL-HOWTO-41.html
SQL BNF
http://www.ooc.com/
Object Oriented Concepts Inc; CORBA applications with examples in source.
http://www.pbc.ottawa.on.ca/
DBWiz; Includes an example of how to manage cursors in VB.
http://keilor.cs.umass.edu/pluribus/
Pluribus is a free search engine that learns to improve the quality of its results over time. Pluribus works by recording which pages a user prefers among those returned for a query. A user votes for a page by selecting it; Pluribus then uses that knowledge to improve the quality of the results when someone else submits the same (or similar) query. Uses PHP and MySQL.
http://www.stopbit.com/
Stopbit - A technology news site using MySQL and PHP.
http://www.linuxsupportline.com/~kalendar/
KDE based calendar manager - The calendar manager has both single user (file based) and multi-user (MySQL database) support.
http://tim.desert.net/~tim/imger/
Example of storing/retrieving images with MySQL and CGI.
http://www.penguinservices.com/scripts
Online shopping cart system.
http://www.city-gallery.com/album/
Old Photo Album - The album is a collaborative popular history of photography project that generates all pages from data stored in a MySQL database. Pages are dynamically generated through a php3 interface to the database content. Users contribute images and descriptions. Contributed images are stored on the web server to avoid storing them in the database as BLOBs. All other information is stored on the shared MySQL server.

1.2.21 General Database Links

http://www.pcslink.com/~ej/dbweb.html
Database Jump Site
http://black.hole-in-the.net/guy/webdb/
Homepage of the webdb-l (Web Databases) mailing list.
http://www.symbolstone.org/technology/perl/DBI/index.html
Perl DBI/DBD modules homepage.
http://www.student.uni-koeln.de/cygwin/
Cygwin tools. Unix on top of Windows.
http://dbasecentral.com/
dbasecentral.com; Development and distribution of powerful and easy-to-use database applications and systems.
http://www.tek-tips.com/
Tek-Tips Forums are 800+ independent peer-to-peer non-commercial support forums for Computer Professionals. Features include automatic e-mail notification of responses, a links library, and member confidentiality guaranteed.
http://www.public.asu.edu/~peterjn/btree/
B-Trees: Balanced Tree Data Structures.
http://www.fit.qut.edu.au/~maire/baobab/lecture/sld001.htm
A lecture about B-Trees.

There are also many Web pages that use MySQL. See section B Some MySQL Users. Send any additions to this list to webmaster@mysql.com. We now require that you show a MySQL logo somewhere if you wish your site to be added. It is okay to have it on a ``used tools'' page or something similar.

1.2.22 MySQL Mailing Lists

This section introduces you to the MySQL mailing lists, and gives some guidelines as to how to use them.

1.2.22.1 The MySQL Mailing Lists

To subscribe to the main MySQL mailing list, send a message to the electronic mail address mysql-subscribe@lists.mysql.com.

To unsubscribe from the main MySQL mailing list, send a message to the electronic mail address mysql-unsubscribe@lists.mysql.com.

Only the address to which you send your messages is significant. The subject line and the body of the message are ignored.

If your reply address is not valid, you can specify your address explicitly. Adding a hyphen to the subscribe or unsubscribe command word, followed by your address with the `@' character in your address replaced by a `='. For example, to subscribe your_name@host.domain, send a message to mysql-subscribe-your_name=host.domain@lists.mysql.com.

Mail to mysql-subscribe@lists.mysql.com or mysql-unsubscribe@lists.mysql.com is handled automatically by the ezmlm mailing list processor. Information about ezmlm is available at The ezmlm Website.

To post a message to the list itself, send your message to mysql@lists.mysql.com. However, please do not send mail about subscribing or unsubscribing to mysql@lists.mysql.com, because any mail sent to that address is distributed automatically to thousands of other users.

Your local site may have many subscribers to mysql@lists.mysql.com. If so, it may have a local mailing list, so that messages sent from lists.mysql.com to your site are propagated to the local list. In such cases, please contact your system administrator to be added to or dropped from the local MySQL list.

If you wish to have traffic for a mailing list go to a separate mailbox in your mail program, set up a filter based on the message headers. You can use either the List-ID: or Delivered-To: headers to identify list messages.

The following MySQL mailing lists exist:

announce-subscribe@lists.mysql.com announce
This is for announcement of new versions of MySQL and related programs. This is a low volume list all MySQL users should subscribe to.
mysql-subscribe@lists.mysql.com mysql
The main list for general MySQL discussion. Please note that some topics are better discussed on the more-specialized lists. If you post to the wrong list, you may not get an answer!
mysql-digest-subscribe@lists.mysql.com mysql-digest
The mysql list in digest form. That means you get all individual messages, sent as one large mail message once a day.
bugs-subscribe@lists.mysql.com bugs
On this list you should only post a full, repeatable bug report using the mysqlbug script (if you are running on Windows, you should include a description of the operating system and the MySQL version). Preferably, you should test the problem using the latest stable or development version of MySQL before posting! Anyone should be able to repeat the bug by just using mysql test < script on the included test case. All bugs posted on this list will be corrected or documented in the next MySQL release! If there are only small code changes involved, we will also post a patch that fixes the problem.
bugs-digest-subscribe@lists.mysql.com bugs-digest
The bugs list in digest form.
internals-subscribe@lists.mysql.com internals
A list for people who work on the MySQL code. On this list one can also discuss MySQL development and post patches.
internals-digest-subscribe@lists.mysql.com internals-digest
A digest version of the internals list.
java-subscribe@lists.mysql.com java
Discussion about MySQL and Java. Mostly about the JDBC drivers.
java-digest-subscribe@lists.mysql.com java-digest
A digest version of the java list.
win32-subscribe@lists.mysql.com win32
All things concerning MySQL on Microsoft operating systems such as Win95, Win98, NT, and Win2000.
win32-digest-subscribe@lists.mysql.com win32-digest
A digest version of the win32 list.
myodbc-subscribe@lists.mysql.com myodbc
All things about connecting to MySQL with ODBC.
myodbc-digest-subscribe@lists.mysql.com myodbc-digest
A digest version of the myodbc list.
plusplus-subscribe@lists.mysql.com plusplus
All things concerning programming with the C++ API to MySQL.
plusplus-digest-subscribe@lists.mysql.com plusplus-digest
A digest version of the plusplus list.
msql-mysql-modules-subscribe@lists.mysql.com msql-mysql-modules
A list about the Perl support in MySQL. msql-mysql-modules
msql-mysql-modules-digest-subscribe@lists.mysql.com msql-mysql-modules-digest
A digest version of the msql-mysql-modules list.

You subscribe or unsubscribe to all lists in the same way as described above. In your subscribe or unsubscribe message, just put the appropriate mailing list name rather than mysql. For example, to subscribe to or unsubscribe from the myodbc list, send a message to myodbc-subscribe@lists.mysql.com or myodbc-unsubscribe@lists.mysql.com.

If you can't get an answer for your questions from the mailing list, one option is to pay for support from MySQL AB, which will put you in direct contact with MySQL developers. See section 1.3.5 Types of Commercial Support.

The following table shows some MySQL mailing in other languages than English. Note that these are not operated by MySQL AB, so we can't guarantee the quality on these.

mysql-france-subscribe@yahoogroups.com A French mailing list
list@tinc.net A Korean mailing list
Email subscribe mysql your@email.address to this list.
mysql-de-request@lists.4t2.com A German mailing list
Email subscribe mysql-de your@email.address to this list. You can find information about this mailing list at http://www.4t2.com/mysql.
mysql-br-request@listas.linkway.com.br A Portugese mailing list
Email subscribe mysql-br your@email.address to this list.
mysql-alta@elistas.net A Spanish mailing list
Email subscribe mysql your@email.address to this list.

1.2.22.2 Asking Questions or Reporting Bugs

Before posting a bug report or question, please do the following:

If you can't find an answer in the manual or the archives, check with your local MySQL expert. If you still can't find an answer to your question, go ahead and read the next section about how to send mail to mysql@lists.mysql.com.

1.2.22.3 How to Report Bugs or Problems

Writing a good bug report takes patience, but doing it right the first time saves time for us and for you. A good bug report containing a full test case for the bug will make it very likely that we will fix it in the next release. This section will help you write your report correctly so that you don't waste your time doing things that may not help us much or at all.

We encourage everyone to use the mysqlbug script to generate a bug report (or a report about any problem), if possible. mysqlbug can be found in the `scripts' directory in the source distribution, or, for a binary distribution, in the `bin' directory under your MySQL installation directory. If you are unable to use mysqlbug, you should still include all the necessary information listed in this section.

The mysqlbug script helps you generate a report by determining much of the following information automatically, but if something important is missing, please include it with your message! Please read this section carefully and make sure that all the information described here is included in your report.

The normal place to report bugs and problems is mysql@lists.mysql.com. If you can make a test case that clearly demonstrates the bug, you should post it to the bugs@lists.mysql.com list. Note that on this list you should only post a full, repeatable bug report using the mysqlbug script. If you are running on Windows, you should include a description of the operating system and the MySQL version. Preferably, you should test the problem using the latest stable or development version of MySQL before posting! Anyone should be able to repeat the bug by just using ``mysql test < script'' on the included test case or run the shell or perl script that is included in the bug report. All bugs posted on the bugs list will be corrected or documented in the next MySQL release! If there are only small code changes involved to correct this problem, we will also post a patch that fixes the problem.

Remember that it is possible to respond to a message containing too much information, but not to one containing too little. Often people omit facts because they think they know the cause of a problem and assume that some details don't matter. A good principle is: if you are in doubt about stating something, state it! It is a thousand times faster and less troublesome to write a couple of lines more in your report than to be forced to ask again and wait for the answer because you didn't include enough information the first time.

The most common errors are that people don't indicate the version number of the MySQL distribution they are using, or don't indicate what platform they have MySQL installed on (including the platform version number). This is highly relevant information, and in 99 cases out of 100 the bug report is useless without it! Very often we get questions like, ``Why doesn't this work for me?'' then we find that the feature requested wasn't implemented in that MySQL version, or that a bug described in a report has been fixed already in newer MySQL versions. Sometimes the error is platform dependent; in such cases, it is next to impossible to fix anything without knowing the operating system and the version number of the platform.

Remember also to provide information about your compiler, if it is related to the problem. Often people find bugs in compilers and think the problem is MySQL-related. Most compilers are under development all the time and become better version by version. To determine whether or not your problem depends on your compiler, we need to know what compiler is used. Note that every compiling problem should be regarded as a bug report and reported accordingly.

It is most helpful when a good description of the problem is included in the bug report. That is, a good example of all the things you did that led to the problem and the problem itself exactly described. The best reports are those that include a full example showing how to reproduce the bug or problem. See section G.1.6 Making a test case when you experience table corruption.

If a program produces an error message, it is very important to include the message in your report! If we try to search for something from the archives using programs, it is better that the error message reported exactly matches the one that the program produces. (Even the case should be observed!) You should never try to remember what the error message was; instead, copy and paste the entire message into your report!

If you have a problem with MyODBC, you should try to generate a MyODBC trace file. See section 8.3.7 Reporting Problems with MyODBC.

Please remember that many of the people who will read your report will do so using an 80-column display. When generating reports or examples using the mysql command line tool, you should therefore use the --vertical option (or the \G statement terminator) for output that would exceed the available width for such a display (for example, with the EXPLAIN SELECT statement; see the example below).

Please include the following information in your report:

If you are a support customer, please cross-post the bug report to mysql-support@mysql.com for higher priority treatment, as well as to the appropriate mailing list to see if someone else has experienced (and perhaps solved) the problem.

For information on reporting bugs in MyODBC, see section 8.3.4 How to Report Problems with MyODBC.

For solutions to some common problems, see See section A Problems and Common Errors.

When answers are sent to you individually and not to the mailing list, it is considered good etiquette to summarize the answers and send the summary to the mailing list so that others may have the benefit of responses you received that helped you solve your problem!

1.2.22.4 Guidelines for Answering Question on the Mailing List

If you consider your answer to have broad interest, you may want to post it to the mailing list instead of replying directly to the individual who asked. Try to make your answer general enough that people other than the original poster may benefit from it. When you post to the list, please make sure that your answer is not a duplication of a previous answer.

Try to summarize the essential part of the question in your reply; don't feel obliged to quote the entire original message.

Please don't post mail messages from your browser with HTML mode turned on! Many users don't read mail with a browser!

1.3 MySQL Licensing and Support

This section describes MySQL support and licensing arrangements:

1.3.1 MySQL Licensing Policy

The formal terms of the GPL license can be found at section K GNU GENERAL PUBLIC LICENSE. Basically, our licensing policy and interpretation of the GPL is as follows:

Note that older versions of MySQL are still using a more strict license. See the documentation for that version for more information. If you need a commercial MySQL license, because the GPL license doesn't suit your application, you can buy one at https://order.mysql.com/.

For normal internal use, MySQL costs nothing. You do not have to pay us if you do not want to.

A license is required if:

A license is NOT required if:

For circumstances under which a MySQL license is required, you need a license per machine that runs the mysqld server. However, a multiple-CPU machine counts as a single machine, and there is no restriction on the number of MySQL servers that run on one machine, or on the number of clients concurrently connected to a server running on that machine!

If you have any questions as to whether or not a license is required for your particular use of MySQL, please read this again and then contact us. See section 1.3.4.2 Contact Information.

If you require a MySQL license, the easiest way to pay for it is to use the license form on MySQL's secure server at https://order.mysql.com/. Other forms of payment are discussed in section 1.3.4.1 Payment information.

1.3.2 Copyrights Used by MySQL

There are several different copyrights on the MySQL distribution:

  1. The MySQL-specific source needed to build the mysqlclient library is licensed under the LGPL and programs in the `client' directory is GPL. Each file has a header that shows which copyright is used for that file.
  2. The client library and the (GNU getopt) library are covered by the ``GNU LIBRARY GENERAL PUBLIC LICENSE.'' See section L GNU LESSER GENERAL PUBLIC LICENSE.
  3. Some parts of the source (the regexp library) are covered by a Berkeley-style copyright.
  4. All the source in the server and the (GNU readline) library is covered by the ``GNU GENERAL PUBLIC LICENSE.'' See section K GNU GENERAL PUBLIC LICENSE. This is also available as the file `COPYING' in the distributions.

One goal is that the SQL client library should be free enough that it is possible to add MySQL support into commercial products without a license. For this reason, we chose the LGPL license for the client code.

This means that you can use MySQL for free with any program that uses any of the free software licenses. MySQL is also free for any end user for his own or company usage.

However, if you use MySQL for something important to you, you may want to help secure its development by purchasing licenses or a support contract. See section 1.3.5 Types of Commercial Support.

1.3.2.1 Copyright Changes

Version 3.22 of MySQL is still using a more strict license. See the documentation for that version for more information.

1.3.3 Example Licensing Situations

This section describes some situations illustrating whether or not you must license the MySQL server. Generally these examples involve providing MySQL as an integral part of a product.

Note that a single MySQL license covers any number of CPUs and mysqld servers on a machine! There is no artificial limit on the number of clients that connect to the server in any way.

1.3.3.1 Selling Products that use MySQL

To determine whether or not you need a MySQL license when selling your application, you should ask whether the proper functioning of your application is dependent on the use of MySQL and whether you include the MySQL server with your product. There are several cases to consider:

1.3.3.2 ISP MySQL Services

Internet Service Providers (ISPs) often host MySQL servers for their customers. With the GPL license this does not require a license.

On the other hand, we do encourage people to use ISPs that have MySQL support, as this will give them the confidence that if they have some problem with their MySQL installation, their ISP will be able to solve the problem for them (in some cases with the help from the MySQL development team).

All ISPs that want to keep themselves up-to-date should subscribe to our announce mailing list so that they can be aware of fatal issues that may be relevant for their MySQL installations.

Note that if the ISP doesn't have a license for MySQL, it should give its customers at least read access to the source of the MySQL installation so that its customer can verify that it is patched correctly.

1.3.3.3 Running a Web Server Using MySQL

If you use MySQL in conjunction with a Web server on Unix, you don't have to pay for a license.

This is true even if you run a commercial Web server that uses MySQL, because you are not selling an embedded MySQL version yourself. However, in this case we would like you to purchase MySQL support, because MySQL is helping your enterprise.

1.3.4 MySQL Licensing and Support Costs

Our current license prices are shown below. To make a purchase, please visit https://order.mysql.com/.

If you pay by credit card, the currency is EURO (European Union Euro) so the prices will differ slightly.

Number of licenses Per copy
1-9 230 EURO
10-24 138 EURO
25-49 117 EURO
50-99 102 EURO
100-249 91 EURO
250-499 76 EURO
500-999 66 EURO

For high volume (OEM) purchases, please contact sales@mysql.com.

For OEM purchases, you must act as the middle-man for eventual problems or extension requests from your users. We also require that OEM customers have at least an extended e-mail support contract. Note that OEM licenses only apply for products where the user doesn't have direct access to the MySQL server (embedded system). In other words, the MySQL server should only be used with the application that was supplied you.

If you have a low-margin, high-volume product, you can always talk to us about other terms (for example, a percent of the sale price). If you do, please be informative about your product, pricing, market, and any other information that may be relevant.

A full-price license is not a support agreement and includes very minimal support. This means that we try to answer any relevant questions. If the answer is in the documentation, we will direct you to the appropriate section. If you have not purchased a license or support, we probably will not answer at all.

If you discover what we consider a real bug, we are likely to fix it in any case. But if you pay for support we will notify you about the fix status instead of just fixing it in a later release.

More comprehensive support is sold separately. Descriptions of what each level of support includes are given in section 1.3.5 Types of Commercial Support. Costs for the various types of commercial support are shown below. Support level prices are in EURO (European Union Euro). One EURO is about 1.06 USD.

Type of support Cost per year
Basic e-mail support. See section 1.3.5.1 Basic E-mail Support. EURO 200
Extended e-mail support See section 1.3.5.2 Extended E-mail Support. EURO 1000
Login support See section 1.3.5.3 Login Support. EURO 2000
Extended login support See section 1.3.5.4 Extended Login Support. EURO 5000
Telephone support See section 1.3.5.5 Telephone Support. EURO 12000

You may upgrade from any lower level of support to a higher level of support for the difference in price between the two support levels.

We do also provide telephone support (mostly emergency support but also 24/7 support). This support option doesn't however have a fixed price but is negotiated for case to case. If you are interested in this option you can email sales@mysql.com and tell us about your needs.

Note that as our sales staff is very busy, it may take some time until your request is handled. Our support staff does however always answer promptly to support questions!

1.3.4.1 Payment information

Currently we can take SWIFT payments, checks, or credit cards.

Payment should be made to:

Postgirot Bank AB
105 06 STOCKHOLM, SWEDEN

MySQL AB
BOX 6434
11382 STOCKHOLM, SWEDEN

SWIFT address: PGSI SESS
Account number: 96 77 06 - 3

Specify: license and/or support and your name and e-mail address.

In Europe and Japan you can use EuroGiro (that should be less expensive) to the same account.

If you want to pay by check, make it payable to ``MySQL Finland AB'' and mail it to the address below:

MySQL AB
BOX 6434, Torsgatan 21
11382 STOCKHOLM, SWEDEN

If you want to pay by credit card over the Internet, you can use MySQL AB's secure license form.

You can also print a copy of the license form, fill it in, and send it by fax to:

+46-8-729 69 05

If you want us to bill you, you can use the license form and write ``bill us'' in the comment field. You can also mail a message to sales@mysql.com (not mysql@lists.mysql.com!) with your company information and ask us to bill you.

1.3.4.2 Contact Information

For commercial licensing, please contact the MySQL licensing team. The much preferred method is by e-mail to licensing@mysql.com. Fax is also possible but handling of these may take much longer (Fax +46-8-729 69 05).

If you represent a business that is interested in partnering with MySQL, please send e-mail to partner@mysql.com.

For timely, precise answers to technical questions about MySQL you should order one of our support contracts. MySQL support is provided by the MySQL developers so the standard is extremely high.

If you are interested in placing a banner advertisement on our Web site, please send e-mail to advertising@mysql.com.

If you are interested in any of the jobs listed in our jobs section, please send e-mail to jobs@mysql.com.

For general discussion amongst our many users, please direct your attention to the appropriate mailing list.

For general information inquires, please send e-mail to info@mysql.com.

For questions or comments about the workings or content of the Web site, please send e-mail to webmaster@mysql.com.

1.3.5 Types of Commercial Support

The following is true of all support options:

1.3.5.1 Basic E-mail Support

Basic e-mail support is a very inexpensive support option and should be thought of more as a way to support our development of MySQL than as a real support option. We at MySQL do give a lot of free support in all the different MySQL lists, and the money we get from basic e-mail support is largely used to make this possible.

At this support level, the MySQL mailing lists are the preferred means of communication. Questions normally should be mailed to the primary mailing list (mysql@lists.mysql.com) or one of the other regular lists (for example, win32@lists.mysql.com for Windows-related MySQL questions), as someone else already may have experienced and solved the problem you have. See section 1.2.22.2 Asking Questions or Reporting Bugs.

However, by purchasing basic e-mail support, you also have access to the support address mysql-support@mysql.com, which is not available as part of the minimal support that you get by purchasing a MySQL license. This means that for especially critical questions, you can cross-post your message to mysql-support@mysql.com. (If the message contains sensitive data, you should post only to mysql-support@mysql.com.)

REMEMBER! to ALWAYS include your registration number and expiration date when you send a message to mysql-support@mysql.com.

Note that if you have encountered a critical, repeatable bug, and follow the rules outlined in the manual section of how to report bugs and send it to bugs@lists.mysql.com, we promise to try to fix this as soon as possible, regardless of your support level! See section 1.2.22.3 How to Report Bugs or Problems.

Basic e-mail support includes the following types of service:

1.3.5.2 Extended E-mail Support

Extended e-mail support includes everything in basic e-mail support with these additions:

1.3.5.3 Login Support

Login support includes everything in extended e-mail support with these additions:

1.3.5.4 Extended Login Support

Extended login support includes everything in login support with these additions:

1.3.5.5 Telephone Support

Telephone support includes everything in extended login support with these additions:

1.3.5.6 Support for other table handlers

To get support for BDB tables, InnoDB tables you have to pay an additional 30% on the standard support price for each of the table handlers you would like to have support for.

We at MySQL AB will help you create a proper bug report for the table handler and submit it to the developers for the specific table handler. We will also do our best to ensure that you will get a timely answer or solution from the developers of the table handler.

Even if we are quite confident that we can solve most problems within a timely manner, we can't guarantee a quick solution for any problems you can get with the different table handlers. We will however do our best to help you get the problem solved.

1.4 How Standards-compatible Is MySQL?

This section describes how MySQL relates to the ANSI SQL standards. MySQL has many extensions to the ANSI SQL standards, and here you will find out what they are, and how to use them. You will also find information about functionality missing from MySQL, and how to work around some differences.

1.4.1 MySQL Extensions to ANSI SQL92

MySQL includes some extensions that you probably will not find in other SQL databases. Be warned that if you use them, your code will not be portable to other SQL servers. In some cases, you can write code that includes MySQL extensions, but is still portable, by using comments of the form /*! ... */. In this case, MySQL will parse and execute the code within the comment as it would any other MySQL statement, but other SQL servers will ignore the extensions. For example:

SELECT /*! STRAIGHT_JOIN */ col_name FROM table1,table2 WHERE ...

If you add a version number after the '!', the syntax will only be executed if the MySQL version is equal to or newer than the used version number:

CREATE /*!32302 TEMPORARY */ TABLE (a int);

The above means that if you have Version 3.23.02 or newer, then MySQL will use the TEMPORARY keyword.

MySQL extensions are listed below:

1.4.2 MySQL Differences Compared to ANSI SQL92

We try to make MySQL follow the ANSI SQL standard and the ODBC SQL standard, but in some cases MySQL does some things differently:

1.4.3 Running MySQL in ANSI Mode

If you start mysqld with the --ansi option, the following behavior of MySQL changes:

This is the same as using --sql-mode=REAL_AS_FLOAT,PIPES_AS_CONCAT,ANSI_QUOTES,IGNORE_SPACE,SERIALIZE,ONLY_FULL_GROUP_BY.

1.4.4 Functionality Missing from MySQL

The following functionality is missing in the current version of MySQL. For a prioritized list indicating when new extensions may be added to MySQL, you should consult the online MySQL TODO list. That is the latest version of the TODO list in this manual. See section 1.6 MySQL and the future (The TODO).

1.4.4.1 Sub-selects

MySQL currently only supports sub selects of the form INSERT ... SELECT ... and REPLACE ... SELECT .... You can however use the function IN() in other contexts.

In many cases you can rewrite the query without a sub-select:

SELECT * FROM table1 WHERE id IN (SELECT id FROM table2);

This can be re-written as:

SELECT table1.* FROM table1,table2 WHERE table1.id=table2.id;

The queries:

SELECT * FROM table1 WHERE id NOT IN (SELECT id FROM table2);
SELECT * FROM table1 WHERE NOT EXISTS (SELECT id FROM table2 where table1.id=table2.id);

Can be rewritten as:

SELECT table1.* FROM table1 LEFT JOIN table2 ON table1.id=table2.id where table2.id IS NULL

For more complicated subqueries you can often create temporary tables to hold the subquery. In some cases, however this option will not work. The most frequently encountered of these cases arises with DELETE statements, for which standard SQL does not support joins (except in sub-selects). For this situation there are two options available until subqueries are supported by MySQL.

The first option is to use a procedural programming language (such as Perl or PHP) to submit a SELECT query to obtain the primary keys for the records to be deleted, and then use these values to construct the DELETE statement (DELETE FROM ... WHERE ... IN (key1, key2, ...)).

The second option is to use interactive SQL to contruct a set of DELETE statements automatically, using the MySQL extension CONCAT() (in lieu of the standard || operator). For example:

SELECT CONCAT('DELETE FROM tab1 WHERE pkid = ', tab1.pkid, ';')
  FROM tab1, tab2
 WHERE tab1.col1 = tab2.col2;

You can place this query in a script file and redirect input from it to the mysql command-line interpreter, piping its output back to a second instance of the interpreter:

prompt> mysql --skip-column-names mydb < myscript.sql | mysql mydb

MySQL 4.0 supports multi-table deletes that can be used to efficiently delete rows based on information from one table or even from many tables at the same time.

1.4.4.2 SELECT INTO TABLE

MySQL doesn't yet support the Oracle SQL extension: SELECT ... INTO TABLE .... MySQL supports instead the ANSI SQL syntax INSERT INTO ... SELECT ..., which is basically the same thing. See section 6.4.2.1 INSERT ... SELECT Syntax.

INSERT INTO tblTemp2 (fldID) SELECT tblTemp1.fldOrder_ID FROM tblTemp1 WHERE
tblTemp1.fldOrder_ID > 100;

Alternatively, you can use SELECT INTO OUTFILE... or CREATE TABLE ... SELECT to solve your problem.

1.4.4.3 Transactions

As MySQL does nowadays support transactions, the following discussion is only valid if you are only using the non-transaction-safe table types. See section 6.7.1 BEGIN/COMMIT/ROLLBACK Syntax.

The question is often asked, by the curious and the critical, ``Why is MySQL not a transactional database?'' or ``Why does MySQL not support transactions?''

MySQL has made a conscious decision to support another paradigm for data integrity, ``atomic operations.'' It is our thinking and experience that atomic operations offer equal or even better integrity with much better performance. We, nonetheless, appreciate and understand the transactional database paradigm and plan, within the next few releases, to introduce transaction-safe tables on a per table basis. We will be giving our users the possibility to decide if they need the speed of atomic operations or if they need to use transactional features in their applications.

How does one use the features of MySQL to maintain rigorous integrity and how do these features compare with the transactional paradigm?

First, in the transactional paradigm, if your applications are written in a way that is dependent on the calling of ``rollback'' instead of ``commit'' in critical situations, then transactions are more convenient. Moreover, transactions ensure that unfinished updates or corrupting activities are not committed to the database; the server is given the opportunity to do an automatic rollback and your database is saved.

MySQL, in almost all cases, allows you to solve for potential problems by including simple checks before updates and by running simple scripts that check the databases for inconsistencies and automatically repair or warn if such occurs. Note that just by using the MySQL log or even adding one extra log, one can normally fix tables perfectly with no data integrity loss.

Moreover, fatal transactional updates can be rewritten to be atomic. In fact,we will go so far as to say that all integrity problems that transactions solve can be done with LOCK TABLES or atomic updates, ensuring that you never will get an automatic abort from the database, which is a common problem with transactional databases.

Not even transactions can prevent all loss if the server goes down. In such cases even a transactional system can lose data. The difference between different systems lies in just how small the time-lap is where they could lose data. No system is 100% secure, only ``secure enough.'' Even Oracle, reputed to be the safest of transactional databases, is reported to sometimes lose data in such situations.

To be safe with MySQL, you only need to have backups and have the update logging turned on. With this you can recover from any situation that you could with any transactional database. It is, of course, always good to have backups, independent of which database you use.

The transactional paradigm has its benefits and its drawbacks. Many users and application developers depend on the ease with which they can code around problems where an abort appears to be, or is necessary, and they may have to do a little more work with MySQL to either think differently or write more. If you are new to the atomic operations paradigm, or more familiar or more comfortable with transactions, do not jump to the conclusion that MySQL has not addressed these issues. Reliability and integrity are foremost in our minds. Recent estimates indicate that there are more than 1,000,000 mysqld servers currently running, many of which are in production environments. We hear very, very seldom from our users that they have lost any data, and in almost all of those cases user error is involved. This is, in our opinion, the best proof of MySQL's stability and reliability.

Lastly, in situations where integrity is of highest importance, MySQL's current features allow for transaction-level or better reliability and integrity. If you lock tables with LOCK TABLES, all updates will stall until any integrity checks are made. If you only obtain a read lock (as opposed to a write lock), then reads and inserts are still allowed to happen. The new inserted records will not be seen by any of the clients that have a READ lock until they release their read locks. With INSERT DELAYED you can queue inserts into a local queue, until the locks are released, without having the client wait for the insert to complete. See section 6.4.3 INSERT DELAYED syntax.

``Atomic,'' in the sense that we mean it, is nothing magical. It only means that you can be sure that while each specific update is running, no other user can interfere with it, and there will never be an automatic rollback (which can happen on transaction based systems if you are not very careful). MySQL also guarantees that there will not be any dirty reads. You can find some example of how to write atomic updates in the commit-rollback section. See section 1.4.6 How to Cope Without COMMIT/ROLLBACK.

We have thought quite a bit about integrity and performance, and we believe that our atomic operations paradigm allows for both high reliability and extremely high performance, on the order of three to five times the speed of the fastest and most optimally tuned of transactional databases. We didn't leave out transactions because they are hard to do. The main reason we went with atomic operations as opposed to transactions is that by doing this we could apply many speed optimizations that would not otherwise have been possible.

Many of our users who have speed foremost in their minds are not at all concerned about transactions. For them transactions are not an issue. For those of our users who are concerned with or have wondered about transactions vis-a-vis MySQL, there is a ``MySQL way'' as we have outlined above. For those where safety is more important than speed, we recommend them to use the BDB, or InnoDB tables for all their critical data. See section 7 MySQL Table Types.

One final note: We are currently working on a safe replication schema that we believe to be better than any commercial replication system we know of. This system will work most reliably under the atomic operations, non-transactional, paradigm. Stay tuned.

1.4.4.4 Stored Procedures and Triggers

A stored procedure is a set of SQL commands that can be compiled and stored in the server. Once this has been done, clients don't need to keep reissuing the entire query but can refer to the stored procedure. This provides better performance because the query has to be parsed only once, and less information needs to be sent between the server and the client. You can also raise the conceptual level by having libraries of functions in the server.

A trigger is a stored procedure that is invoked when a particular event occurs. For example, you can install a stored procedure that is triggered each time a record is deleted from a transaction table and that automatically deletes the corresponding customer from a customer table when all his transactions are deleted.

The planned update language will be able to handle stored procedures, but without triggers. Triggers usually slow down everything, even queries for which they are not needed.

To see when MySQL might get stored procedures, see section 1.6 MySQL and the future (The TODO).

1.4.4.5 Foreign Keys

Note that foreign keys in SQL are not used to join tables, but are used mostly for checking referential integrity (foreign key constraints). If you want to get results from multiple tables from a SELECT statement, you do this by joining tables:

SELECT * from table1,table2 where table1.id = table2.id;

See section 6.4.1.1 JOIN Syntax. See section 3.5.6 Using Foreign Keys.

The FOREIGN KEY syntax in MySQL exists only for compatibility with other SQL vendors' CREATE TABLE commands; it doesn't do anything. The FOREIGN KEY syntax without ON DELETE ... is mostly used for documentation purposes. Some ODBC applications may use this to produce automatic WHERE clauses, but this is usually easy to override. FOREIGN KEY is sometimes used as a constraint check, but this check is unnecessary in practice if rows are inserted into the tables in the right order. MySQL only supports these clauses because some applications require them to exist (regardless of whether or not they work).

In MySQL, you can work around the problem of ON DELETE ... not being implemented by adding the appropriate DELETE statement to an application when you delete records from a table that has a foreign key. In practice this is as quick (in some cases quicker) and much more portable than using foreign keys.

In the near future we will extend the FOREIGN KEY implementation so that at least the information will be saved in the table specification file and may be retrieved by mysqldump and ODBC. At a later stage we will implement the foreign key constraints for application that can't easily be coded to avoid them.

1.4.4.6 Reasons NOT to Use Foreign Keys constraints

There are so many problems with foreign key constraints that we don't know where to start:

The only nice aspect of FOREIGN KEY is that it gives ODBC and some other client programs the ability to see how a table is connected and to use this to show connection diagrams and to help in building applications.

MySQL will soon store FOREIGN KEY definitions so that a client can ask for and receive an answer about how the original connection was made. The current `.frm' file format does not have any place for it. At a later stage we will implement the foreign key constraints for application that can't easily be coded to avoid them.

1.4.4.7 Views

MySQL doesn't yet support views, but we plan to implement these to about 4.1.

Views are mostly useful for letting users access a set of relations as one table (in read-only mode). Many SQL databases don't allow one to update any rows in a view, but you have to do the updates in the separate tables.

As MySQL is mostly used in applications and on web system where the application writer has full control on the database usage, most of our users haven't regarded views to be very important. (At least no one has been interested enough in this to be prepared to finance the implementation of views).

One doesn't need views in MySQL to restrict access to columns as MySQL has a very sophisticated privilege system. See section 4.2 General Security Issues and the MySQL Access Privilege System.

1.4.4.8 `--' as the Start of a Comment

Some other SQL databases use `--' to start comments. MySQL has `#' as the start comment character, even if the mysql command-line tool removes all lines that start with `--'. You can also use the C comment style /* this is a comment */ with MySQL. See section 6.1.5 Comment Syntax.

MySQL Version 3.23.3 and above supports the `--' comment style only if the comment is followed by a space. This is because this degenerate comment style has caused many problems with automatically generated SQL queries that have used something like the following code, where we automatically insert the value of the payment for !payment!:

UPDATE tbl_name SET credit=credit-!payment!

What do you think will happen when the value of payment is negative?

Because 1--1 is legal in SQL, we think it is terrible that `--' means start comment.

In MySQL Version 3.23 you can, however, use: 1-- This is a comment

The following discussion only concerns you if you are running a MySQL version earlier than Version 3.23:

If you have a SQL program in a text file that contains `--' comments you should use:

shell> replace " --" " #" < text-file-with-funny-comments.sql \
         | mysql database

instead of the usual:

shell> mysql database < text-file-with-funny-comments.sql

You can also edit the command file ``in place'' to change the `--' comments to `#' comments:

shell> replace " --" " #" -- text-file-with-funny-comments.sql

Change them back with this command:

shell> replace " #" " --" -- text-file-with-funny-comments.sql

1.4.5 What Standards Does MySQL Follow?

Entry level SQL92. ODBC levels 0-2.

1.4.6 How to Cope Without COMMIT/ROLLBACK

The following mostly applies only for ISAM, MyISAM, and HEAP tables. If you only use transaction-safe tables (BDB, or InnoDB tables) in an an update, you can do COMMIT and ROLLBACK also with MySQL. See section 6.7.1 BEGIN/COMMIT/ROLLBACK Syntax.

The problem with handling COMMIT-ROLLBACK efficiently with the above table types would require a completely different table layout than MySQL uses today. The table type would also need extra threads that do automatic cleanups on the tables, and the disk usage would be much higher. This would make these table types about 2-4 times slower than they are today.

For the moment, we prefer implementing the SQL server language (something like stored procedures). With this you would very seldom really need COMMIT-ROLLBACK. This would also give much better performance.

Loops that need transactions normally can be coded with the help of LOCK TABLES, and you don't need cursors when you can update records on the fly.

We at TcX had a greater need for a real fast database than a 100% general database. Whenever we find a way to implement these features without any speed loss, we will probably do it. For the moment, there are many more important things to do. Check the TODO for how we prioritize things at the moment. (Customers with higher levels of support can alter this, so things may be reprioritized.)

The current problem is actually ROLLBACK. Without ROLLBACK, you can do any kind of COMMIT action with LOCK TABLES. To support ROLLBACK with the above table types, MySQL would have to be changed to store all old records that were updated and revert everything back to the starting point if ROLLBACK was issued. For simple cases, this isn't that hard to do (the current isamlog could be used for this purpose), but it would be much more difficult to implement ROLLBACK for ALTER/DROP/CREATE TABLE.

To avoid using ROLLBACK, you can use the following strategy:

  1. Use LOCK TABLES ... to lock all the tables you want to access.
  2. Test conditions.
  3. Update if everything is okay.
  4. Use UNLOCK TABLES to release your locks.

This is usually a much faster method than using transactions with possible ROLLBACKs, although not always. The only situation this solution doesn't handle is when someone kills the threads in the middle of an update. In this case, all locks will be released but some of the updates may not have been executed.

You can also use functions to update records in a single operation. You can get a very efficient application by using the following techniques:

For example, when we are doing updates to some customer information, we update only the customer data that has changed and test only that none of the changed data, or data that depend on the changed data, has changed compared to the original row. The test for changed data is done with the WHERE clause in the UPDATE statement. If the record wasn't updated, we give the client a message: "Some of the data you have changed have been changed by another user". Then we show the old row versus the new row in a window, so the user can decide which version of the customer record he should use.

This gives us something that is similar to column locking but is actually even better, because we only update some of the columns, using values that are relative to their current values. This means that typical UPDATE statements look something like these:

UPDATE tablename SET pay_back=pay_back+'relative change';

UPDATE customer
  SET
    customer_date='current_date',
    address='new address',
    phone='new phone',
    money_he_owes_us=money_he_owes_us+'new_money'
  WHERE
    customer_id=id AND address='old address' AND phone='old phone';

As you can see, this is very efficient and works even if another client has changed the values in the pay_back or money_he_owes_us columns.

In many cases, users have wanted ROLLBACK and/or LOCK TABLES for the purpose of managing unique identifiers for some tables. This can be handled much more efficiently by using an AUTO_INCREMENT column and either the SQL function LAST_INSERT_ID() or the C API function mysql_insert_id(). See section 8.4.3.126 mysql_insert_id().

At MySQL AB, we have never had any need for row-level locking because we have always been able to code around it. Some cases really need row locking, but they are very few. If you want row-level locking, you can use a flag column in the table and do something like this:

UPDATE tbl_name SET row_flag=1 WHERE id=ID;

MySQL returns 1 for the number of affected rows if the row was found and row_flag wasn't already 1 in the original row.

You can think of it as MySQL changed the above query to:

UPDATE tbl_name SET row_flag=1 WHERE id=ID and row_flag <> 1;

1.4.7 Known errors and design deficiencies in MySQL

The following problems are known and have a very high priority to get fixed:

The following problems are known and will be fixed in due time:

The following are known bugs in earlier versions of MySQL:

For platform-specific bugs, see the sections about compiling and porting.

1.5 How MySQL Compares to Other Databases

This section compares MySQL to other popular databases.

This section has been written by the MySQL developers, so it should be read with that in mind. There are no factual errors contained in this section that we know of. If you find something which you believe to be an error, please contact us about it at docs@mysql.com.

For a list of all supported limits, functions, and types, see the crash-me Web page at http://www.mysql.com/information/crash-me.php.

1.5.1 How MySQL Compares to mSQL

Performance
For a true comparison of speed, consult the growing MySQL benchmark suite. See section 5.1.4 The MySQL Benchmark Suite. Because there is no thread creation overhead, a small parser, few features, and simple security, mSQL should be quicker at: Because these operations are so simple, it is hard to be better at them when you have a higher startup overhead. After the connection is established, MySQL should perform much better. On the other hand, MySQL is much faster than mSQL (and most other SQL implementations) on the following:
SQL Features
Disk Space Efficiency
That is, how small can you make your tables? MySQL has very precise types, so you can create tables that take very little space. An example of a useful MySQL datatype is the MEDIUMINT that is 3 bytes long. If you have 100,000,000 records, saving even one byte per record is very important. mSQL2 has a more limited set of column types, so it is more difficult to get small tables.
Stability
This is harder to judge objectively. For a discussion of MySQL stability, see section 1.1.7 How Stable Is MySQL?. We have no experience with mSQL stability, so we cannot say anything about that.
Price
Another important issue is the license. MySQL has a more flexible license than mSQL, and is also less expensive than mSQL. Whichever product you choose to use, remember to at least consider paying for a license or e-mail support. (You are required to get a license if you include MySQL with a product that you sell, of course.)
Perl Interfaces
MySQL has basically the same interfaces to Perl as mSQL with some added features.
JDBC (Java)
MySQL currently has a lot of different JDBC drivers: The recommended driver is the mm driver. The Resin driver may also be good (at least the benchmarks looks good), but we haven't received that much information about this yet. We know that mSQL has a JDBC driver, but we have too little experience with it to compare.
Rate of Development
MySQL has a very small team of developers, but we are quite used to coding C and C++ very rapidly. Because threads, functions, GROUP BY, and so on are still not implemented in mSQL, it has a lot of catching up to do. To get some perspective on this, you can view the mSQL `HISTORY' file for the last year and compare it with the News section of the MySQL Reference Manual (see section F MySQL change history). It should be pretty obvious which one has developed most rapidly.
Utility Programs
Both mSQL and MySQL have many interesting third-party tools. Because it is very easy to port upward (from mSQL to MySQL), almost all the interesting applications that are available for mSQL are also available for MySQL. MySQL comes with a simple msql2mysql program that fixes differences in spelling between mSQL and MySQL for the most-used C API functions. For example, it changes instances of msqlConnect() to mysql_connect(). Converting a client program from mSQL to MySQL usually takes a couple of minutes.

1.5.1.1 How to Convert mSQL Tools for MySQL

According to our experience, it would just take a few hours to convert tools such as msql-tcl and msqljava that use the mSQL C API so that they work with the MySQL C API.

The conversion procedure is:

  1. Run the shell script msql2mysql on the source. This requires the replace program, which is distributed with MySQL.
  2. Compile.
  3. Fix all compiler errors.

Differences between the mSQL C API and the MySQL C API are:

1.5.1.2 How mSQL and MySQL Client/Server Communications Protocols Differ

There are enough differences that it is impossible (or at least not easy) to support both.

The most significant ways in which the MySQL protocol differs from the mSQL protocol are listed below:

1.5.1.3 How mSQL 2.0 SQL Syntax Differs from MySQL

Column types

MySQL
Has the following additional types (among others; see section 6.5.3 CREATE TABLE Syntax):
MySQL also supports the following additional type attributes:
mSQL2
mSQL column types correspond to the MySQL types shown below:
mSQL type Corresponding MySQL type
CHAR(len) CHAR(len)
TEXT(len) TEXT(len). len is the maximal length. And LIKE works.
INT INT. With many more options!
REAL REAL. Or FLOAT. Both 4- and 8-byte versions are available.
UINT INT UNSIGNED
DATE DATE. Uses ANSI SQL format rather than mSQL's own format.
TIME TIME
MONEY DECIMAL(12,2). A fixed-point value with two decimals.

Index Creation

MySQL
Indexes may be specified at table creation time with the CREATE TABLE statement.
mSQL
Indexes must be created after the table has been created, with separate CREATE INDEX statements.

To Insert a Unique Identifier into a Table

MySQL
Use AUTO_INCREMENT as a column type specifier. See section 8.4.3.126 mysql_insert_id().
mSQL
Create a SEQUENCE on a table and select the _seq column.

To Obtain a Unique Identifier for a Row

MySQL
Add a PRIMARY KEY or UNIQUE key to the table and use this. New in Version 3.23.11: If the PRIMARY or UNIQUE key consists of only one column and this is of type integer, one can also refer to it as _rowid.
mSQL
Use the _rowid column. Observe that _rowid may change over time depending on many factors.

To Get the Time a Column Was Last Modified

MySQL
Add a TIMESTAMP column to the table. This column is automatically set to the current date and time for INSERT or UPDATE statements if you don't give the column a value or if you give it a NULL value.
mSQL
Use the _timestamp column.

NULL Value Comparisons

MySQL
MySQL follows ANSI SQL, and a comparison with NULL is always NULL.
mSQL
In mSQL, NULL = NULL is TRUE. You must change =NULL to IS NULL and <>NULL to IS NOT NULL when porting old code from mSQL to MySQL.

String Comparisons

MySQL
Normally, string comparisons are performed in case-independent fashion with the sort order determined by the current character set (ISO-8859-1 Latin1 by default). If you don't like this, declare your columns with the BINARY attribute, which causes comparisons to be done according to the ASCII order used on the MySQL server host.
mSQL
All string comparisons are performed in case-sensitive fashion with sorting in ASCII order.

Case-insensitive Searching

MySQL
LIKE is a case-insensitive or case-sensitive operator, depending on the columns involved. If possible, MySQL uses indexes if the LIKE argument doesn't start with a wild-card character.
mSQL
Use CLIKE.

Handling of Trailing Spaces

MySQL
Strips all spaces at the end of CHAR and VARCHAR columns. Use a TEXT column if this behavior is not desired.
mSQL
Retains trailing space.

WHERE Clauses

MySQL
MySQL correctly prioritizes everything (AND is evaluated before OR). To get mSQL behavior in MySQL, use parentheses (as shown in an example below).
mSQL
Evaluates everything from left to right. This means that some logical calculations with more than three arguments cannot be expressed in any way. It also means you must change some queries when you upgrade to MySQL. You do this easily by adding parentheses. Suppose you have the following mSQL query:
mysql> SELECT * FROM table WHERE a=1 AND b=2 OR a=3 AND b=4;
To make MySQL evaluate this the way that mSQL would, you must add parentheses:
mysql> SELECT * FROM table WHERE (a=1 AND (b=2 OR (a=3 AND (b=4))));

Access Control

MySQL
Has tables to store grant (permission) options per user, host, and database. See section 4.2.5 How the Privilege System Works.
mSQL
Has a file `mSQL.acl' in which you can grant read/write privileges for users.

1.5.2 How MySQL Compares to PostgreSQL

When reading the following, please note that both products are continually evolving. We at MySQL AB and the PostgreSQL developers are both working on making our respective database as good as possible, so we are both a serious choice to any commercial database.

The following comparison is made by us at MySQL AB. We have tried to be as accurate and fair as possible, but because we don't have a full knowledge of all PostgreSQL features while we know MySQL througly, we may have got some things wrong. We will however correct these when they come to our attention.

We would first like to note that PostgreSQL and MySQL are both widely used products, but with different design goals, even if we are both striving to be ANSI SQL compatible. This means that for some applications MySQL is more suited, while for others PostgreSQL is more suited. When choosing which database to use, you should first check if the database's feature set satisfies your application. If you need raw speed, MySQL is probably your best choice. If you need some of the extra features that only PostgreSQL can offer, you should use PostgreSQL.

1.5.2.1 MySQL and PostgreSQL development strategies

When adding things to MySQL we take pride to do an optimal, definite solution. The code should be so good that we shouldn't have any need to change it in the foreseeable future. We also do not like to sacrifice speed for features but instead will do our utmost to find a solution that will give maximal throughput. This means that development will take a little longer, but the end result will be well worth this. This kind of development is only possible because all server code are checked by one of a few (currently two) persons before it's included in the MySQL server.

We at MySQL AB believe in frequent releases to be able to push out new features quickly to our users. Because of this we do a new small release about every three weeks, and a major branch every year. All releases are throughly tested with our testing tools on a lot of different platforms.

PostgreSQL is based on a kernel with lots of contributors. In this setup it makes sense to prioritize adding a lot of new features, instead of implementing them optimally, because one can always optimize things later if there arises a need for this.

Another big difference between MySQL and PostgreSQL is that nearly all of the code in the MySQL server are coded by developers that are employed by MySQL AB and are still working on the server code. The exceptions are the transaction engines, and the regexp library.

This is in sharp contrast to the PostgreSQL code where the majority of the code is coded by a big group of people with different backgrounds. It was only recently that the PostgreSQL developers announced that their current developer group had finally had time to take a look at all the code in the current PostgreSQL release.

Both of the above development methods has it's own merits and drawbacks. We here at MySQL AB think of course that our model is better because our model gives better code consistency, more optimal and reusable code, and in our opinion, fewer bugs. Because we are the authors of the MySQL server code, we are better able to coordinate new features and releases.

1.5.2.2 Featurewise Comparison of MySQL and PostgreSQL

On the crash-me page you can find a list of those database constructs and limits that one can detect automatically with a program. Note however that a lot of the numerical limits may be changed with startup options for respective database. The above web page is however extremely useful when you want to ensure that your applications works with many different databases or when you want to convert your application from one datbase to another.

MySQL offers the following advantages over PostgreSQL:

Drawbacks with MySQL compared to PostgreSQL:

PostgreSQL currently offers the following advantages over MySQL:

Note that because we know the MySQL road map, we have included in the following table the version when MySQL should support this feature. Unfortunately we couldn't do this for previous comparison, because we don't know the PostgreSQL roadmap.

Feature MySQL version
Subselects 4.1
Foreign keys 4.0 and 4.1
Views 4.2
Stored procedures 4.1
Extensible type system Not planned
Unions 4.0
Full join 4.0 or 4.1
Triggers 4.1
Constrainst 4.1
Cursors 4.1 or 4.2
Extensible index types like R-trees R-trees are planned for 4.2
Inherited tables Not planned

Other reasons to use PostgreSQL:

Drawbacks with PostgreSQL compared to MySQL:

For a complete list of drawbacks, you should also examine the first table in this section.

1.5.2.3 Benchmarking MySQL and PostgreSQL

The only open source benchmark that we know of that can be used to benchmark MySQL and PostgreSQL (and other databases) is our own. It can be found at http://www.mysql.com/information/benchmarks.html.

We have many times asked the PostgreSQL developers and some PostgreSQL users to help us extend this benchmark to make it the definitive benchmark for databases, but unfortunately we haven't gotten any feedback for this.

We the MySQL developers have, because of this, spent a lot of hours to get maximum performance from PostgreSQL for the benchmarks, but because we don't know PostgreSQL intimately, we are sure that there are things that we have missed. We have on the benchmark page documented exactly how we did run the benchmark so that it should be easy for anyone to repeat and verify our results.

The benchmarks are usually run with and without the --fast option. When run with --fast we are trying to use every trick the server can do to get the code to execute as fast as possible. The idea is that the normal run should show how the server would work in a default setup and the --fast run shows how the server would do if the application developer would use extensions in the server to make his application run faster.

When running with PostgreSQL and --fast we do a VACUUM() after every major table UPDATE and DROP TABLE to make the database in perfect shape for the following SELECTs. The time for VACUUM() is measured separately.

When running with PostgreSQL 7.1.1 we could, however, not run with --fast because during the INSERT test, the postmaster (the PostgreSQL deamon) died and the database was so corrupted that it was impossible to restart postmaster. After this happened twice, we decided to postpone the --fast test until next PostgreSQL release. The details about the machine we run the benchmark can be found on the benchmark page.

Before going to the other benchmarks we know of, we would like to give some background on benchmarks:

It's very easy to write a test that shows ANY database to be best database in the world, by just restricting the test to something the database is very good at and not test anything that the database is not good at. If one after this publishes the result with a single figure, things are even easier.

This would be like us measuring the speed of MySQL compared to PostgreSQL by looking at the summary time of the MySQL benchmarks on our web page. Based on this MySQL would be more than 40 times faster than PostgreSQL, something that is of course not true. We could make things even worse by just taking the test where PostgreSQL performs worst and claim that MySQL is more than 2000 times faster than PostgreSQL.

The case is that MySQL does a lot of optimizations that PostgreSQL doesn't do. This is of course also true the other way around. An SQL optimizer is a very complex thing, and a company could spend years on just making the optimizer faster and faster.

When looking at the benchmark results you should look for things that you do in your application and just use these results to decide which database would be best suited for your application. The benchmark results also shows things a particular database is not good at and should give you a notion about things to avoid and what you may have to do in other ways.

We know of two benchmark tests that claims that PostgreSQL performs better than MySQL. These both where multi-user tests, a test that we here at MySQL AB haven't had time to write and include in the benchmark suite, mainly because it's a big task to do this in a manner that is fair against all databases.

One is the benchmark paid for by Great Bridge, which you can read about at: http://www.greatbridge.com/about/press.php?content_id=4.

This is the probably worst benchmark we have ever seen anyone conduct. This was not only tuned to only test what PostgreSQL is absolutely best at, it was also totally unfair against every other database involved in the test.

NOTE: We know that not even some of the main PostgreSQL developers did like the way Great Bridge conducted the benchmark, so we don't blame them for the way the benchmark was made.

This benchmark has been condemned in a lot of postings and newsgroups so we will here just shortly repeat some things that where wrong with it.

Tim Perdue, a long time PostgreSQL fan and a reluctant MySQL user published a comparison on phpbuider.

When we became aware of the comparison, we phoned Tim Perdue about this because there were a lot of strange things in his results. For example, he claimed that MySQL had a problem with five users in his tests, when we know that there are users with similar machines as his that are using MySQL with 2000 simultaneous connections doing 400 queries per second. (In this case the limit was the web bandwidth, not the database.)

It sounded like he was using a Linux kernel that either had some problems with many threads, such as kernels before 2.4, which had a problem with many threads on multi-CPU machines. We have documented in this manual how to fix this and Tim should be aware of this problem.

The other possible problem could have been an old glibc library and that Tim didn't use a MySQL binary from our site, which is linked with a corrected glibc library, but had compiled a version of his own with. In any of the above cases, the symptom would have been exactly what Tim had measured.

We asked Tim if we could get access to his data so that we could repeat the benchmark and if he could check the MySQL version on the machine to find out what was wrong and he promised to come back to us about this. He has not done that yet.

Because of this we can't put any trust in this benchmark either :(

Over time things also changes and the above benchmarks are not that relevant anymore. MySQL now have a couple of different table handlers with different speed/concurrency tradeoffs. See section 7 MySQL Table Types. It would be interesting to see how the above tests would run with the different transactional table types in MySQL. PostgreSQL has of course also got new features since the test was made. As the above test are not publicly available there is no way for us to know how the database would preform in the same tests today.

Conclusion:

The only benchmarks that exist today that anyone can download and run against MySQL and PostgreSQL is the MySQL benchmarks. We here at MySQL believe that open source databases should be tested with open source tools! This is the only way to ensure that no one does tests that nobody can reproduce and use this to claim that a database is better than another. Without knowing all the facts it's impossible to answer the claims of the tester.

The thing we find strange is that every test we have seen about PostgreSQL, that is impossible to reproduce, claims that PostgreSQL is better in most cases while our tests, which anyone can reproduce, clearly shows otherwise. With this we don't want to say that PostgreSQL isn't good at many things (it is!) or that it isn't faster than MySQL under certain conditions. We would just like to see a fair test where they are very good so that we could get some friendly competition going!

For more information about our benchmarks suite See section 5.1.4 The MySQL Benchmark Suite.

We are working on an even better benchmark suite, including multi user tests, and a better documentation of what the individual tests really do and how to add more tests to the suite.

1.6 MySQL and the future (The TODO)

This appendix lists the features that we plan to implement in MySQL.

Everything in this list is approximately in the order it will be done. If you want to affect the priority order, please register a license or support us and tell us what you want to have done more quickly. See section 1.3 MySQL Licensing and Support.

The plan is that we in the future will support the full ANSI SQL99 standard, but with a lot of useful extensions. The challenge is to do this without sacrifying the speed or compromise the code.

1.6.1 Things that should be in 4.0

We plan to make MySQL Version 4.0 a ``quick'' release where we only add some new stuff to enable others to help us with developing new features into Version 4.1. The MySQL 4.0 version should only take us about a month to make after which we want to stabilize it and start working on Version 4.1. Version 4.0 should have the following new features:

The news section for 4.0 includes a list of the features we have already implemented in the 4.0 tree. See section F.1 Changes in release 4.0.x (Development; Alpha).

1.6.2 Things that must be done in the real near future

1.6.3 Things that have to be done sometime

Time is given according to amount of work, not real time.

1.6.4 Some things we don't have any plans to do

2 MySQL Installation

This chapter describes how to obtain and install MySQL:

2.1 Quick Standard Installation of MySQL

2.1.1 Installing MySQL on Linux

The recommended way to install MySQL on Linux is by using an RPM file. The MySQL RPMs are currently being built on a RedHat Version 6.2 system but should work on other versions of Linux that support rpm and use glibc.

If you have problems with an RPM file, for example, if you receive the error ``Sorry, the host 'xxxx' could not be looked up'', see section 2.6.1.1 Linux Notes for Binary Distributions.

The RPM files you may want to use are:

To see all files in an RPM package, run:

shell> rpm -qpl MySQL-VERSION.i386.rpm

To perform a standard minimal installation, run:

shell> rpm -i MySQL-VERSION.i386.rpm MySQL-client-VERSION.i386.rpm

To install just the client package, run:

shell> rpm -i MySQL-client-VERSION.i386.rpm

The RPM places data in `/var/lib/mysql'. The RPM also creates the appropriate entries in `/etc/rc.d/' to start the server automatically at boot time. (This means that if you have performed a previous installation, you may want to make a copy of your previously installed MySQL startup file if you made any changes to it, so you don't lose your changes.)

After installing the RPM file(s), the mysqld daemon should be running and you should now be able to start using MySQL. See section 2.4 Post-installation Setup and Testing.

If something goes wrong, you can find more information in the binary installation chapter. See section M.1 Installing a MySQL Binary Distribution.

2.1.2 Installing MySQL on Windows

The following instructions apply to precompiled binary distributions. If you download a source distribution, you will have to compile and install it yourself.

If you don't have a copy of the MySQL distribution, you should first download one from http://www.mysql.com/downloads/mysql-3.23.html.

If you plan to connect to MySQL from some other program, you will probably also need the MyODBC driver. You can find this at the MyODBC download page (http://www.mysql.com/downloads/api-myodbc.html).

To install either distribution, unzip it in some empty directory and run the Setup.exe program.

By default, MySQL-Windows is configured to be installed in `C:\mysql'. If you want to install MySQL elsewhere, install it in `C:\mysql' first, then move the installation to where you want it. If you do move MySQL, you must indicate where everything is located by supplying a --basedir option when you start the server. For example, if you have moved the MySQL distribution to `D:\programs\mysql', you must start mysqld like this:

C:\> D:\programs\mysql\bin\mysqld --basedir D:\programs\mysql

Use mysqld --help to display all the options that mysqld understands!

With all newer MySQL versions, you can also create a `C:\my.cnf' file that holds any default options for the MySQL server. Copy the file `\mysql\my-xxxxx.cnf' to `C:\my.cnf' and edit it to suit your setup. Note that you should specify all paths with `/' instead of `\'. If you use `\', you need to specify it twice, because `\' is the escape character in MySQL. See section 4.1.2 my.cnf Option Files.

Starting with MySQL 3.23.38, the Windows distribution includes both the normal and the MySQL-Max binaries. The main benefit of using the normal mysqld.exe binary is that it's a little faster and uses less resources.

Here is a list of the different MySQL servers you can use:

mysqld Compiled with full debugging and automatic memory allocation checking, symbolic links, BDB and InnoDB tables.
mysqld-opt Optimized binary with no support for transactional tables.
mysqld-nt Optimized binary for NT with support for named pipes. You can run this version on Win98, but in this case no named pipes are created and you must have TCP/IP installed.
mysqld-max Optimized binary with support for symbolic links, BDB and InnoDB tables.
mysqld-max-nt Like mysqld-max, but compiled with support for named pipes.

All of the above binaries are optimized for the Pentium Pro processor but should work on any Intel processor >= i386.

NOTE: If you want to use InnoDB tables, there are certain startup options that must be specified in your `my.ini' file! See section 7.6.2 InnoDB startup options.

2.2 General Installation Issues

2.2.1 How to Get MySQL

Check the MySQL home page for information about the current version and for downloading instructions.

Our main download mirror is located at:

http://download.sourceforge.net/mirrors/mysql/

If you are interested in becoming a MySQL mirror site, you may anonymously rsync with: rsync://download.sourceforge.net/mysql/. Please send e-mail to webmaster@mysql.com notifying us of your mirror to be added to the list below.

If you have problems downloading from our main site, try using one of the mirrors listed below.

Please report bad or out-of-date mirrors to webmaster@mysql.com.

Europe:

North America:

South America:

Asia:

Africa:

2.2.2 Operating Systems Supported by MySQL

We use GNU Autoconf, so it is possible to port MySQL to all modern systems with working Posix threads and a C++ compiler. (To compile only the client code, a C++ compiler is required but not threads.) We use and develop the software ourselves primarily on Sun Solaris (Versions 2.5 - 2.7) and SuSE Linux Version 7.x.

Note that for many operating systems, the native thread support works only in the latest versions. MySQL has been reported to compile successfully on the following operating system/thread package combinations:

Note that not all platforms are suited equally well for running MySQL. How well a certain platform is suited for a high-load mission critical MySQL server is determined by the following factors:

Based on the above criteria, the best platforms for running MySQL at this point are x86 with SuSE Linux 7.1, 2.4 kernel and ReiserFS (or any similar Linux distribution) and Sparc with Solaris 2.7 or 2.8. FreeBSD comes third, but we really hope it will join the top club once the thread library is improved. We also hope that at some point we will be able to include all other platforms on which MySQL compiles, runs ok, but not quite with the same level of stability and performance, into the top category. This will require some effort on our part in cooperation with the developers of the OS/library components MySQL depends upon. If you are interested in making one of those components better, are in a position to influence their development, and need more detailed instructions on what MySQL needs to run better, send an e-mail to internals@lists.mysql.com.

Please note that the comparison above is not to say that one OS is better or worse than the other in general. We are talking about choosing a particular OS for a dedicated purpose - running MySQL, and compare platforms in that regard only. With this in mind, the result of this comparison would be different if we included more issues into it. And in some cases, the reason one OS is better than the other could simply be that we have put forth more effort into testing on and optimizing for that particular platform. We are just stating our observations to help you make a decision on which platform to use MySQL on in your setup.

2.2.3 Which MySQL Version to Use

The first decision to make is whether you want to use the latest development release or the last stable release:

The second decision to make is whether you want to use a source distribution or a binary distribution. In most cases you should probably use a binary distribution, if one exists for your platform, as this generally will be easier to install than a source distribution.

In the following cases you probably will be better off with a source installation:

The MySQL naming scheme uses release numbers that consist of three numbers and a suffix. For example, a release name like mysql-3.21.17-beta is interpreted like this:

All versions of MySQL are run through our standard tests and benchmarks to ensure that they are relatively safe to use. Because the standard tests are extended over time to check for all previously found bugs, the test suite keeps getting better.

Note that all releases have been tested at least with:

An internal test suite
This is part of a production system for a customer. It has many tables with hundreds of megabytes of data.
The MySQL benchmark suite
This runs a range of common queries. It is also a test to see whether the latest batch of optimizations actually made the code faster. See section 5.1.4 The MySQL Benchmark Suite.
The crash-me test
This tries to determine what features the database supports and what its capabilities and limitations are. See section 5.1.4 The MySQL Benchmark Suite.

Another test is that we use the newest MySQL version in our internal production environment, on at least one machine. We have more than 100 gigabytes of data to work with.

2.2.4 Installation Layouts

This section describes the default layout of the directories created by installing binary and source distributions.

A binary distribution is installed by unpacking it at the installation location you choose (typically `/usr/local/mysql') and creates the following directories in that location:

Directory Contents of directory
`bin' Client programs and the mysqld server
`data' Log files, databases
`include' Include (header) files
`lib' Libraries
`scripts' mysql_install_db
`share/mysql' Error message files
`sql-bench' Benchmarks

A source distribution is installed after you configure and compile it. By default, the installation step installs files under `/usr/local', in the following subdirectories:

Directory Contents of directory
`bin' Client programs and scripts
`include/mysql' Include (header) files
`info' Documentation in Info format
`lib/mysql' Libraries
`libexec' The mysqld server
`share/mysql' Error message files
`sql-bench' Benchmarks and crash-me test
`var' Databases and log files

Within an installation directory, the layout of a source installation differs from that of a binary installation in the following ways:

You can create your own binary installation from a compiled source distribution by executing the script `scripts/make_binary_distribution'.

2.2.5 How and When Updates Are Released

MySQL is evolving quite rapidly here at MySQL AB and we want to share this with other MySQL users. We try to make a release when we have very useful features that others seem to have a need for.

We also try to help out users who request features that are easy to implement. We take note of what our licensed users want to have, and we especially take note of what our extended e-mail supported customers want and try to help them out.

No one has to download a new release. The News section will tell you if the new release has something you really want. See section F MySQL change history.

We use the following policy when updating MySQL:

The current stable release is Version 3.23; We have already moved active development to Version 4.0. Bugs will still be fixed in the stable version. We don't believe in a complete freeze, as this also leaves out bug fixes and things that ``must be done.'' ``Somewhat frozen'' means that we may add small things that ``almost surely will not affect anything that's already working.''

2.2.6 MySQL Binaries Compiled by MySQL AB

As a service, we at MySQL AB provide a set of binary distributions of MySQL that are compiled at our site or at sites where customers kindly have given us access to their machines.

These distributions are generated with scripts/make_binary_distribution and are configured with the following compilers and options:

SunOS 4.1.4 2 sun4c with gcc 2.7.2.1
CC=gcc CXX=gcc CXXFLAGS="-O3 -felide-constructors" ./configure --prefix=/usr/local/mysql --disable-shared --with-extra-charsets=complex --enable-assembler
SunOS 5.5.1 (and above) sun4u with egcs 1.0.3a or 2.90.27 or gcc 2.95.2 and newer
CC=gcc CFLAGS="-O3" CXX=gcc CXXFLAGS="-O3 -felide-constructors -fno-exceptions -fno-rtti" ./configure --prefix=/usr/local/mysql --with-low-memory --with-extra-charsets=complex --enable-assembler
SunOS 5.6 i86pc with gcc 2.8.1
CC=gcc CXX=gcc CXXFLAGS=-O3 ./configure --prefix=/usr/local/mysql --with-low-memory --with-extra-charsets=complex
Linux 2.0.33 i386 with pgcc 2.90.29 (egcs 1.0.3a)
CFLAGS="-O3 -mpentium -mstack-align-double" CXX=gcc CXXFLAGS="-O3 -mpentium -mstack-align-double -felide-constructors -fno-exceptions -fno-rtti" ./configure --prefix=/usr/local/mysql --enable-assembler --with-mysqld-ldflags=-all-static --with-extra-charsets=complex
Linux 2.2.x with x686 with gcc 2.95.2
CFLAGS="-O3 -mpentiumpro" CXX=gcc CXXFLAGS="-O3 -mpentiumpro -felide-constructors -fno-exceptions -fno-rtti" ./configure --prefix=/usr/local/mysql --enable-assembler --with-mysqld-ldflags=-all-static --disable-shared --with-extra-charset=complex
SCO 3.2v5.0.4 i386 with gcc 2.7-95q4
CC=gcc CXX=gcc CXXFLAGS=-O3 ./configure --prefix=/usr/local/mysql --with-extra-charsets=complex
AIX 2 4 with gcc 2.7.2.2
CC=gcc CXX=gcc CXXFLAGS=-O3 ./configure --prefix=/usr/local/mysql --with-extra-charsets=complex
OSF1 V4.0 564 alpha with gcc 2.8.1
CC=gcc CFLAGS=-O CXX=gcc CXXFLAGS=-O3 ./configure --prefix=/usr/local/mysql --with-low-memory --with-extra-charsets=complex
Irix 6.3 IP32 with gcc 2.8.0
CC=gcc CXX=gcc CXXFLAGS=-O3 ./configure --prefix=/usr/local/mysql --with-extra-charsets=complex
BSDI BSD/OS 3.1 i386 with gcc 2.7.2.1
CC=gcc CXX=gcc CXXFLAGS=-O ./configure --prefix=/usr/local/mysql --with-extra-charsets=complex
BSDI BSD/OS 2.1 i386 with gcc 2.7.2
CC=gcc CXX=gcc CXXFLAGS=-O3 ./configure --prefix=/usr/local/mysql --with-extra-charsets=complex

Anyone who has more optimal options for any of the configurations listed above can always mail them to the developer's mailing list at internals@lists.mysql.com.

RPM distributions prior to MySQL Version 3.22 are user-contributed. Beginning with Version 3.22, the RPMs are generated by us at MySQL AB.

If you want to compile a debug version of MySQL, you should add --with-debug or --with-debug=full to the above configure lines and remove any -fomit-frame-pointer options.

2.3 Installing a MySQL Source Distribution

Before you proceed with the source installation, check first to see if our binary is available for your platform and if it will work for you. We put in a lot of effort into making sure that our binaries are built with the best possible options.

You need the following tools to build and install MySQL from source:

If you are using a recent version of gcc, recent enough to understand -fno-exceptions option, it is VERY IMPORTANT that you use it. Otherwise, you may compile a binary that crashes randomly. We also recommend that you use -felide-contructors and -fno-rtti along with -fno-exceptions. When in doubt, do the following:


CFLAGS="-O3" CXX=gcc CXXFLAGS="-O3 -felide-constructors -fno-exceptions -fno-rtti" ./configure --prefix=/usr/local/mysql --enable-assembler --with-mysqld-ldflags=-all-static

On most systems this will give you a fast and stable binary.

If you run into problems, PLEASE ALWAYS USE mysqlbug when posting questions to mysql@lists.mysql.com. Even if the problem isn't a bug, mysqlbug gathers system information that will help others solve your problem. By not using mysqlbug, you lessen the likelihood of getting a solution to your problem! You will find mysqlbug in the `scripts' directory after you unpack the distribution. See section 1.2.22.3 How to Report Bugs or Problems.

2.3.1 Quick Installation Overview

The basic commands you must execute to install a MySQL source distribution are:

shell> groupadd mysql
shell> useradd -g mysql mysql
shell> gunzip < mysql-VERSION.tar.gz | tar -xvf -
shell> cd mysql-VERSION
shell> ./configure --prefix=/usr/local/mysql
shell> make
shell> make install
shell> scripts/mysql_install_db
shell> chown -R root  /usr/local/mysql
shell> chown -R mysql /usr/local/mysql/var
shell> chgrp -R mysql /usr/local/mysql
shell> cp support-files/my-medium.cnf /etc/my.cnf
shell> /usr/local/mysql/bin/safe_mysqld --user=mysql &

If you want have support for InnoDB tables, you should edit the /etc/my.cnf file and remove the # character before the parameters that starts with innodb_.... See section 4.1.2 my.cnf Option Files. See section 7.6.2 InnoDB startup options.

If you start from a source RPM, then do the following:

shell> rpm --rebuild MySQL-VERSION.src.rpm

This will make a binary RPM that you can install.

You can add new users using the bin/mysql_setpermission script if you install the DBI and Msql-Mysql-modules Perl modules.

A more detailed description follows.

To install a source distribution, follow the steps below, then proceed to section 2.4 Post-installation Setup and Testing, for post-installation initialization and testing:

  1. Pick the directory under which you want to unpack the distribution, and move into it.
  2. Obtain a distribution file from one of the sites listed in section 2.2.1 How to Get MySQL.
  3. If you are interested in using Berkeley DB tables with MySQL, you will need to obtain a patched version of the Berkeley DB source code. Please read the chapter on Berkeley DB tables before proceeding. See section 7.5 BDB or Berkeley_DB Tables. MySQL source distributions are provided as compressed tar archives and have names like `mysql-VERSION.tar.gz', where VERSION is a number like 3.23.42.
  4. Add a user and group for mysqld to run as:
    shell> groupadd mysql
    shell> useradd -g mysql mysql
    
    These commands add the mysql group, and the mysql user. The syntax for useradd and groupadd may differ slightly on different versions of Unix. They may also be called adduser and addgroup. You may wish to call the user and group something else instead of mysql.
  5. Unpack the distribution into the current directory:
    shell> gunzip < /path/to/mysql-VERSION.tar.gz | tar xvf -
    
    This command creates a directory named `mysql-VERSION'.
  6. Change into the top-level directory of the unpacked distribution:
    shell> cd mysql-VERSION
    
    Note that currently you must configure and build MySQL from this top-level directory. You can not build it in a different directory.
  7. Configure the release and compile everything:
    shell> ./configure --prefix=/usr/local/mysql
    shell> make
    
    When you run configure, you might want to specify some options. Run ./configure --help for a list of options. section 2.3.3 Typical configure Options, discusses some of the more useful options. If configure fails, and you are going to send mail to mysql@lists.mysql.com to ask for assistance, please include any lines from `config.log' that you think can help solve the problem. Also include the last couple of lines of output from configure if configure aborts. Post the bug report using the mysqlbug script. See section 1.2.22.3 How to Report Bugs or Problems. If the compile fails, see section 2.3.5 Problems Compiling?, for help with a number of common problems.
  8. Install everything:
    shell> make install
    
    You might need to run this command as root.
  9. Create the MySQL grant tables (necessary only if you haven't installed MySQL before):
    shell> scripts/mysql_install_db
    
    Note that MySQL versions older than Version 3.22.10 started the MySQL server when you run mysql_install_db. This is no longer true!
  10. Change ownership of binaries to root and ownership of the data directory to the user that you will run mysqld as:
    shell> chown -R root  /usr/local/mysql
    shell> chown -R mysql /usr/local/mysql/var
    shell> chgrp -R mysql /usr/local/mysql
    
    The first command changes the owner attribute of the files to the root user, the second one changes the owner attribute of the data directory to the mysql user, and the third one changes the group attribute to the mysql group.
  11. If you want to install support for the Perl DBI/DBD interface, see section M.2 Perl Installation Comments.
  12. If you would like MySQL to start automatically when you boot your machine, you can copy support-files/mysql.server to the location where your system has its startup files. More information can be found in the support-files/mysql.server script itself and in section 2.4.3 Starting and Stopping MySQL Automatically.

After everything has been installed, you should initialize and test your distribution:

shell> /usr/local/mysql/bin/safe_mysqld --user=mysql &

If that command fails immediately with mysqld daemon ended then you can find some information in the file `mysql-data-directory/'hostname'.err'. The likely reason is that you already have another mysqld server running. See section 4.1.4 Running Multiple MySQL Servers on the Same Machine.

See section 2.4 Post-installation Setup and Testing.

2.3.2 Applying Patches

Sometimes patches appear on the mailing list or are placed in the patches area of the MySQL Web site.

To apply a patch from the mailing list, save the message in which the patch appears in a file, change into the top-level directory of your MySQL source tree, and run these commands:

shell> patch -p1 < patch-file-name
shell> rm config.cache
shell> make clean

Patches from the FTP site are distributed as plain text files or as files compressed with gzip. Apply a plain patch as shown above for mailing list patches. To apply a compressed patch, change into the top-level directory of your MySQL source tree and run these commands:

shell> gunzip < patch-file-name.gz | patch -p1
shell> rm config.cache
shell> make clean

After applying a patch, follow the instructions for a normal source install, beginning with the ./configure step. After running the make install step, restart your MySQL server.

You may need to bring down any currently running server before you run make install. (Use mysqladmin shutdown to do this.) Some systems do not allow you to install a new version of a program if it replaces the version that is currently executing.

2.3.3 Typical configure Options

The configure script gives you a great deal of control over how you configure your MySQL distribution. Typically you do this using options on the configure command line. You can also affect configure using certain environment variables. See section H Environment Variables. For a list of options supported by configure, run this command:

shell> ./configure --help

Some of the more commonly-used configure options are described below:

2.3.4 Installing from the Development Source Tree

CAUTION: You should read this section only if you are interested in helping us test our new code. If you just want to get MySQL up and running on your system, you should use a standard release distribution (either a source or binary distribution will do).

To obtain our most recent development source tree, use these instructions:

  1. Download BitKeeper from http://www.bitmover.com/cgi-bin/download.cgi. You will need Bitkeeper 2.0 or newer to access our repository.
  2. Follow the instructions to install it.
  3. After BitKeeper is installed, use this command if you want to clone the MySQL 3.23 branch:
    shell> bk clone bk://work.mysql.com:7000 mysql
    
    To clone the 4.0 branch, use this command instead:
    shell> bk clone bk://work.mysql.com:7001 mysql-4.0
    
    The initial download of the source tree may take a while, depending on the speed of your connection; be patient.
  4. You will need GNU autoconf, automake, libtool, and m4 to run the next set of commands. If you get some strange error during this stage, check that you really have libtool installed!
    shell> cd mysql
    shell> bk -r edit
    shell> aclocal; autoheader; autoconf;  automake;
    shell> ./configure  # Add your favorite options here
    shell> make
    
    A collection of our standard configure scripts is located in the `BUILD/' subdirectory. If you are lazy, you can use `BUILD/compile-pentium-debug'. To compile on a different architecture, modify the script removing flags that are Pentium-specific.
  5. When the build is done, run make install. Be careful with this on a production machine; the command may overwrite your live release installation. If you have another installation of MySQL, we recommand that you run ./configure with different values for the prefix, tcp-port, and unix-socket-path options than those used for your production server.
  6. Play hard with your new installation and try to make the new features crash. Start by running make test. See section 9.3.2 MySQL Test Suite.
  7. If you have gotten to the make stage and the distribution does not compile, please report it to bugs@lists.mysql.com. If you have installed the latest versions of the required GNU tools, and they crash trying to process our configuration files, please report that also. However, if you execute aclocal and get a command not found error or a similar problem, do not report it. Instead, make sure all the necessary tools are installed and that your PATH variable is set correctly so your shell can find them.
  8. After the initial bk clone operation to get the source tree, you should run bk pull periodically to get the updates.
  9. You can examine the change history for the tree with all the diffs by using bk sccstool. If you see some funny diffs or code that you have a question about, do not hesitate to send e-mail to internals@lists.mysql.com. Also, if you think you have a better idea on how to do something, send an email to the same address with a patch. bk diffs will produce a patch for you after you have made changes to the source. If you do not have the time to code your idea, just send a description.
  10. BitKeeper has a nice help utility that you can access via bk helptool.

2.3.5 Problems Compiling?

All MySQL programs compile cleanly for us with no warnings on Solaris using gcc. On other systems, warnings may occur due to differences in system include files. See section 2.3.6 MIT-pthreads Notes for warnings that may occur when using MIT-pthreads. For other problems, check the list below.

The solution to many problems involves reconfiguring. If you do need to reconfigure, take note of the following:

To prevent old configuration information or object files from being used, run these commands before rerunning configure:

shell> rm config.cache
shell> make clean

Alternatively, you can run make distclean.

The list below describes some of the problems compiling MySQL that have been found to occur most often:

2.3.6 MIT-pthreads Notes

This section describes some of the issues involved in using MIT-pthreads.

Note that on Linux you should NOT use MIT-pthreads but install LinuxThreads! See section 2.6.1 Linux Notes (All Linux Versions).

If your system does not provide native thread support, you will need to build MySQL using the MIT-pthreads package. This includes older FreeBSD systems, SunOS 4.x, Solaris 2.4 and earlier, and some others. See section 2.2.2 Operating Systems Supported by MySQL.

2.4 Post-installation Setup and Testing

Once you've installed MySQL (from either a binary or source distribution), you need to initialize the grant tables, start the server, and make sure that the server works okay. You may also wish to arrange for the server to be started and stopped automatically when your system starts up and shuts down.

Normally you install the grant tables and start the server like this for installation from a source distribution:

shell> ./scripts/mysql_install_db
shell> cd mysql_installation_directory
shell> ./bin/safe_mysqld --user=mysql &

For a binary distribution (not RPM or pkg packages), do this:

shell> cd mysql_installation_directory
shell> ./bin/mysql_install_db
shell> ./bin/safe_mysqld --user=mysql &

This creates the mysql database which will hold all database privileges, the test database which you can use to test MySQL and also privilege entries for the user that run mysql_install_db and a root user (without any passwords). This also starts the mysqld server.

mysql_install_db will not overwrite any old privilege tables, so it should be safe to run in any circumstances. If you don't want to have the test database you can remove it with mysqladmin -u root drop test.

Testing is most easily done from the top-level directory of the MySQL distribution. For a binary distribution, this is your installation directory (typically something like `/usr/local/mysql'). For a source distribution, this is the main directory of your MySQL source tree.

In the commands shown below in this section and in the following subsections, BINDIR is the path to the location in which programs like mysqladmin and safe_mysqld are installed. For a binary distribution, this is the `bin' directory within the distribution. For a source distribution, BINDIR is probably `/usr/local/bin', unless you specified an installation directory other than `/usr/local' when you ran configure. EXECDIR is the location in which the mysqld server is installed. For a binary distribution, this is the same as BINDIR. For a source distribution, EXECDIR is probably `/usr/local/libexec'.

Testing is described in detail below:

  1. If necessary, start the mysqld server and set up the initial MySQL grant tables containing the privileges that determine how users are allowed to connect to the server. This is normally done with the mysql_install_db script:
    shell> scripts/mysql_install_db
    
    Typically, mysql_install_db needs to be run only the first time you install MySQL. Therefore, if you are upgrading an existing installation, you can skip this step. (However, mysql_install_db is quite safe to use and will not update any tables that already exist, so if you are unsure of what to do, you can always run mysql_install_db.) mysql_install_db creates six tables (user, db, host, tables_priv, columns_priv, and func) in the mysql database. A description of the initial privileges is given in section 4.3.4 Setting Up the Initial MySQL Privileges. Briefly, these privileges allow the MySQL root user to do anything, and allow anybody to create or use databases with a name of 'test' or starting with 'test_'. If you don't set up the grant tables, the following error will appear in the log file when you start the server:
    mysqld: Can't find file: 'host.frm'
    
    The above may also happen with a binary MySQL distribution if you don't start MySQL by executing exactly ./bin/safe_mysqld! See section 4.7.2 safe_mysqld, the wrapper around mysqld. You might need to run mysql_install_db as root. However, if you prefer, you can run the MySQL server as an unprivileged (non-root) user, provided that user can read and write files in the database directory. Instructions for running MySQL as an unprivileged user are given in section A.3.2 How to Run MySQL As a Normal User. If you have problems with mysql_install_db, see section 2.4.1 Problems Running mysql_install_db. There are some alternatives to running the mysql_install_db script as it is provided in the MySQL distribution: For more information about these alternatives, see section 4.3.4 Setting Up the Initial MySQL Privileges.
  2. Start the MySQL server like this:
    shell> cd mysql_installation_directory
    shell> bin/safe_mysqld &
    
    If you have problems starting the server, see section 2.4.2 Problems Starting the MySQL Server.
  3. Use mysqladmin to verify that the server is running. The following commands provide a simple test to check that the server is up and responding to connections:
    shell> BINDIR/mysqladmin version
    shell> BINDIR/mysqladmin variables
    
    The output from mysqladmin version varies slightly depending on your platform and version of MySQL, but should be similar to that shown below:
    shell> BINDIR/mysqladmin version
    mysqladmin  Ver 8.14 Distrib 3.23.32, for linux on i586
    Copyright (C) 2000 MySQL AB & MySQL Finland AB & TCX DataKonsult AB
    This software comes with ABSOLUTELY NO WARRANTY. This is free software,
    and you are welcome to modify and redistribute it under the GPL license
    
    Server version          3.23.32-debug
    Protocol version        10
    Connection              Localhost via Unix socket
    TCP port                3306
    UNIX socket             /tmp/mysql.sock
    Uptime:                 16 sec
    
    Threads: 1  Questions: 9  Slow queries: 0  Opens: 7  Flush tables: 2  Open tables: 0 Queries per second avg: 0.000  Memory in use: 132K  Max memory used: 16773K
    
    To get a feeling for what else you can do with BINDIR/mysqladmin, invoke it with the --help option.
  4. Verify that you can shut down the server:
    shell> BINDIR/mysqladmin -u root shutdown
    
  5. Verify that you can restart the server. Do this using safe_mysqld or by invoking mysqld directly. For example:
    shell> BINDIR/safe_mysqld --log &
    
    If safe_mysqld fails, try running it from the MySQL installation directory (if you are not already there). If that doesn't work, see section 2.4.2 Problems Starting the MySQL Server.
  6. Run some simple tests to verify that the server is working. The output should be similar to what is shown below:
    shell> BINDIR/mysqlshow
    +-----------+
    | Databases |
    +-----------+
    | mysql     |
    +-----------+
    
    shell> BINDIR/mysqlshow mysql
    Database: mysql
    +--------------+
    |    Tables    |
    +--------------+
    | columns_priv |
    | db           |
    | func         |
    | host         |
    | tables_priv  |
    | user         |
    +--------------+
    
    shell> BINDIR/mysql -e "select host,db,user from db" mysql
    +------+--------+------+
    | host | db     | user |
    +------+--------+------+
    | %    | test   |      |
    | %    | test_% |      |
    +------+--------+------+
    
    There is also a benchmark suite in the `sql-bench' directory (under the MySQL installation directory) that you can use to compare how MySQL performs on different platforms. The `sql-bench/Results' directory contains the results from many runs against different databases and platforms. To run all tests, execute these commands:
    shell> cd sql-bench
    shell> run-all-tests
    
    If you don't have the `sql-bench' directory, you are probably using an RPM for a binary distribution. (Source distribution RPMs include the benchmark directory.) In this case, you must first install the benchmark suite before you can use it. Beginning with MySQL Version 3.22, there are benchmark RPM files named `mysql-bench-VERSION-i386.rpm' that contain benchmark code and data. If you have a source distribution, you can also run the tests in the `tests' subdirectory. For example, to run `auto_increment.tst', do this:
    shell> BINDIR/mysql -vvf test < ./tests/auto_increment.tst
    
    The expected results are shown in the `./tests/auto_increment.res' file.

2.4.1 Problems Running mysql_install_db

The purpose of the mysql_install_db script is to generate new MySQL privilege tables. It will not affect any other data! It will also not do anything if you already have MySQL privilege tables installed!

If you want to re-create your privilege tables, you should take down the mysqld server, if it's running, and then do something like:

mv mysql-data-directory/mysql mysql-data-directory/mysql-old
mysql_install_db

This section lists problems you might encounter when you run mysql_install_db:

mysql_install_db doesn't install the grant tables
You may find that mysql_install_db fails to install the grant tables and terminates after displaying the following messages:
starting mysqld daemon with databases from XXXXXX
mysql daemon ended
In this case, you should examine the log file very carefully! The log should be located in the directory `XXXXXX' named by the error message, and should indicate why mysqld didn't start. If you don't understand what happened, include the log when you post a bug report using mysqlbug! See section 1.2.22.3 How to Report Bugs or Problems.
There is already a mysqld daemon running
In this case, you probably don't have to run mysql_install_db at all. You have to run mysql_install_db only once, when you install MySQL the first time.
Installing a second mysqld daemon doesn't work when one daemon is running
This can happen when you already have an existing MySQL installation, but want to put a new installation in a different place (for example, for testing, or perhaps you simply want to run two installations at the same time). Generally the problem that occurs when you try to run the second server is that it tries to use the same socket and port as the old one. In this case you will get the error message: Can't start server: Bind on TCP/IP port: Address already in use or Can't start server : Bind on unix socket.... See section 4.1.3 Installing Many Servers on the Same Machine.
You don't have write access to `/tmp'
If you don't have write access to create a socket file at the default place (in `/tmp') or permission to create temporary files in `/tmp,' you will get an error when running mysql_install_db or when starting or using mysqld. You can specify a different socket and temporary directory as follows:
shell> TMPDIR=/some_tmp_dir/
shell> MYSQL_UNIX_PORT=/some_tmp_dir/mysqld.sock
shell> export TMPDIR MYSQL_UNIX_PORT
`some_tmp_dir' should be the path to some directory for which you have write permission. See section H Environment Variables. After this you should be able to run mysql_install_db and start the server with these commands:
shell> scripts/mysql_install_db
shell> BINDIR/safe_mysqld &
mysqld crashes immediately
If you are running RedHat Version 5.0 with a version of glibc older than 2.0.7-5, you should make sure you have installed all glibc patches! There is a lot of information about this in the MySQL mail archives. Links to the mail archives are available online at http://www.mysql.com/documentation/. Also, see section 2.6.1 Linux Notes (All Linux Versions). You can also start mysqld manually using the --skip-grant-tables option and add the privilege information yourself using mysql:
shell> BINDIR/safe_mysqld --skip-grant-tables &
shell> BINDIR/mysql -u root mysql
From mysql, manually execute the SQL commands in mysql_install_db. Make sure you run mysqladmin flush-privileges or mysqladmin reload afterward to tell the server to reload the grant tables.

2.4.2 Problems Starting the MySQL Server

If you are going to use tables that support transactions (BDB, InnoDB), you should first create a my.cnf file and set startup options for the table types you plan to use. See section 7 MySQL Table Types.

Generally, you start the mysqld server in one of three ways:

When the mysqld daemon starts up, it changes directory to the data directory. This is where it expects to write log files and the pid (process ID) file, and where it expects to find databases.

The data directory location is hardwired in when the distribution is compiled. However, if mysqld expects to find the data directory somewhere other than where it really is on your system, it will not work properly. If you have problems with incorrect paths, you can find out what options mysqld allows and what the default path settings are by invoking mysqld with the --help option. You can override the defaults by specifying the correct pathnames as command-line arguments to mysqld. (These options can be used with safe_mysqld as well.)

Normally you should need to tell mysqld only the base directory under which MySQL is installed. You can do this with the --basedir option. You can also use --help to check the effect of changing path options (note that --help must be the final option of the mysqld command). For example:

shell> EXECDIR/mysqld --basedir=/usr/local --help

Once you determine the path settings you want, start the server without the --help option.

Whichever method you use to start the server, if it fails to start up correctly, check the log file to see if you can find out why. Log files are located in the data directory (typically `/usr/local/mysql/data' for a binary distribution, `/usr/local/var' for a source distribution, `\mysql\data\mysql.err' on Windows.) Look in the data directory for files with names of the form `host_name.err' and `host_name.log' where host_name is the name of your server host. Then check the last few lines of these files:

shell> tail host_name.err
shell> tail host_name.log

If you find something like the following in the log file:

000729 14:50:10  bdb:  Recovery function for LSN 1 27595 failed
000729 14:50:10  bdb:  warning: ./test/t1.db: No such file or directory
000729 14:50:10  Can't init databases

This means that you didn't start mysqld with --bdb-no-recover and Berkeley DB found something wrong with its log files when it tried to recover your databases. To be able to continue, you should move away the old Berkeley DB log file from the database directory to some other place, where you can later examine these. The log files are named `log.0000000001', where the number will increase over time.

If you are running mysqld with BDB table support and mysqld core dumps at start this could be because of some problems with the BDB recover log. In this case you can try starting mysqld with --bdb-no-recover. If this helps, then you should remove all `log.*' files from the data directory and try starting mysqld again.

If you get the following error, it means that some other program (or another mysqld server) is already using the TCP/IP port or socket mysqld is trying to use:

Can't start server: Bind on TCP/IP port: Address already in use
  or
Can't start server : Bind on unix socket...

Use ps to make sure that you don't have another mysqld server running. If you can't find another server running, you can try to execute the command telnet your-host-name tcp-ip-port-number and press RETURN a couple of times. If you don't get an error message like telnet: Unable to connect to remote host: Connection refused, something is using the TCP/IP port mysqld is trying to use. See section 2.4.1 Problems Running mysql_install_db and section 4.1.4 Running Multiple MySQL Servers on the Same Machine.

If mysqld is currently running, you can find out what path settings it is using by executing this command:

shell> mysqladmin variables

or

shell> mysqladmin -h 'your-host-name' variables

If safe_mysqld starts the server but you can't connect to it, you should make sure you have an entry in `/etc/hosts' that looks like this:

127.0.0.1       localhost

This problem occurs only on systems that don't have a working thread library and for which MySQL must be configured to use MIT-pthreads.

If you can't get mysqld to start you can try to make a trace file to find the problem. See section G.1.2 Creating trace files.

If you are using InnoDB tables, refer to the InnoDB-specific startup options. See section 7.6.2 InnoDB startup options.

If you are using BDB (Berkeley DB) tables, you should familiarize yourself with the different BDB specific startup options. See section 7.5.3 BDB startup options.

2.4.3 Starting and Stopping MySQL Automatically

The mysql.server and safe_mysqld scripts can be used to start the server automatically at system startup time. mysql.server can also be used to stop the server.

The mysql.server script can be used to start or stop the server by invoking it with start or stop arguments:

shell> mysql.server start
shell> mysql.server stop

mysql.server can be found in the `share/mysql' directory under the MySQL installation directory or in the `support-files' directory of the MySQL source tree.

Before mysql.server starts the server, it changes directory to the MySQL installation directory, then invokes safe_mysqld. You might need to edit mysql.server if you have a binary distribution that you've installed in a non-standard location. Modify it to cd into the proper directory before it runs safe_mysqld. If you want the server to run as some specific user, add an appropriate user line to the `/etc/my.cnf' file, as shown later in this section.

mysql.server stop brings down the server by sending a signal to it. You can take down the server manually by executing mysqladmin shutdown.

You might want to add these start and stop commands to the appropriate places in your `/etc/rc*' files when you start using MySQL for production applications. Note that if you modify mysql.server, then upgrade MySQL sometime, your modified version will be overwritten, so you should make a copy of your edited version that you can reinstall.

If your system uses `/etc/rc.local' to start external scripts, you should append the following to it:

/bin/sh -c 'cd /usr/local/mysql ; ./bin/safe_mysqld --user=mysql &'

You can also add options for mysql.server in a global `/etc/my.cnf' file. A typical `/etc/my.cnf' file might look like this:

[mysqld]
datadir=/usr/local/mysql/var
socket=/tmp/mysqld.sock
port=3306
user=mysql

[mysql.server]
basedir=/usr/local/mysql

The mysql.server script understands the following options: datadir, basedir, and pid-file.

The following table shows which option groups each of the startup scripts read from option files:

Script Option groups
mysqld mysqld and server
mysql.server mysql.server, mysqld, and server
safe_mysqld mysql.server, mysqld, and server

See section 4.1.2 my.cnf Option Files.

2.5 Upgrading/Downgrading MySQL

You can always move the MySQL form and data files between different versions on the same architecture as long as you have the same base version of MySQL. The current base version is 3. If you change the character set when running MySQL (which may also change the sort order), you must run myisamchk -r -q on all tables. Otherwise your indexes may not be ordered correctly.

If you are afraid of new versions, you can always rename your old mysqld to something like mysqld-'old-version-number'. If your new mysqld then does something unexpected, you can simply shut it down and restart with your old mysqld!

When you do an upgrade you should also back up your old databases, of course.

If after an upgrade, you experience problems with recompiled client programs, like Commands out of sync or unexpected core dumps, you probably have used an old header or library file when compiling your programs. In this case you should check the date for your `mysql.h' file and `libmysqlclient.a' library to verify that they are from the new MySQL distribution. If not, please recompile your programs!

If you get some problems that the new mysqld server doesn't want to start or that you can't connect without a password, check that you don't have some old `my.cnf' file from your old installation! You can check this with: program-name --print-defaults. If this outputs anything other than the program name, you have an active my.cnf file that will affect things!

It is a good idea to rebuild and reinstall the Msql-Mysql-modules distribution whenever you install a new release of MySQL, particularly if you notice symptoms such as all your DBI scripts dumping core after you upgrade MySQL.

2.5.1 Upgrading From Version 3.22 to Version 3.23

MySQL Version 3.23 supports tables of the new MyISAM type and the old ISAM type. You don't have to convert your old tables to use these with Version 3.23. By default, all new tables will be created with type MyISAM (unless you start mysqld with the --default-table-type=isam option). You can change an ISAM table to a MyISAM table with ALTER TABLE table_name TYPE=MyISAM or the Perl script mysql_convert_table_format.

Version 3.22 and 3.21 clients will work without any problems with a Version 3.23 server.

The following lists tell what you have to watch out for when upgrading to Version 3.23:

2.5.2 Upgrading from Version 3.21 to Version 3.22

Nothing that affects compatibility has changed between Version 3.21 and 3.22. The only pitfall is that new tables that are created with DATE type columns will use the new way to store the date. You can't access these new fields from an old version of mysqld.

After installing MySQL Version 3.22, you should start the new server and then run the mysql_fix_privilege_tables script. This will add the new privileges that you need to use the GRANT command. If you forget this, you will get Access denied when you try to use ALTER TABLE, CREATE INDEX, or DROP INDEX. If your MySQL root user requires a password, you should give this as an argument to mysql_fix_privilege_tables.

The C API interface to mysql_real_connect() has changed. If you have an old client program that calls this function, you must place a 0 for the new db argument (or recode the client to send the db element for faster connections). You must also call mysql_init() before calling mysql_real_connect()! This change was done to allow the new mysql_options() function to save options in the MYSQL handler structure.

The mysqld variable key_buffer has changed names to key_buffer_size, but you can still use the old name in your startup files.

2.5.3 Upgrading from Version 3.20 to Version 3.21

If you are running a version older than Version 3.20.28 and want to switch to Version 3.21, you need to do the following:

You can start the mysqld Version 3.21 server with safe_mysqld --old-protocol to use it with clients from a Version 3.20 distribution. In this case, the new client function mysql_errno() will not return any server error, only CR_UNKNOWN_ERROR (but it works for client errors), and the server uses the old password() checking rather than the new one.

If you are NOT using the --old-protocol option to mysqld, you will need to make the following changes:

MySQL Version 3.20.28 and above can handle the new user table format without affecting clients. If you have a MySQL version earlier than Version 3.20.28, passwords will no longer work with it if you convert the user table. So to be safe, you should first upgrade to at least Version 3.20.28 and then upgrade to Version 3.21.

The new client code works with a 3.20.x mysqld server, so if you experience problems with 3.21.x, you can use the old 3.20.x server without having to recompile the clients again.

If you are not using the --old-protocol option to mysqld, old clients will issue the error message:

ERROR: Protocol mismatch. Server Version = 10 Client Version = 9

The new Perl DBI/DBD interface also supports the old mysqlperl interface. The only change you have to make if you use mysqlperl is to change the arguments to the connect() function. The new arguments are: host, database, user, password (the user and password arguments have changed places). See section 8.2.2 The DBI Interface.

The following changes may affect queries in old applications:

2.5.4 Upgrading to Another Architecture

If you are using MySQL Version 3.23, you can copy the .frm, .MYI, and .MYD files between different architectures that support the same floating-point format. (MySQL takes care of any byte swapping issues.)

The MySQL ISAM data and index files (`.ISD' and `*.ISM', respectively) are architecture-dependent and in some cases OS-dependent. If you want to move your applications to another machine that has a different architecture or OS than your current machine, you should not try to move a database by simply copying the files to the other machine. Use mysqldump instead.

By default, mysqldump will create a file full of SQL statements. You can then transfer the file to the other machine and feed it as input to the mysql client.

Try mysqldump --help to see what options are available. If you are moving the data to a newer version of MySQL, you should use mysqldump --opt with the newer version to get a fast, compact dump.

The easiest (although not the fastest) way to move a database between two machines is to run the following commands on the machine on which the database is located:

shell> mysqladmin -h 'other hostname' create db_name
shell> mysqldump --opt db_name \
        | mysql -h 'other hostname' db_name

If you want to copy a database from a remote machine over a slow network, you can use:

shell> mysqladmin create db_name
shell> mysqldump -h 'other hostname' --opt --compress db_name \
        | mysql db_name

You can also store the result in a file, then transfer the file to the target machine and load the file into the database there. For example, you can dump a database to a file on the source machine like this:

shell> mysqldump --quick db_name | gzip > db_name.contents.gz

(The file created in this example is compressed.) Transfer the file containing the database contents to the target machine and run these commands there:

shell> mysqladmin create db_name
shell> gunzip < db_name.contents.gz | mysql db_name

You can also use mysqldump and mysqlimport to accomplish the database transfer. For big tables, this is much faster than simply using mysqldump. In the commands shown below, DUMPDIR represents the full pathname of the directory you use to store the output from mysqldump.

First, create the directory for the output files and dump the database:

shell> mkdir DUMPDIR
shell> mysqldump --tab=DUMPDIR db_name

Then transfer the files in the DUMPDIR directory to some corresponding directory on the target machine and load the files into MySQL there:

shell> mysqladmin create db_name           # create database
shell> cat DUMPDIR/*.sql | mysql db_name   # create tables in database
shell> mysqlimport db_name DUMPDIR/*.txt   # load data into tables

Also, don't forget to copy the mysql database, because that's where the grant tables (user, db, host) are stored. You may have to run commands as the MySQL root user on the new machine until you have the mysql database in place.

After you import the mysql database on the new machine, execute mysqladmin flush-privileges so that the server reloads the grant table information.

2.6 Operating System Specific Notes

2.6.1 Linux Notes (All Linux Versions)

The notes below regarding glibc apply only to the situation when you build MySQL yourself. If you are running Linux on an x86 machine, in most cases it is much better for you to just use our binary. We link our binaries against the best patched version of glibc we can come up with and with the best compiler options, in an attempt to make it suitable for a high-load server. So if you read the text below, and are in doubt about what you should do, try our binary first to see if it meets your needs, and worry about your own build only after you have discovered that our binary is not good enough. In that case, we would appreciate a note about it, so we can build a better binary next time. For a typical user, even for setups with a lot of concurrent connections and/or tables exceeding 2GB limit, our binary in most cases is the best choice.

MySQL uses LinuxThreads on Linux. If you are using an old Linux version that doesn't have glibc2, you must install LinuxThreads before trying to compile MySQL. You can get LinuxThreads at http://www.mysql.com/Downloads/Linux.

NOTE: We have seen some strange problems with Linux 2.2.14 and MySQL on SMP systems; If you have a SMP system, we recommend you to upgrade to Linux 2.4 ASAP! Your system will be faster and more stable by doing this!

Note that glibc versions before and including Version 2.1.1 have a fatal bug in pthread_mutex_timedwait handling, which is used when you do INSERT DELAYED. We recommend you to not use INSERT DELAYED before upgrading glibc.

If you plan to have 1000+ concurrent connections, you will need to make some changes to LinuxThreads, recompile it, and relink MySQL against the new `libpthread.a'. Increase PTHREAD_THREADS_MAX in `sysdeps/unix/sysv/linux/bits/local_lim.h' to 4096 and decrease STACK_SIZE in `linuxthreads/internals.h' to 256 KB. The paths are relative to the root of glibc Note that MySQL will not be stable with around 600-1000 connections if STACK_SIZE is the default of 2 MB.

If you have a problem with that MySQL can't open enough files, or connections, it may be that you haven't configured Linux to handle enough files.

In Linux 2.2 and forwards, you can check the number of allocated file handlers by doing:

cat /proc/sys/fs/file-max
cat /proc/sys/fs/dquot-max 
cat /proc/sys/fs/super-max

If you have more than 16M of memory, you should add something like the following in your boot script (`/etc/rc/boot.local' on SuSE):

echo 65536 > /proc/sys/fs/file-max
echo 8192 > /proc/sys/fs/dquot-max
echo 1024 > /proc/sys/fs/super-max

You can also run the above from the command line as root, but in this case your old limits will be used next time your computer reboots.

You should also add /etc/my.cnf:

[safe_mysqld]
open-files-limit=8192

The above should allow MySQL to create up to 8192 connections + files.

The STACK_SIZE constant in LinuxThreads controls the spacing of thread stacks in the address space. It needs to be large enough so that there will be plenty of room for the stack of each individual thread, but small enough to keep the stack of some thread from running into the global mysqld data. Unfortunately, the Linux implementation of mmap(), as we have experimentally discovered, will successfully unmap an already mapped region if you ask it to map out an address already in use, zeroing out the data on the entire page, instead of returning an error. So, the safety of mysqld or any other threaded application depends on the "gentleman" behavior of the code that creates threads. The user must take measures to make sure the number of running threads at any time is sufficiently low for thread stacks to stay away from the global heap. With mysqld, you should enforce this "gentleman" behavior by setting a reasonable value for the max_connections variable.

If you build MySQL yourself and do not want to mess with patching LinuxThreads, you should set max_connections to a value no higher than 500. It should be even less if you have a large key buffer, large heap tables, or some other things that make mysqld allocate a lot of memory or if you are running a 2.2 kernel with a 2GB patch. If you are using our binary or RPM version 3.23.25 or later, you can safely set max_connections at 1500, assuming no large key buffer or heap tables with lots of data. The more you reduce STACK_SIZE in LinuxThreads the more threads you can safely create. We recommend the values between 128K and 256K.

If you use a lot of concurrent connections, you may suffer from a "feature" in the 2.2 kernel that penalizes a process for forking or cloning a child in an attempt to prevent a fork bomb attack. This will cause MySQL not to scale well as you increase the number of concurrent clients. On single CPU systems, we have seen this manifested in a very slow thread creation, which means it may take a long time to connect to MySQL (as long as 1 minute), and it may take just as long to shut it down. On multiple CPU systems, we have observed a gradual drop in query speed as the number of clients increases. In the process of trying to find a solution, we have received a kernel patch from one of our users, who claimed it made a lot of difference for his site. The patch is available here (http://www.mysql.com/Downloads/Patches/linux-fork.patch). We have now done rather extensive testing of this patch on both development and production systems. It has significantly improved MySQL performance without causing any problems and we now recommend it to our users who are still running high-load servers on 2.2 kernels. This issue has been fixed in the 2.4 kernel, so if you are not satisfied with the current performance of your system, rather than patching your 2.2 kernel, it might be easier to just upgrade to 2.4, which will also give you a nice SMP boost in addition to fixing this fairness bug.

We have tested MySQL on the 2.4 kernel on a 2 CPU machine and found MySQL scales MUCH better - there was virtually no slowdown on query throughput all the way up to 1000 clients, and MySQL scaling factor ( computed as the ratio of maximum throughput to the throughput with one client) was 180%. We have observed similar results on a 4-CPU system - virtually no slowdown as the number of clients was increased up to 1000, and 300% scaling factor. So for a high-load SMP server we would definitely recommend the 2.4 kernel at this point. We have discovered that it is essential to run mysqld process with the highest possible priority on the 2.4 kernel to achieve maximum performance. This can be done by adding renice -20 $$ command to safe_mysqld. In our testing on a 4-CPU machine, increasing the priority gave 60% increase in throughput with 400 clients.

We are currently also trying to collect more info on how well MySQL performs on 2.4 kernel on 4-way and 8-way systems. If you have access such a system and have done some benchmarks, please send a mail to docs@mysql.com with the results - we will include them in the manual.

There is another issue that greatly hurts MySQL performance, especially on SMP systems. The implementation of mutex in LinuxThreads in glibc-2.1 is very bad for programs with many threads that only hold the mutex for a short time. On an SMP system, ironic as it is, if you link MySQL against unmodified LinuxThreads, removing processors from the machine improves MySQL performance in many cases. We have made a patch available for glibc 2.1.3, linuxthreads-2.1-patch to correct this behavior.

With glibc-2.2.2 MySQL version 3.23.36 will use the adaptive mutex, which is much better than even the patched one in glibc-2.1.3. Be warned, however, that under some conditions, the current mutex code in glibc-2.2.2 overspins, which hurts MySQL performance. The chance of this condition can be reduced by renicing mysqld process to the highest priority. We have also been able to correct the overspin behavior with a patch, available here. It combines the correction of overspin, maximum number of threads, and stack spacing all in one. You will need to apply it in the linuxthreads directory with patch -p0 </tmp/linuxthreads-2.2.2.patch. We hope it will be included in some form in to the future releases of glibc-2.2. In any case, if you link against glibc-2.2.2 you still need to correct STACK_SIZE and PTHREAD_THREADS_MAX. We hope that the defaults will be corrected to some more acceptable values for high-load MySQL setup in the future, so that your own build can be reduced to ./configure; make; make install.

We recommend that you use the above patches to build a special static version of libpthread.a and use it only for statically linking against MySQL. We know that the patches are safe for MySQL and significantly improve its performance, but we cannot say anything about other applications. If you link other applications against the patched version of the library, or build a patched shared version and install it on your system, you are doing it at your own risk with regard to other applications that depend on LinuxThreads.

If you experience any strange problems during the installation of MySQL, or with some common utilties hanging, it is very likely that they are either library or compiler related. If this is the case, using our binary will resolve them.

One known problem with the binary distribution is that with older Linux systems that use libc (like RedHat 4.x or Slackware), you will get some non-fatal problems with hostname resolution. See section 2.6.1.1 Linux Notes for Binary Distributions.

When using LinuxThreads you will see a minimum of three processes running. These are in fact threads. There will be one thread for the LinuxThreads manager, one thread to handle connections, and one thread to handle alarms and signals.

Note that the Linux kernel and the LinuxThread library can by default only have 1024 threads. This means that you can only have up to 1021 connections to MySQL on an unpatched system. The page http://www.volano.com/linuxnotes.html contains information how to go around this limit.

If you see a dead mysqld daemon process with ps, this usually means that you have found a bug in MySQL or you have a corrupted table. See section A.4.1 What To Do If MySQL Keeps Crashing.

To get a core dump on Linux if mysqld dies with a SIGSEGV signal, you can start mysqld with the --core-file option. Note that you also probably need to raise the core file size by adding ulimit -c 1000000 to safe_mysqld or starting safe_mysqld with --core-file-sizes=1000000. See section 4.7.2 safe_mysqld, the wrapper around mysqld.

To get a core dump on Linux if mysqld dies with a SIGSEGV signal, you can start mysqld with the --core-file option. Note that you also probably need to raise the core file size by adding ulimit -c 1000000 to safe_mysqld or starting safe_mysqld with --core-file-sizes=1000000. See section 4.7.2 safe_mysqld, the wrapper around mysqld.

If you are linking your own MySQL client and get the error:

ld.so.1: ./my: fatal: libmysqlclient.so.4: open failed: No such file or directory

When executing them, the problem can be avoided by one of the following methods:

If you are using the Fujitsu compiler (fcc / FCC) you will have some problems compiling MySQL because the Linux header files are very gcc oriented.

The following configure line should work with fcc/FCC:

CC=fcc CFLAGS="-O -K fast -K lib -K omitfp -Kpreex -D_GNU_SOURCE -DCONST=const -DNO_STRTOLL_PROTO" CXX=FCC CXXFLAGS="-O -K fast -K lib  -K omitfp -K preex --no_exceptions --no_rtti -D_GNU_SOURCE -DCONST=const -Dalloca=__builtin_alloca -DNO_STRTOLL_PROTO '-D_EXTERN_INLINE=static __inline'" ./configure --prefix=/usr/local/mysql --enable-assembler --with-mysqld-ldflags=-all-static --disable-shared --with-low-memory

2.6.1.1 Linux Notes for Binary Distributions

MySQL needs at least Linux Version 2.0.

The binary release is linked with -static, which means you do not normally need to worry about which version of the system libraries you have. You need not install LinuxThreads, either. A program linked with -static is slightly bigger than a dynamically linked program but also slightly faster (3-5%). One problem, however, is that you can't use user-definable functions (UDFs) with a statically linked program. If you are going to write or use UDF functions (this is something only for C or C++ programmers), you must compile MySQL yourself, using dynamic linking.

If you are using a libc-based system (instead of a glibc2 system), you will probably get some problems with hostname resolving and getpwnam() with the binary release. (This is because glibc unfortunately depends on some external libraries to resolve hostnames and getpwent(), even when compiled with -static). In this case you probably get the following error message when you run mysql_install_db:

Sorry, the host 'xxxx' could not be looked up

or the following error when you try to run mysqld with the --user option:

getpwnam: No such file or directory

You can solve this problem in one of the following ways:

The Linux-Intel binary and RPM releases of MySQL are configured for the highest possible speed. We are always trying to use the fastest stable compiler available.

MySQL Perl support requires Version Perl 5.004_03 or newer.

On some Linux 2.2 versions, you may get the error Resource temporarily unavailable when you do a lot of new connections to a mysqld server over TCP/IP.

The problem is that Linux has a delay between when you close a TCP/IP socket and until this is actually freed by the system. As there is only room for a finite number of TCP/IP slots, you will get the above error if you try to do too many new TCP/IP connections during a small time, like when you run the MySQL `test-connect' benchmark over TCP/IP.

We have mailed about this problem a couple of times to different Linux mailing lists but have never been able to resolve this properly.

The only known 'fix' to this problem is to use persistent connections in your clients or use sockets, if you are running the database server and clients on the same machine. We hope that the Linux 2.4 kernel will fix this problem in the future.

2.6.1.2 Linux x86 Notes

MySQL requires libc Version 5.4.12 or newer. It's known to work with libc 5.4.46. glibc Version 2.0.6 and later should also work. There have been some problems with the glibc RPMs from RedHat, so if you have problems, check whether or not there are any updates! The glibc 2.0.7-19 and 2.0.7-29 RPMs are known to work.

On some older Linux distributions, configure may produce an error like this:

Syntax error in sched.h. Change _P to __P in the /usr/include/sched.h file.
See the Installation chapter in the Reference Manual.

Just do what the error message says and add an extra underscore to the _P macro that has only one underscore, then try again.

You may get some warnings when compiling; those shown below can be ignored:

mysqld.cc -o objs-thread/mysqld.o
mysqld.cc: In function `void init_signals()':
mysqld.cc:315: warning: assignment of negative value `-1' to `long unsigned int'
mysqld.cc: In function `void * signal_hand(void *)':
mysqld.cc:346: warning: assignment of negative value `-1' to `long unsigned int'

In Debian GNU/Linux, if you want MySQL to start automatically when the system boots, do the following:

shell> cp support-files/mysql.server /etc/init.d/mysql.server
shell> /usr/sbin/update-rc.d mysql.server defaults 99

mysql.server can be found in the `share/mysql' directory under the MySQL installation directory or in the `support-files' directory of the MySQL source tree.

If mysqld always core dumps when it starts up, the problem may be that you have an old `/lib/libc.a'. Try renaming it, then remove `sql/mysqld' and do a new make install and try again. This problem has been reported on some Slackware installations.

If you get the following error when linking mysqld, it means that your `libg++.a' is not installed correctly:

/usr/lib/libc.a(putc.o): In function `_IO_putc':
putc.o(.text+0x0): multiple definition of `_IO_putc'

You can avoid using `libg++.a' by running configure like this:

shell> CXX=gcc ./configure

2.6.1.3 Linux SPARC Notes

In some implementations, readdir_r() is broken. The symptom is that SHOW DATABASES always returns an empty set. This can be fixed by removing HAVE_READDIR_R from `config.h' after configuring and before compiling.

Some problems will require patching your Linux installation. The patch can be found at http://www.mysql.com/Downloads/patches/Linux-sparc-2.0.30.diff. This patch is against the Linux distribution `sparclinux-2.0.30.tar.gz' that is available at vger.rutgers.edu (a version of Linux that was never merged with the official 2.0.30). You must also install LinuxThreads Version 0.6 or newer.

2.6.1.4 Linux Alpha Notes

MySQL Version 3.23.12 is the first MySQL version that is tested on Linux-Alpha. If you plan to use MySQL on Linux-Alpha, you should ensure that you have this version or newer.

We have tested MySQL on Alpha with our benchmarks and test suite, and it appears to work nicely. The main thing we haven't yet had time to test is how things works with many concurrent users.

When we compiled the standard MySQL binary we are using SuSE 6.4, kernel 2.2.13-SMP, Compaq C compiler (V6.2-504) and Compaq C++ compiler (V6.3-005) on a Comaq DS20 machine with an Alpha EV6 processor.

You can find the above compilers at http://www.support.compaq.com/alpha-tools). By using these compilers, instead of gcc, we get about 9-14 % better performance with MySQL.

Note that the configure line optimized the binary for the current CPU; This means you can only use our binary if you have an Alpha EV6 processor. We also compile statically to avoid library problems.

CC=ccc CFLAGS="-fast" CXX=cxx CXXFLAGS="-fast -noexceptions -nortti" ./configure --prefix=/usr/local/mysql --disable-shared --with-extra-charsets=complex --enable-thread-safe-client --with-mysqld-ldflags=-non_shared --with-client-ldflags=-non_shared

If you want to use egcs the following configure line worked for us:

CFLAGS="-O3 -fomit-frame-pointer" CXX=gcc CXXFLAGS="-O3 -fomit-frame-pointer -felide-constructors -fno-exceptions -fno-rtti" ./configure --prefix=/usr/local/mysql --disable-shared

Some known problems when running MySQL on Linux-Alpha:

2.6.1.5 Linux PowerPC Notes

MySQL should work on MkLinux with the newest glibc package (tested with glibc 2.0.7).

2.6.1.6 Linux MIPS Notes

To get MySQL to work on Qube2, (Linux Mips), you need the newest glibc libraries (glibc-2.0.7-29C2 is known to work). You must also use the egcs C++ compiler (egcs-1.0.2-9, gcc 2.95.2 or newer).

2.6.1.7 Linux IA64 Notes

To get MySQL to compile on Linux Ia64, we had to do the following (we assume that this will be easier when next gcc version for ia64 is released).

Using gcc-2.9-final:

CFLAGS="-O2" CXX=gcc CXXFLAGS="-O2 -felide-constructors -fno-exceptions -fno-rtti" ./configure --prefix=/usr/local/mysql --enable-assembler --with-mysqld-ldflags=-all-static --disable-shared --with-extra-charsets=complex

After make you will get an error that sql/opt_range.cc will not compile (internal compiler error). To fix this, go to the sql directory and type make again. Copy the compile line, but change -O2 to -O0. The file should now compile.

Now you can do:

cd ..
make
make_install

and mysqld should be ready to run.

On Ia64 the MySQL client binaries are using shared libraries. This means that if you install our binary distribution in some other place than `/usr/local/mysql' you need to either modify `/etc/ld.so.conf' or add the path to the directory where you have `libmysqlclient.so' to the LD_LIBRARY_PATH environment variable.

See section A.3.1 Problems When Linking with the MySQL Client Library.

2.6.2 Windows Notes

This section describes installation and use of MySQL on Windows. This information is also provided in the `README' file that comes with the MySQL Windows distribution.

2.6.2.1 Starting MySQL on Windows 95 or Windows 98

MySQL uses TCP/IP to connect a client to a server. (This will allow any machine on your network to connect to your MySQL server.) Because of this, you must install TCP/IP on your machine before starting MySQL. You can find TCP/IP on your Windows CD-ROM.

Note that if you are using an old Win95 release (for example OSR2), it's likely that you have an old Winsock package! MySQL requires Winsock 2! You can get the newest Winsock from http://www.microsoft.com/. Win98 has the new Winsock 2 library, so the above doesn't apply for Win98.

To start the mysqld server, you should start an MS-DOS window and type:

C:\> C:\mysql\bin\mysqld

This will start mysqld in the background without a window.

You can kill the MySQL server by executing:

C:\> C:\mysql\bin\mysqladmin -u root shutdown

Note that Win95 and Win98 don't support creation of named pipes. On Win95 and Win98, you can only use named pipes to connect to a remote MySQL server running on a Windows NT server host. (The MySQL server must also support named pipes, of course. For example, using mysqld-opt under NT will not allow named pipe connections. You should use either mysqld-nt or mysqld-max-nt.)

If mysqld doesn't start, please check the `\mysql\data\mysql.err' file to see if the server wrote any message there to indicate the cause of the problem. You can also try to start the server with mysqld --standalone; In this case, you may get some useful information on the screen that may help solve the problem.

The last option is to start mysqld with --standalone --debug. In this case mysqld will write a log file `C:\mysqld.trace' that should contain the reason why mysqld doesn't start. See section G.1.2 Creating trace files.

2.6.2.2 Starting MySQL on Windows NT or Windows 2000

The Win95/Win98 section also applies to MySQL on NT/Win2000, with the following differences:

To get MySQL to work with TCP/IP on NT, you must install service pack 3 (or newer)!

Note that everything in the following that applies for NT also applies for Win2000!

For NT/Win2000, the server name is mysqld-nt. Normally you should install MySQL as a service on NT/Win2000:

C:\> C:\mysql\bin\mysqld-nt --install

or

C:\> C:\mysql\bin\mysqld-max-nt --install

(Under Windows NT, you can actually install any of the server binaries as a service, but only those having names that end with -nt.exe provide support for named pipes.)

You can start and stop the MySQL service with these commands:

C:\> NET START mysql
C:\> NET STOP mysql

Note that in this case you can't use any other options for mysqld-nt!

You can also run mysqld-nt as a stand-alone program on NT if you need to start mysqld-nt with any options! If you start mysqld-nt without options on NT, mysqld-nt tries to start itself as a service with the default service options. If you have stopped mysqld-nt, you have to start it with NET START mysql.

The service is installed with the name MySQL. Once installed, it must be started using the Services Control Manager (SCM) Utility found in the Control Panel, or by using the NET START MySQL command. If any options are desired, they must be specified as ``Startup parameters'' in the SCM utility before you start the MySQL service. Once running, mysqld-nt can be stopped using mysqladmin, or from the SCM utility or by using the command NET STOP MySQL. If you use SCM to stop mysqld-nt, there is a strange message from SCM about mysqld shutdown normally. When run as a service, mysqld-nt has no access to a console and so no messages can be seen.

On NT you can get the following service error messages:

Permission Denied Means that it cannot find mysqld-nt.exe.
Cannot Register Means that the path is incorrect.
Failed to install service. Means that the service is already installed or that the Service Control Manager is in bad state.

If you have problems installing mysqld-nt as a service, try starting it with the full path:

C:\> C:\mysql\bin\mysqld-nt --install

If this doesn't work, you can get mysqld-nt to start properly by fixing the path in the registry!

If you don't want to start mysqld-nt as a service, you can start it as follows:

C:\> C:\mysql\bin\mysqld-nt --standalone

or

C:\> C:\mysql\bin\mysqld --standalone --debug

The last version gives you a debug trace in `C:\mysqld.trace'. See section G.1.2 Creating trace files.

2.6.2.3 Running MySQL on Windows

MySQL supports TCP/IP on all Windows platforms and named pipes on NT. The default is to use named pipes for local connections on NT and TCP/IP for all other cases if the client has TCP/IP installed. The host name specifies which protocol is used:

Host name
Protocol
NULL (none) On NT, try named pipes first; if that doesn't work, use TCP/IP. On Win95/Win98, TCP/IP is used.
. Named pipes
localhost TCP/IP to current host
hostname TCP/IP

You can force a MySQL client to use named pipes by specifying the --pipe option or by specifying . as the host name. Use the --socket option to specify the name of the pipe.

You can test whether or not MySQL is working by executing the following commands:

C:\> C:\mysql\bin\mysqlshow
C:\> C:\mysql\bin\mysqlshow -u root mysql
C:\> C:\mysql\bin\mysqladmin version status proc
C:\> C:\mysql\bin\mysql test

If mysqld is slow to answer to connections on Win95/Win98, there is probably a problem with your DNS. In this case, start mysqld with --skip-name-resolve and use only localhost and IP numbers in the MySQL grant tables. You can also avoid DNS when connecting to a mysqld-nt MySQL server running on NT by using the --pipe argument to specify use of named pipes. This works for most MySQL clients.

There are two versions of the MySQL command-line tool:
mysql Compiled on native Windows, which offers very limited text editing capabilities.
mysqlc Compiled with the Cygnus GNU compiler and libraries, which offers readline editing.

If you want to use mysqlc.exe, you must copy `C:\mysql\lib\cygwinb19.dll' to your Windows system directory (`\windows\system' or similar place).

The default privileges on Windows give all local users full privileges to all databases without specifying a password. To make MySQL more secure, you should set a password for all users and remove the row in the mysql.user table that has Host='localhost' and User=''.

You should also add a password for the root user. The following example starts by removing the anonymous user that can be used by anyone to access the test database, then sets a root user password:

C:\> C:\mysql\bin\mysql mysql
mysql> DELETE FROM user WHERE Host='localhost' AND User='';
mysql> QUIT
C:\> C:\mysql\bin\mysqladmin reload
C:\> C:\mysql\bin\mysqladmin -u root password your_password

After you've set the password, if you want to take down the mysqld server, you can do so using this command:

C:\> mysqladmin --user=root --password=your_password shutdown

If you are using the old shareware version of MySQL Version 3.21 under Windows, the above command will fail with an error: parse error near 'SET OPTION password'. The fix is in to upgrade to the current MySQL version, which is freely available.

With the current MySQL versions you can easily add new users and change privileges with GRANT and REVOKE commands. See section 4.3.1 GRANT and REVOKE Syntax.

2.6.2.4 Connecting to a Remote MySQL from Windows with SSH

Here is a note about how to connect to get a secure connection to remote MySQL server with SSH (by David Carlson dcarlson@mplcomm.com):

You should now have an ODBC connection to MySQL, encrypted using SSH.

2.6.2.5 Splitting Data Across Different Disks on Windows

Beginning with MySQL Version 3.23.16, the mysqld-max and mysql-max-nt servers in the MySQL distribution are compiled with the -DUSE_SYMDIR option. This allows you to put a database on different disk by adding a symbolic link to it (in a manner similar to the way that symbolic links work on Unix).

On Windows, you make a symbolic link to a database by creating a file that contains the path to the destination directory and saving this in the `mysql_data' directory under the filename `database.sym'. Note that the symbolic link will be used only if the directory `mysql_data_dir\database' doesn't exist.

For example, if the MySQL data directory is `C:\mysql\data' and you want to have database foo located at `D:\data\foo', you should create the file `C:\mysql\data\foo.sym' that contains the text D:\data\foo\. After that, all tables created in the database foo will be created in `D:\data\foo'.

Note that because of the speed penalty you get when opening every table, we have not enabled this by default even if you have compiled MySQL with support for this. To enable symlinks you should put in your my.cnf or my.ini file the following entry:

[mysqld]
use-symbolic-links

In MySQL 4.0 we will enable symlinks by default. Then you should instead use the skip-symlink option if you want to disable this.

2.6.2.6 Compiling MySQL Clients on Windows

In your source files, you should include `windows.h' before you include `mysql.h':

#if defined(_WIN32) || defined(_WIN64)
#include <windows.h>
#endif
#include <mysql.h>

You can either link your code with the dynamic `libmysql.lib' library, which is just a wrapper to load in `libmysql.dll' on demand, or link with the static `mysqlclient.lib' library.

Note that as the mysqlclient libraries are compiled as threaded libraries, you should also compile your code to be multi-threaded!

2.6.2.7 MySQL-Windows Compared to Unix MySQL

MySQL-Windows has by now proven itself to be very stable. This version of MySQL has the same features as the corresponding Unix version with the following exceptions:

Win95 and threads
Win95 leaks about 200 bytes of main memory for each thread creation. Each connection in MySQL creates a new thread, so you shouldn't run mysqld for an extended time on Win95 if your server handles many connections! WinNT and Win98 don't suffer from this bug.
Concurrent reads
MySQL depends on the pread() and pwrite() calls to be able to mix INSERT and SELECT. Currently we use mutexes to emulate pread()/pwrite(). We will, in the long run, replace the file level interface with a virtual interface so that we can use the readfile()/writefile() interface on NT to get more speed. The current implementation limits the number of open files MySQL can use to 1024, which means that you will not be able to run as many concurrent threads on NT as on Unix.
Blocking read
MySQL uses a blocking read for each connection. This means that: We plan to fix this problem when our Windows developers have figured out a nice workaround.
UDF functions
For the moment, MySQL-Windows does not support user-definable functions.
DROP DATABASE
You can't drop a database that is in use by some thread.
Killing MySQL from the task manager
You can't kill MySQL from the task manager or with the shutdown utility in Win95. You must take it down with mysqladmin shutdown.
Case-insensitive names
Filenames are case insensitive on Windows, so database and table names are also case insensitive in MySQL for Windows. The only restriction is that database and table names must be specified using the same case throughout a given statement. See section 6.1.3 Case Sensitivity in Names.
The `\' directory character
Pathname components in Win95 are separated by the `\' character, which is also the escape character in MySQL. If you are using LOAD DATA INFILE or SELECT ... INTO OUTFILE, you must double the `\' character:
mysql> LOAD DATA INFILE "C:\\tmp\\skr.txt" INTO TABLE skr;
mysql> SELECT * INTO OUTFILE 'C:\\tmp\\skr.txt' FROM skr;
Alternatively, use Unix style filenames with `/' characters:
mysql> LOAD DATA INFILE "C:/tmp/skr.txt" INTO TABLE skr;
mysql> SELECT * INTO OUTFILE 'C:/tmp/skr.txt' FROM skr;
Can't open named pipe error
If you use a MySQL 3.22 version on NT with the newest mysql-clients you will get the following error:
error 2017: can't open named pipe to host: . pipe...
This is because the release version of MySQL uses named pipes on NT by default. You can avoid this error by using the --host=localhost option to the new MySQL clients or create an option file `C:\my.cnf' that contains the following information:
[client]
host = localhost
Access denied for user error
If you get the error Access denied for user: 'some-user@unknown' to database 'mysql' when accessing a MySQL server on the same machine, this means that MySQL can't resolve your host name properly. To fix this, you should create a file `\windows\hosts' with the following information:
127.0.0.1       localhost
ALTER TABLE
While you are executing an ALTER TABLE statement, the table is locked from usage by other threads. This has to do with the fact that on Windows, you can't delete a file that is in use by another threads. (In the future, we may find some way to work around this problem.)
DROP TABLE on a table that is in use by a MERGE table will not work
The MERGE handler does its table mapping hidden from MySQL. Because Windows doesn't allow you to drop files that are open, you first must flush all MERGE tables (with FLUSH TABLES) or drop the MERGE table before dropping the table. We will fix this at the same time we introduce VIEWs.

Here are some open issues for anyone who might want to help us with the Windows release:

Other Windows-specific issues are described in the `README' file that comes with the MySQL-Windows distribution.

2.6.3 Solaris Notes

On Solaris, you may run into trouble even before you get the MySQL distribution unpacked! Solaris tar can't handle long file names, so you may see an error like this when you unpack MySQL:

x mysql-3.22.12-beta/bench/Results/ATIS-mysql_odbc-NT_4.0-cmp-db2,informix,ms-sql,mysql,oracle,solid,sybase, 0 bytes, 0 tape blocks
tar: directory checksum error

In this case, you must use GNU tar (gtar) to unpack the distribution. You can find a precompiled copy for Solaris at http://www.mysql.com/Downloads/.

Sun native threads work only on Solaris 2.5 and higher. For Version 2.4 and earlier, MySQL will automatically use MIT-pthreads. See section 2.3.6 MIT-pthreads Notes.

If you get the following error from configure:

checking for restartable system calls... configure: error can not run test
programs while cross compiling

This means that you have something wrong with your compiler installation! In this case you should upgrade your compiler to a newer version. You may also be able to solve this problem by inserting the following row into the `config.cache' file:

ac_cv_sys_restartable_syscalls=${ac_cv_sys_restartable_syscalls='no'}

If you are using Solaris on a SPARC, the recommended compiler is gcc 2.95.2. You can find this at http://gcc.gnu.org/. Note that egcs 1.1.1 and gcc 2.8.1 don't work reliably on SPARC!

The recommended configure line when using gcc 2.95.2 is:

CC=gcc CFLAGS="-O3" \
CXX=gcc CXXFLAGS="-O3 -felide-constructors -fno-exceptions -fno-rtti" \
./configure --prefix=/usr/local/mysql --with-low-memory --enable-assembler

If you have a ultra sparc, you can get 4 % more performance by adding "-mcpu=v8 -Wa,-xarch=v8plusa" to CFLAGS and CXXFLAGS.

If you have Sun Workshop (Fortre) 5.3 (or newer) compiler, you can run configure like this:

CC=cc CFLAGS="-Xa -fast -xO4 -native -xstrconst -mt" \
CXX=CC CXXFLAGS="-noex -xO4 -mt" \
./configure --prefix=/usr/local/mysql --enable-assembler

In the MySQL benchmarks, we got a 6 % speedup on an Ultrasparc when using Sun Workshop 5.3 compared to using gcc with -mcpu flags.

If you get a problem with fdatasync or sched_yield, you can fix this by adding LIBS=-lrt to the configure line

The following paragraph is only relevant for older compilers than WorkShop 5.3:

You may also have to edit the configure script to change this line:

#if !defined(__STDC__) || __STDC__ != 1

to this:

#if !defined(__STDC__)

If you turn on __STDC__ with the -Xc option, the Sun compiler can't compile with the Solaris `pthread.h' header file. This is a Sun bug (broken compiler or broken include file).

If mysqld issues the error message shown below when you run it, you have tried to compile MySQL with the Sun compiler without enabling the multi-thread option (-mt):

libc internal error: _rmutex_unlock: rmutex not held

Add -mt to CFLAGS and CXXFLAGS and try again.

If you get the following error when compiling MySQL with gcc, it means that your gcc is not configured for your version of Solaris:

shell> gcc -O3 -g -O2 -DDBUG_OFF  -o thr_alarm ...
./thr_alarm.c: In function `signal_hand':
./thr_alarm.c:556: too many arguments to function `sigwait'

The proper thing to do in this case is to get the newest version of gcc and compile it with your current gcc compiler! At least for Solaris 2.5, almost all binary versions of gcc have old, unusable include files that will break all programs that use threads (and possibly other programs)!

Solaris doesn't provide static versions of all system libraries (libpthreads and libdl), so you can't compile MySQL with --static. If you try to do so, you will get the error:

ld: fatal: library -ldl: not found

or

undefined reference to `dlopen'

or

cannot find -lrt

If too many processes try to connect very rapidly to mysqld, you will see this error in the MySQL log:

Error in accept: Protocol error

You might try starting the server with the --set-variable back_log=50 option as a workaround for this. See section 4.1.1 mysqld Command-line Options.

If you are linking your own MySQL client, you might get the following error when you try to execute it:

ld.so.1: ./my: fatal: libmysqlclient.so.#: open failed: No such file or directory

The problem can be avoided by one of the following methods:

When using the --with-libwrap configure option, you must also include the libraries that `libwrap.a' needs:

--with-libwrap="/opt/NUtcpwrapper-7.6/lib/libwrap.a -lnsl -lsocket

If you have problems with configure trying to link with -lz and you don't have zlib installed, you have two options:

If you are using gcc and have problems with loading UDF functions into MySQL, try adding -lgcc to the link line for the UDF function.

If you would like MySQL to start automatically, you can copy `support-files/mysql.server' to `/etc/init.d' and create a symbolic link to it named `/etc/rc3.d/S99mysql.server'.

2.6.3.1 Solaris 2.7/2.8 Notes

You can normally use a Solaris 2.6 binary on Solaris 2.7 and 2.8. Most of the Solaris 2.6 issues also apply for Solaris 2.7 and 2.8.

Note that MySQL Version 3.23.4 and above should be able to autodetect new versions of Solaris and enable workarounds for the following problems!

Solaris 2.7 / 2.8 has some bugs in the include files. You may see the following error when you use gcc:

/usr/include/widec.h:42: warning: `getwc' redefined
/usr/include/wchar.h:326: warning: this is the location of the previous
definition

If this occurs, you can do the following to fix the problem:

Copy /usr/include/widec.h to .../lib/gcc-lib/os/gcc-version/include and change line 41 from:

#if     !defined(lint) && !defined(__lint)

to

#if     !defined(lint) && !defined(__lint) && !defined(getwc)

Alternatively, you can edit `/usr/include/widec.h' directly. Either way, after you make the fix, you should remove `config.cache' and run configure again!

If you get errors like this when you run make, it's because configure didn't detect the `curses.h' file (probably because of the error in `/usr/include/widec.h'):

In file included from mysql.cc:50:
/usr/include/term.h:1060: syntax error before `,'
/usr/include/term.h:1081: syntax error before `;'

The solution to this is to do one of the following:

If you get a problem that your linker can't find -lz when linking your client program, the problem is probably that your `libz.so' file is installed in `/usr/local/lib'. You can fix this by one of the following methods:

2.6.3.2 Solaris x86 Notes

On Solaris 2.8 on x86, mysqld will core dump if you run 'strip' in.

If you are using gcc or egcs on Solaris x86 and you experience problems with core dumps under load, you should use the following configure command:

CC=gcc CFLAGS="-O3 -fomit-frame-pointer -DHAVE_CURSES_H" \
CXX=gcc \
CXXFLAGS="-O3 -fomit-frame-pointer -felide-constructors -fno-exceptions -fno-rtti -DHAVE_CURSES_H" \
./configure --prefix=/usr/local/mysql

This will avoid problems with the libstdc++ library and with C++ exceptions.

If this doesn't help, you should compile a debug version and run it with a trace file or under gdb. See section G.1.3 Debugging mysqld under gdb.

2.6.4 BSD Notes

2.6.4.1 FreeBSD Notes

FreeBSD 3.x is recommended for running MySQL since the thread package is much more integrated.

The easiest and therefor the preferred way to install is to use the mysql-server and mysql-client ports available on http://www.freebsd.org.

Using these gives you:

It is recommended you use MIT-pthreads on FreeBSD 2.x and native threads on Versions 3 and up. It is possible to run with native threads on some late 2.2.x versions but you may encounter problems shutting down mysqld.

The MySQL `Makefile's require GNU make (gmake) to work. If you want to compile MySQL you need to install GNU make first.

Be sure to have your name resolver setup correct. Otherwise you may experience resolver delays or failures when connecting to mysqld.

Make sure that the localhost entry in the `/etc/hosts' file is correct (otherwise you will have problems connecting to the database). The `/etc/hosts' file should start with a line:

127.0.0.1       localhost localhost.your.domain

If you notice that configure will use MIT-pthreads, you should read the MIT-pthreads notes. See section 2.3.6 MIT-pthreads Notes.

If you get an error from make install that it can't find `/usr/include/pthreads', configure didn't detect that you need MIT-pthreads. This is fixed by executing these commands:

shell> rm config.cache
shell> ./configure --with-mit-threads

FreeBSD is also known to have a very low default file handle limit. See section A.2.16 File Not Found. Uncomment the ulimit -n section in safe_mysqld or raise the limits for the mysqld user in /etc/login.conf (and rebuild it with cap_mkdb /etc/login.conf). Also be sure you set the appropriate class for this user in the password file if you are not using the default (use: chpass mysqld-user-name). See section 4.7.2 safe_mysqld, the wrapper around mysqld.

If you get problems with the current date in MySQL, setting the TZ variable will probably help. See section H Environment Variables.

To get a secure and stable system you should only use FreeBSD kernels that are marked -STABLE.

2.6.4.2 NetBSD notes

To compile on NetBSD you need GNU make. Otherwise the compile will crash when make tries to run lint on C++ files.

2.6.4.3 OpenBSD Notes

2.6.4.4 OpenBSD 2.5 Notes

On OpenBSD Version 2.5, you can compile MySQL with native threads with the following options:

CFLAGS=-pthread CXXFLAGS=-pthread ./configure --with-mit-threads=no

2.6.4.5 OpenBSD 2.8 Notes

Our users have reported that OpenBSD 2.8 has a threading bug which causes problems with MySQL. The OpenBSD Developers have fixed the problem, but as of January 25th, 2001, it's only available in the ``-current'' branch. The symptoms of this threading bug are: slow response, high load, high CPU usage, and crashes.

2.6.4.6 BSD/OS Notes

2.6.4.7 BSD/OS Version 2.x Notes

If you get the following error when compiling MySQL, your ulimit value for virtual memory is too low:

item_func.h: In method `Item_func_ge::Item_func_ge(const Item_func_ge &)':
item_func.h:28: virtual memory exhausted
make[2]: *** [item_func.o] Error 1

Try using ulimit -v 80000 and run make again. If this doesn't work and you are using bash, try switching to csh or sh; some BSDI users have reported problems with bash and ulimit.

If you are using gcc, you may also use have to use the --with-low-memory flag for configure to be able to compile `sql_yacc.cc'.

If you get problems with the current date in MySQL, setting the TZ variable will probably help. See section H Environment Variables.

2.6.4.8 BSD/OS Version 3.x Notes

Upgrade to BSD/OS Version 3.1. If that is not possible, install BSDIpatch M300-038.

Use the following command when configuring MySQL:

shell> env CXX=shlicc++ CC=shlicc2 \
       ./configure \
           --prefix=/usr/local/mysql \
           --localstatedir=/var/mysql \
           --without-perl \
           --with-unix-socket-path=/var/mysql/mysql.sock

The following is also known to work:

shell> env CC=gcc CXX=gcc CXXFLAGS=-O3 \
       ./configure \
           --prefix=/usr/local/mysql \
           --with-unix-socket-path=/var/mysql/mysql.sock

You can change the directory locations if you wish, or just use the defaults by not specifying any locations.

If you have problems with performance under heavy load, try using the --skip-thread-priority option to mysqld! This will run all threads with the same priority; on BSDI Version 3.1, this gives better performance (at least until BSDI fixes their thread scheduler).

If you get the error virtual memory exhausted while compiling, you should try using ulimit -v 80000 and run make again. If this doesn't work and you are using bash, try switching to csh or sh; some BSDI users have reported problems with bash and ulimit.

2.6.4.9 BSD/OS Version 4.x Notes

BSDI Version 4.x has some thread-related bugs. If you want to use MySQL on this, you should install all thread-related patches. At least M400-023 should be installed.

On some BSDI Version 4.x systems, you may get problems with shared libraries. The symptom is that you can't execute any client programs, for example, mysqladmin. In this case you need to reconfigure not to use shared libraries with the --disable-shared option to configure.

Some customers have had problems on BSDI 4.0.1 that the mysqld binary after a while can't open tables. This is because some library/system related bug causes mysqld to change current directory without asking for this!

The fix is to either upgrade to 3.23.34 or after running configure remove the line #define HAVE_REALPATH from config.h before running make.

Note that the above means that you can't symbolic link a database directories to another database directory or symbolic link a table to another database on BSDI! (Making a symbolic link to another disk is ok).

2.6.5 Mac OS X Notes

2.6.5.1 Mac OS X Public Beta

MySQL should work without any problems on Mac OS X Public Beta (Darwin). You don't need the pthread patches for this OS!

2.6.5.2 Mac OS X Server

Before trying to configure MySQL on Mac OS X server you must first install the pthread package from http://www.prnet.de/RegEx/mysql.html.

Our binary for Mac OS X is compiled on Rhapsody 5.5 with the following configure line:

CC=gcc CFLAGS="-O2 -fomit-frame-pointer" CXX=gcc CXXFLAGS="-O2 -fomit-frame-pointer" ./configure --prefix=/usr/local/mysql "--with-comment=Official MySQL binary" --with-extra-charsets=complex  --disable-shared

You might want to also add aliases to your shell's resource file to access mysql and mysqladmin from the command line:

alias mysql '/usr/local/mysql/bin/mysql'
alias mysqladmin '/usr/local/mysql/bin/mysqladmin'

2.6.6 Other Unix Notes

2.6.6.1 HP-UX Notes for Binary Distributions

Some of the binary distributions of MySQL for HP-UX is distributed as an HP depot file and as a tar file. To use the depot file you must be running at least HP-UX 10.x to have access to HP's software depot tools.

The HP version of MySQL was compiled on an HP 9000/8xx server under HP-UX 10.20, and uses MIT-pthreads. It is known to work well under this configuration. MySQL Version 3.22.26 and newer can also be built with HP's native thread package.

Other configurations that may work:

The following configurations almost definitely won't work:

To install the distribution, use one of the commands below, where /path/to/depot is the full pathname of the depot file:

The depot places binaries and libraries in `/opt/mysql' and data in `/var/opt/mysql'. The depot also creates the appropriate entries in `/etc/init.d' and `/etc/rc2.d' to start the server automatically at boot time. Obviously, this entails being root to install.

To install the HP-UX tar.gz distribution, you must have a copy of GNU tar.

2.6.6.2 HP-UX Version 10.20 Notes

There are a couple of small problems when compiling MySQL on HP-UX. We recommend that you use gcc instead of the HP-UX native compiler, because gcc produces better code!

We recommend using gcc 2.95 on HP-UX. Don't use high optimization flags (like -O6) as this may not be safe on HP-UX.

Note that MIT-pthreads can't be compiled with the HP-UX compiler because it can't compile .S (assembler) files.

The following configure line should work:

CFLAGS="-DHPUX -I/opt/dce/include" CXXFLAGS="-DHPUX -I/opt/dce/include -felide-constructors -fno-exceptions -fno-rtti" CXX=gcc ./configure --with-pthread --with-named-thread-libs='-ldce' --prefix=/usr/local/mysql --disable-shared

If you are compiling gcc 2.95 yourself, you should NOT link it with the DCE libraries (libdce.a or libcma.a) if you want to compile MySQL with MIT-pthreads. If you mix the DCE and MIT-pthreads packages you will get a mysqld to which you cannot connect. Remove the DCE libraries while you compile gcc 2.95!

2.6.6.3 HP-UX Version 11.x Notes

For HP-UX Version 11.x we recommend MySQL Version 3.23.15 or later.

Because of some critical bugs in the standard HP-UX libraries, you should install the following patches before trying to run MySQL on HP-UX 11.0:

PHKL_22840 Streams cumulative
PHNE_22397 ARPA cumulative

This will solve a problem that one gets EWOULDBLOCK from recv() and EBADF from accept() in threaded applications.

If you are using gcc 2.95.1 on an unpatched HP-UX 11.x system, you will get the error:

In file included from /usr/include/unistd.h:11,
                 from ../include/global.h:125,
                 from mysql_priv.h:15,
                 from item.cc:19:
/usr/include/sys/unistd.h:184: declaration of C function ...
/usr/include/sys/pthread.h:440: previous declaration ...
In file included from item.h:306,
                 from mysql_priv.h:158,
                 from item.cc:19:

The problem is that HP-UX doesn't define pthreads_atfork() consistently. It has conflicting prototypes in `/usr/include/sys/unistd.h':184 and `/usr/include/sys/pthread.h':440 (details below).

One solution is to copy `/usr/include/sys/unistd.h' into `mysql/include' and edit `unistd.h' and change it to match the definition in `pthread.h'. Here's the diff:

183,184c183,184
<      extern int pthread_atfork(void (*prepare)(), void (*parent)(),
<                                                void (*child)());
---
>      extern int pthread_atfork(void (*prepare)(void), void (*parent)(void),
>                                                void (*child)(void));

After this, the following configure line should work:

CFLAGS="-fomit-frame-pointer -O3 -fpic" CXX=gcc CXXFLAGS="-felide-constructors -fno-exceptions -fno-rtti -O3" ./configure --prefix=/usr/local/mysql --disable-shared

Here is some information that a HP-UX Version 11.x user sent us about compiling MySQL with HP-UX:x compiler:

 Environment:
      proper compilers.
         setenv CC cc
         setenv CXX aCC
      flags
         setenv CFLAGS -D_REENTRANT
         setenv CXXFLAGS -D_REENTRANT
         setenv CPPFLAGS -D_REENTRANT
     % aCC -V
     aCC: HP ANSI C++ B3910B X.03.14.06
     % cc -V /tmp/empty.c
     cpp.ansi: HP92453-01 A.11.02.00 HP C Preprocessor (ANSI)
     ccom: HP92453-01 A.11.01.00 HP C Compiler
     cc: "/tmp/empty.c", line 1: warning 501: Empty source file.

  configuration:
     ./configure  --with-pthread        \
     --prefix=/source-control/mysql     \
     --with-named-thread-libs=-lpthread \
     --with-low-memory

    added '#define _CTYPE_INCLUDED' to include/m_ctype.h. This
    symbol is the one defined in HP's /usr/include/ctype.h:

     /* Don't include std ctype.h when this is included */
     #define _CTYPE_H
     #define __CTYPE_INCLUDED
     #define _CTYPE_INCLUDED
     #define _CTYPE_USING   /* Don't put names in global namespace. */

If you get the following error from configure

checking for cc option to accept ANSI C... no
configure: error: MySQL requires a ANSI C compiler (and a C++ compiler). Try gcc. See the Installation chapter in the Reference Manual.

Check that you don't have the path to the K&R compiler before the path to the HP-UX C and C++ compiler.

2.6.6.4 IBM-AIX notes

Automatic detection of xlC is missing from Autoconf, so a configure command something like this is needed when compiling MySQL (This example uses the IBM compiler):

export CC="xlc_r -ma -O3 -qstrict -qoptimize=3 -qmaxmem=8192 "
export CXX="xlC_r -ma -O3 -qstrict -qoptimize=3 -qmaxmem=8192"
export CFLAGS="-I /usr/local/include"
export LDLFAGS="-L /usr/local/lib"
export CPPFLAGS=$CFLAGS
export CXXFLAGS=$CFLAGS

./configure --prefix=/usr/local \
		--localstatedir=/var/mysql \
		--sysconfdir=/etc/mysql \
		--sbindir='/usr/local/bin' \
		--libexecdir='/usr/local/bin' \
		--enable-thread-safe-client \
		--enable-large-files

Above are the options used to compile the MySQL distribution that can be found at http://www-frec.bull.com/.

If you change the -O3 to -O2 in the above configure line, you must also remove the -qstrict option (this is a limitation in the IBM C compiler).

If you are using gcc or egcs to compile MySQL, you MUST use the -fno-exceptions flag, as the exception handling in gcc/egcs is not thread safe! (This is tested with egcs 1.1.). There are also some known problems with IBM's assembler, which may cause it to generate bad code when used with gcc.

We recommend the following configure line with egcs and gcc 2.95 on AIX:

CC="gcc -pipe -mcpu=power -Wa,-many" \
CXX="gcc -pipe -mcpu=power -Wa,-many" \
CXXFLAGS="-felide-constructors -fno-exceptions -fno-rtti" \
./configure --prefix=/usr/local/mysql --with-low-memory

The -Wa,-many is necessary for the compile to be successful. IBM is aware of this problem but is in to hurry to fix it because of the workaround available. We don't know if the -fno-exceptions is required with gcc 2.95, but as MySQL doesn't use exceptions and the above option generates faster code, we recommend that you should always use this option with egcs / gcc.

If you get a problem with assembler code try changing the -mcpu=xxx to match your cpu. Typically power2, power, or powerpc may need to be used, alternatively you might need to use 604 or 604e. I'm not positive but I would think using "power" would likely be safe most of the time, even on a power2 machine.

If you don't know what your cpu is then do a "uname -m", this will give you back a string that looks like "000514676700", with a format of xxyyyyyymmss where xx and ss are always 0's, yyyyyy is a unique system id and mm is the id of the CPU Planar. A chart of these values can be found at http://www.rs6000.ibm.com/doc_link/en_US/a_doc_lib/cmds/aixcmds5/uname.htm. This will give you a machine type and a machine model you can use to determine what type of cpu you have.

If you have problems with signals (MySQL dies unexpectedly under high load) you may have found an OS bug with threads and signals. In this case you can tell MySQL not to use signals by configuring with:

shell> CFLAGS=-DDONT_USE_THR_ALARM CXX=gcc \
       CXXFLAGS="-felide-constructors -fno-exceptions -fno-rtti -DDONT_USE_THR_ALARM" \
       ./configure --prefix=/usr/local/mysql --with-debug --with-low-memory

This doesn't affect the performance of MySQL, but has the side effect that you can't kill clients that are ``sleeping'' on a connection with mysqladmin kill or mysqladmin shutdown. Instead, the client will die when it issues its next command.

On some versions of AIX, linking with libbind.a makes getservbyname core dump. This is an AIX bug and should be reported to IBM.

For AIX 4.2.1 and gcc you have to do the following changes.

After configuring, edit `config.h' and `include/my_config.h' and change the line that says

#define HAVE_SNPRINTF 1

to

#undef HAVE_SNPRINTF

And finally, in `mysqld.cc' you need to add a prototype for initgoups.

#ifdef _AIX41
extern "C" int initgroups(const char *,int);
#endif

2.6.6.5 SunOS 4 Notes

On SunOS 4, MIT-pthreads is needed to compile MySQL, which in turn means you will need GNU make.

Some SunOS 4 systems have problems with dynamic libraries and libtool. You can use the following configure line to avoid this problem:

shell> ./configure --disable-shared --with-mysqld-ldflags=-all-static

When compiling readline, you may get warnings about duplicate defines. These may be ignored.

When compiling mysqld, there will be some implicit declaration of function warnings. These may be ignored.

2.6.6.6 Alpha-DEC-UNIX Notes (Tru64)

If you are using egcs 1.1.2 on Digital Unix, you should upgrade to gcc 2.95.2, as egcs on DEC has some serious bugs!

When compiling threaded programs under Digital Unix, the documentation recommends using the -pthread option for cc and cxx and the libraries -lmach -lexc (in addition to -lpthread). You should run configure something like this:

CC="cc -pthread" CXX="cxx -pthread -O" \
./configure --with-named-thread-libs="-lpthread -lmach -lexc -lc"

When compiling mysqld, you may see a couple of warnings like this:

mysqld.cc: In function void handle_connections()':
mysqld.cc:626: passing long unsigned int *' as argument 3 of
accept(int,sockadddr *, int *)'

You can safely ignore these warnings. They occur because configure can detect only errors, not warnings.

If you start the server directly from the command line, you may have problems with it dying when you log out. (When you log out, your outstanding processes receive a SIGHUP signal.) If so, try starting the server like this:

shell> nohup mysqld [options] &

nohup causes the command following it to ignore any SIGHUP signal sent from the terminal. Alternatively, start the server by running safe_mysqld, which invokes mysqld using nohup for you. See section 4.7.2 safe_mysqld, the wrapper around mysqld.

If you get a problem when compiling mysys/get_opt.c, just remove the line #define _NO_PROTO from the start of that file!

If you are using Compac's CC compiler, the following configure line should work:

CC="cc -pthread"
CFLAGS="-O4 -ansi_alias -ansi_args -fast -inline speed all -arch host"
CXX="cxx -pthread"
CXXFLAGS="-O4 -ansi_alias -ansi_args -fast -inline speed all -arch host"
export CC CFLAGS CXX CXXFLAGS
./configure \
--prefix=/usr/local/mysql \
--with-low-memory \
--enable-large-files \
--enable-shared=yes \
--with-named-thread-libs="-lpthread -lmach -lexc -lc"
gnumake

If you get a problem with libtool, when compiling with shared libraries as above, when linking mysql, you should be able to get around this by issuing:

cd mysql
/bin/sh ../libtool --mode=link cxx -pthread  -O3 -DDBUG_OFF \
-O4 -ansi_alias -ansi_args -fast -inline speed \
-speculate all \ -arch host  -DUNDEF_HAVE_GETHOSTBYNAME_R \
-o mysql  mysql.o readline.o sql_string.o completion_hash.o \
../readline/libreadline.a -lcurses \
../libmysql/.libs/libmysqlclient.so  -lm
cd ..
gnumake
gnumake install
scripts/mysql_install_db

2.6.6.7 Alpha-DEC-OSF1 Notes

If you have problems compiling and have DEC CC and gcc installed, try running configure like this:

CC=cc CFLAGS=-O CXX=gcc CXXFLAGS=-O3 \
./configure --prefix=/usr/local/mysql

If you get problems with the `c_asm.h' file, you can create and use a 'dummy' `c_asm.h' file with:

touch include/c_asm.h
CC=gcc CFLAGS=-I./include \
CXX=gcc CXXFLAGS=-O3 \
./configure --prefix=/usr/local/mysql

Note that the following problems with the ld program can be fixed by downloading the latest DEC (Compaq) patch kit from: http://ftp.support.compaq.com/public/unix/.

On OSF1 V4.0D and compiler "DEC C V5.6-071 on Digital Unix V4.0 (Rev. 878)" the compiler had some strange behavior (undefined asm symbols). /bin/ld also appears to be broken (problems with _exit undefined errors occuring while linking mysqld). On this system, we have managed to compile MySQL with the following configure line, after replacing /bin/ld with the version from OSF 4.0C:

CC=gcc CXX=gcc CXXFLAGS=-O3 ./configure --prefix=/usr/local/mysql

With the Digital compiler "C++ V6.1-029", the following should work:

CC=cc -pthread
CFLAGS=-O4 -ansi_alias -ansi_args -fast -inline speed -speculate all -arch host
CXX=cxx -pthread
CXXFLAGS=-O4 -ansi_alias -ansi_args -fast -inline speed -speculate all -arch host -noexceptions -nortti
export CC CFLAGS CXX CXXFLAGS
./configure --prefix=/usr/mysql/mysql --with-mysqld-ldflags=-all-static --disable-shared --with-named-thread-libs="-lmach -lexc -lc"

In some versions of OSF1, the alloca() function is broken. Fix this by removing the line in `config.h' that defines 'HAVE_ALLOCA'.

The alloca() function also may have an incorrect prototype in /usr/include/alloca.h. This warning resulting from this can be ignored.

configure will use the following thread libraries automatically: --with-named-thread-libs="-lpthread -lmach -lexc -lc".

When using gcc, you can also try running configure like this:

shell> CFLAGS=-D_PTHREAD_USE_D4 CXX=gcc CXXFLAGS=-O3 ./configure ....

If you have problems with signals (MySQL dies unexpectedly under high load), you may have found an OS bug with threads and signals. In this case you can tell MySQL not to use signals by configuring with:

shell> CFLAGS=-DDONT_USE_THR_ALARM \
       CXXFLAGS=-DDONT_USE_THR_ALARM \
       ./configure ...

This doesn't affect the performance of MySQL, but has the side effect that you can't kill clients that are ``sleeping'' on a connection with mysqladmin kill or mysqladmin shutdown. Instead, the client will die when it issues its next command.

With gcc 2.95.2, you will probably run into the following compile error:

sql_acl.cc:1456: Internal compiler error in `scan_region', at except.c:2566
Please submit a full bug report.

To fix this you should change to the sql directory and do a ``cut and paste'' of the last gcc line, but change -O3 to -O0 (or add -O0 immediately after gcc if you don't have any -O option on your compile line.) After this is done you can just change back to the top-level directly and run make again.

2.6.6.8 SGI Irix Notes

If you are using Irix Version 6.5.3 or newer mysqld will only be able to create threads if you run it as a user with CAP_SCHED_MGT privileges (like root) or give the mysqld server this privilege with the following shell command:

shell> chcap "CAP_SCHED_MGT+epi" /opt/mysql/libexec/mysqld

You may have to undefine some things in `config.h' after running configure and before compiling.

In some Irix implementations, the alloca() function is broken. If the mysqld server dies on some SELECT statements, remove the lines from `config.h' that define HAVE_ALLOC and HAVE_ALLOCA_H. If mysqladmin create doesn't work, remove the line from `config.h' that defines HAVE_READDIR_R. You may have to remove the HAVE_TERM_H line as well.

SGI recommends that you install all of the patches on this page as a set: http://support.sgi.com/surfzone/patches/patchset/6.2_indigo.rps.html

At the very minimum, you should install the latest kernel rollup, the latest rld rollup, and the latest libc rollup.

You definitely need all the POSIX patches on this page, for pthreads support:

http://support.sgi.com/surfzone/patches/patchset/6.2_posix.rps.html

If you get the something like the following error when compiling `mysql.cc':

"/usr/include/curses.h", line 82: error(1084): invalid combination of type

Type the following in the top-level directory of your MySQL source tree:

shell> extra/replace bool curses_bool < /usr/include/curses.h > include/curses.h
shell> make

There have also been reports of scheduling problems. If only one thread is running, things go slow. Avoid this by starting another client. This may lead to a 2-to-10-fold increase in execution speed thereafter for the other thread. This is a poorly understood problem with Irix threads; you may have to improvise to find solutions until this can be fixed.

If you are compiling with gcc, you can use the following configure command:

CC=gcc CXX=gcc CXXFLAGS=-O3 \
./configure --prefix=/usr/local/mysql --with-thread-safe-client --with-named-thread-libs=-lpthread

On Irix 6.5.11 with native Irix C and C++ compilers ver. 7.3.1.2, the following is reported to work

CC=cc CXX=CC CFLAGS='-O3 -n32 -TARG:platform=IP22 -I/usr/local/include \
-L/usr/local/lib' CXXFLAGS='-O3 -n32 -TARG:platform=IP22 \
-I/usr/local/include -L/usr/local/lib' ./configure --prefix=/usr/local/mysql \
--with-berkeley-db --with-innodb \
--with-libwrap=/usr/local --with-named-curses-libs=/usr/local/lib/libncurses.a

2.6.6.9 SCO Notes

The current port is tested only on a ``sco3.2v5.0.4'' and ``sco3.2v5.0.5'' system. There has also been a lot of progress on a port to ``sco 3.2v4.2''.

For the moment the recommended compiler on OpenServer is gcc 2.95.2. With this you should be able to compile MySQL with just:

CC=gcc CXX=gcc ./configure ... (options)
  1. For OpenServer 5.0.X you need to use GDS in Skunkware 95 (95q4c). This is necessary because GNU gcc 2.7.2 in Skunkware 97 does not have GNU as. You can also use egcs 1.1.2 or newer http://www.egcs.com/. If you are using egcs 1.1.2 you have to execute the following command:
    shell> cp -p /usr/include/pthread/stdtypes.h /usr/local/lib/gcc-lib/i386-pc-sco3.2v5.0.5/egcs-2.91.66/include/pthread/
    
  2. You need the port of GCC 2.5.x for this product and the Development system. They are required on this version of SCO Unix. You cannot just use the GCC Dev system.
  3. You should get the FSU Pthreads package and install it first. This can be found at http://www.cs.wustl.edu/~schmidt/ACE_wrappers/FSU-threads.tar.gz. You can also get a precompiled package from http://www.mysql.com/Downloads/SCO/FSU-threads-3.5c.tar.gz.
  4. FSU Pthreads can be compiled with SCO Unix 4.2 with tcpip. Or OpenServer 3.0 or Open Desktop 3.0 (OS 3.0 ODT 3.0), with the SCO Development System installed using a good port of GCC 2.5.x ODT or OS 3.0 you will need a good port of GCC 2.5.x There are a lot of problems without a good port. The port for this product requires the SCO Unix Development system. Without it, you are missing the libraries and the linker that is needed.
  5. To build FSU Pthreads on your system, do the following:
    1. Run ./configure in the `threads/src' directory and select the SCO OpenServer option. This command copies `Makefile.SCO5' to `Makefile'.
    2. Run make.
    3. To install in the default `/usr/include' directory, login as root, then cd to the `thread/src' directory, and run make install.
  6. Remember to use GNU make when making MySQL.
  7. If you don't start safe_mysqld as root, you probably will get only the default 110 open files per process. mysqld will write a note about this in the log file.
  8. With SCO 3.2V5.0.5, you should use FSU Pthreads version 3.5c or newer. You should also use gcc 2.95.2 or newer! The following configure command should work:
    shell> ./configure --prefix=/usr/local/mysql --disable-shared
    
  9. With SCO 3.2V4.2, you should use FSU Pthreads version 3.5c or newer. The following configure command should work:
    shell> CFLAGS="-D_XOPEN_XPG4" CXX=gcc CXXFLAGS="-D_XOPEN_XPG4" \
           ./configure \
               --prefix=/usr/local/mysql \
               --with-named-thread-libs="-lgthreads -lsocket -lgen -lgthreads" \
               --with-named-curses-libs="-lcurses"
    
    You may get some problems with some include files. In this case, you can find new SCO-specific include files at http://www.mysql.com/Downloads/SCO/SCO-3.2v4.2-includes.tar.gz. You should unpack this file in the `include' directory of your MySQL source tree.

SCO development notes:

If you want to install DBI on SCO, you have to edit the `Makefile' in DBI-xxx and each subdirectory.

Note that the following assumes gcc 2.95.2 or newer:

OLD:                                  NEW:
CC = cc                               CC = gcc
CCCDLFLAGS = -KPIC -W1,-Bexport       CCCDLFLAGS = -fpic
CCDLFLAGS = -wl,-Bexport              CCDLFLAGS =

LD = ld                               LD = gcc -G -fpic
LDDLFLAGS = -G -L/usr/local/lib       LDDLFLAGS = -L/usr/local/lib
LDFLAGS = -belf -L/usr/local/lib      LDFLAGS = -L/usr/local/lib

LD = ld                               LD = gcc -G -fpic
OPTIMISE = -Od                        OPTIMISE = -O1

OLD:
CCCFLAGS = -belf -dy -w0 -U M_XENIX -DPERL_SCO5 -I/usr/local/include

NEW:
CCFLAGS = -U M_XENIX -DPERL_SCO5 -I/usr/local/include

This is because the Perl dynaloader will not load the DBI modules if they were compiled with icc or cc.

Perl works best when compiled with cc.

2.6.6.10 SCO Unixware Version 7.0 Notes

You must use a version of MySQL at least as recent as Version 3.22.13 because that version fixes some portability problems under Unixware.

We have been able to compile MySQL with the following configure command on Unixware Version 7.0.1:

CC=cc CXX=CC ./configure --prefix=/usr/local/mysql

If you want to use gcc, you must use gcc 2.95.2 or newer.

2.6.7 OS/2 Notes

MySQL uses quite a few open files. Because of this, you should add something like the following to your `CONFIG.SYS' file:

SET EMXOPT=-c -n -h1024

If you don't do this, you will probably run into the following error:

File 'xxxx' not found (Errcode: 24)

When using MySQL with OS/2 Warp 3, FixPack 29 or above is required. With OS/2 Warp 4, FixPack 4 or above is required. This is a requirement of the Pthreads library. MySQL must be installed in a partition that supports long filenames such as HPFS, FAT32, etc.

The `INSTALL.CMD' script must be run from OS/2's own `CMD.EXE' and may not work with replacement shells such as `4OS2.EXE'.

The `scripts/mysql-install-db' script has been renamed. It is now called `install.cmd' and is a REXX script, which will set up the default MySQL security settings and create the WorkPlace Shell icons for MySQL.

Dynamic module support is compiled in but not fully tested. Dynamic modules should be compiled using the Pthreads run-time library.

gcc -Zdll -Zmt -Zcrtdll=pthrdrtl -I../include -I../regex -I.. \
    -o example udf_example.cc -L../lib -lmysqlclient udf_example.def
mv example.dll example.udf

Note: Due to limitations in OS/2, UDF module name stems must not exceed 8 characters. Modules are stored in the `/mysql2/udf' directory; the safe-mysqld.cmd script will put this directory in the BEGINLIBPATH environment variable. When using UDF modules, specified extensions are ignored -- it is assumed to be `.udf'. For example, in Unix, the shared module might be named `example.so' and you would load a function from it like this:

mysql> CREATE FUNCTION metaphon RETURNS STRING SONAME "example.so";

Is OS/2, the module would be named `example.udf', but you would not specify the module extension:

mysql> CREATE FUNCTION metaphon RETURNS STRING SONAME "example";

2.6.8 BeOS Notes

We are really interested in getting MySQL to work on BeOS, but unfortunately we don't have any person who knows BeOS or has time to do a port.

We are interested in finding someone to do a port, and we will help them with any technical questions they may have while doing the port.

We have previously talked with some BeOS developers that have said that MySQL is 80% ported to BeOS, but we haven't heard from them in a while.

2.6.9 Novell Netware Notes

We are really interested in getting MySQL to work on Netware, but unfortunately we don't have any person who knows Netware or has time to do a port.

We are interested in finding someone to do a port, and we will help them with any technical questions they may have while doing the port.

3 Introduction to MySQL: A MySQL Tutorial

This chapter provides a tutorial introduction to MySQL by showing how to use the mysql client program to create and use a simple database. mysql (sometimes referred to as the ``terminal monitor'' or just ``monitor'') is an interactive program that allows you to connect to a MySQL server, run queries, and view the results. mysql may also be used in batch mode: you place your queries in a file beforehand, then tell mysql to execute the contents of the file. Both ways of using mysql are covered here.

To see a list of options provided by mysql, invoke it with the --help option:

shell> mysql --help

This chapter assumes that mysql is installed on your machine and that a MySQL server is available to which you can connect. If this is not true, contact your MySQL administrator. (If you are the administrator, you will need to consult other sections of this manual.)

This chapter describes the entire process of setting up and using a database. If you are interested only in accessing an already-existing database, you may want to skip over the sections that describe how to create the database and the tables it contains.

Because this chapter is tutorial in nature, many details are necessarily left out. Consult the relevant sections of the manual for more information on the topics covered here.

3.1 Connecting to and Disconnecting from the Server

To connect to the server, you'll usually need to provide a MySQL user name when you invoke mysql and, most likely, a password. If the server runs on a machine other than the one where you log in, you'll also need to specify a hostname. Contact your administrator to find out what connection parameters you should use to connect (that is, what host, user name, and password to use). Once you know the proper parameters, you should be able to connect like this:

shell> mysql -h host -u user -p
Enter password: ********

The ******** represents your password; enter it when mysql displays the Enter password: prompt.

If that works, you should see some introductory information followed by a mysql> prompt:

shell> mysql -h host -u user -p
Enter password: ********
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 459 to server version: 3.22.20a-log

Type 'help' for help.

mysql>

The prompt tells you that mysql is ready for you to enter commands.

Some MySQL installations allow users to connect as the anonymous (unnamed) user to the server running on the local host. If this is the case on your machine, you should be able to connect to that server by invoking mysql without any options:

shell> mysql

After you have connected successfully, you can disconnect any time by typing QUIT at the mysql> prompt:

mysql> QUIT
Bye

You can also disconnect by pressing Control-D.

Most examples in the following sections assume you are connected to the server. They indicate this by the mysql> prompt.

3.2 Entering Queries

Make sure you are connected to the server, as discussed in the previous section. Doing so will not in itself select any database to work with, but that's okay. At this point, it's more important to find out a little about how to issue queries than to jump right in creating tables, loading data into them, and retrieving data from them. This section describes the basic principles of entering commands, using several queries you can try out to familiarize yourself with how mysql works.

Here's a simple command that asks the server to tell you its version number and the current date. Type it in as shown below following the mysql> prompt and hit the RETURN key:

mysql> SELECT VERSION(), CURRENT_DATE;
+--------------+--------------+
| version()    | CURRENT_DATE |
+--------------+--------------+
| 3.22.20a-log | 1999-03-19   |
+--------------+--------------+
1 row in set (0.01 sec)
mysql>

This query illustrates several things about mysql:

Keywords may be entered in any lettercase. The following queries are equivalent:

mysql> SELECT VERSION(), CURRENT_DATE;
mysql> select version(), current_date;
mysql> SeLeCt vErSiOn(), current_DATE;

Here's another query. It demonstrates that you can use mysql as a simple calculator:

mysql> SELECT SIN(PI()/4), (4+1)*5;
+-------------+---------+
| SIN(PI()/4) | (4+1)*5 |
+-------------+---------+
|    0.707107 |      25 |
+-------------+---------+

The commands shown thus far have been relatively short, single-line statements. You can even enter multiple statements on a single line. Just end each one with a semicolon:

mysql> SELECT VERSION(); SELECT NOW();
+--------------+
| version()    |
+--------------+
| 3.22.20a-log |
+--------------+

+---------------------+
| NOW()               |
+---------------------+
| 1999-03-19 00:15:33 |
+---------------------+

A command need not be given all on a single line, so lengthy commands that require several lines are not a problem. mysql determines where your statement ends by looking for the terminating semicolon, not by looking for the end of the input line. (In other words, mysql accepts free-format input: it collects input lines but does not execute them until it sees the semicolon.)

Here's a simple multiple-line statement:

mysql> SELECT
    -> USER()
    -> ,
    -> CURRENT_DATE;
+--------------------+--------------+
| USER()             | CURRENT_DATE |
+--------------------+--------------+
| joesmith@localhost | 1999-03-18   |
+--------------------+--------------+

In this example, notice how the prompt changes from mysql> to -> after you enter the first line of a multiple-line query. This is how mysql indicates that it hasn't seen a complete statement and is waiting for the rest. The prompt is your friend, because it provides valuable feedback. If you use that feedback, you will always be aware of what mysql is waiting for.

If you decide you don't want to execute a command that you are in the process of entering, cancel it by typing \c:

mysql> SELECT
    -> USER()
    -> \c
mysql>

Here, too, notice the prompt. It switches back to mysql> after you type \c, providing feedback to indicate that mysql is ready for a new command.

The following table shows each of the prompts you may see and summarizes what they mean about the state that mysql is in:

Prompt Meaning
mysql> Ready for new command.
-> Waiting for next line of multiple-line command.
'> Waiting for next line, collecting a string that begins with a single quote (`'').
"> Waiting for next line, collecting a string that begins with a double quote (`"').

Multiple-line statements commonly occur by accident when you intend to issue a command on a single line, but forget the terminating semicolon. In this case, mysql waits for more input:

mysql> SELECT USER()
    ->

If this happens to you (you think you've entered a statement but the only response is a -> prompt), most likely mysql is waiting for the semicolon. If you don't notice what the prompt is telling you, you might sit there for a while before realizing what you need to do. Enter a semicolon to complete the statement, and mysql will execute it:

mysql> SELECT USER()
    -> ;
+--------------------+
| USER()             |
+--------------------+
| joesmith@localhost |
+--------------------+

The '> and "> prompts occur during string collection. In MySQL, you can write strings surrounded by either `'' or `"' characters (for example, 'hello' or "goodbye"), and mysql lets you enter strings that span multiple lines. When you see a '> or "> prompt, it means that you've entered a line containing a string that begins with a `'' or `"' quote character, but have not yet entered the matching quote that terminates the string. That's fine if you really are entering a multiple-line string, but how likely is that? Not very. More often, the '> and "> prompts indicate that you've inadvertantly left out a quote character. For example:

mysql> SELECT * FROM my_table WHERE name = "Smith AND age < 30;
    ">

If you enter this SELECT statement, then hit RETURN and wait for the result, nothing will happen. Instead of wondering why this query takes so long, notice the clue provided by the "> prompt. It tells you that mysql expects to see the rest of an unterminated string. (Do you see the error in the statement? The string "Smith is missing the second quote.)

At this point, what do you do? The simplest thing is to cancel the command. However, you cannot just type \c in this case, because mysql interprets it as part of the string that it is collecting! Instead, enter the closing quote character (so mysql knows you've finished the string), then type \c:

mysql> SELECT * FROM my_table WHERE name = "Smith AND age < 30;
    "> "\c
mysql>

The prompt changes back to mysql>, indicating that mysql is ready for a new command.

It's important to know what the '> and "> prompts signify, because if you mistakenly enter an unterminated string, any further lines you type will appear to be ignored by mysql -- including a line containing QUIT! This can be quite confusing, especially if you don't know that you need to supply the terminating quote before you can cancel the current command.

3.3 Creating and Using a Database

Now that you know how to enter commands, it's time to access a database.

Suppose you have several pets in your home (your menagerie) and you'd like to keep track of various types of information about them. You can do so by creating tables to hold your data and loading them with the desired information. Then you can answer different sorts of questions about your animals by retrieving data from the tables. This section shows you how to:

The menagerie database will be simple (deliberately), but it is not difficult to think of real-world situations in which a similar type of database might be used. For example, a database like this could be used by a farmer to keep track of livestock, or by a veterinarian to keep track of patient records. A menagerie distribution containing some of the queries and sample data used in the following sections can be obtained from the MySQL Web site. It's available in either compressed tar format or Zip format.

Use the SHOW statement to find out what databases currently exist on the server:

mysql> SHOW DATABASES;
+----------+
| Database |
+----------+
| mysql    |
| test     |
| tmp      |
+----------+

The list of databases is probably different on your machine, but the mysql and test databases are likely to be among them. The mysql database is required because it describes user access privileges. The test database is often provided as a workspace for users to try things out.

If the test database exists, try to access it:

mysql> USE test
Database changed

Note that USE, like QUIT, does not require a semicolon. (You can terminate such statements with a semicolon if you like; it does no harm.) The USE statement is special in another way, too: it must be given on a single line.

You can use the test database (if you have access to it) for the examples that follow, but anything you create in that database can be removed by anyone else with access to it. For this reason, you should probably ask your MySQL administrator for permission to use a database of your own. Suppose you want to call yours menagerie. The administrator needs to execute a command like this:

mysql> GRANT ALL ON menagerie.* TO your_mysql_name;

where your_mysql_name is the MySQL user name assigned to you.

3.3.1 Creating and Selecting a Database

If the administrator creates your database for you when setting up your permissions, you can begin using it. Otherwise, you need to create it yourself:

mysql> CREATE DATABASE menagerie;

Under Unix, database names are case sensitive (unlike SQL keywords), so you must always refer to your database as menagerie, not as Menagerie, MENAGERIE, or some other variant. This is also true for table names. (Under Windows, this restriction does not apply, although you must refer to databases and tables using the same lettercase throughout a given query.)

Creating a database does not select it for use; you must do that explicitly. To make menagerie the current database, use this command:

mysql> USE menagerie
Database changed

Your database needs to be created only once, but you must select it for use each time you begin a mysql session. You can do this by issuing a USE statement as shown above. Alternatively, you can select the database on the command line when you invoke mysql. Just specify its name after any connection parameters that you might need to provide. For example:

shell> mysql -h host -u user -p menagerie
Enter password: ********

Note that menagerie is not your password on the command just shown. If you want to supply your password on the command line after the -p option, you must do so with no intervening space (for example, as -pmypassword, not as -p mypassword). However, putting your password on the command line is not recommended, because doing so exposes it to snooping by other users logged in on your machine.

3.3.2 Creating a Table

Creating the database is the easy part, but at this point it's empty, as SHOW TABLES will tell you:

mysql> SHOW TABLES;
Empty set (0.00 sec)

The harder part is deciding what the structure of your database should be: what tables you will need and what columns will be in each of them.

You'll want a table that contains a record for each of your pets. This can be called the pet table, and it should contain, as a bare minimum, each animal's name. Because the name by itself is not very interesting, the table should contain other information. For example, if more than one person in your family keeps pets, you might want to list each animal's owner. You might also want to record some basic descriptive information such as species and sex.

How about age? That might be of interest, but it's not a good thing to store in a database. Age changes as time passes, which means you'd have to update your records often. Instead, it's better to store a fixed value such as date of birth. Then, whenever you need age, you can calculate it as the difference between the current date and the birth date. MySQL provides functions for doing date arithmetic, so this is not difficult. Storing birth date rather than age has other advantages, too:

You can probably think of other types of information that would be useful in the pet table, but the ones identified so far are sufficient for now: name, owner, species, sex, birth, and death.

Use a CREATE TABLE statement to specify the layout of your table:

mysql> CREATE TABLE pet (name VARCHAR(20), owner VARCHAR(20),
    -> species VARCHAR(20), sex CHAR(1), birth DATE, death DATE);

VARCHAR is a good choice for the name, owner, and species columns because the column values will vary in length. The lengths of those columns need not all be the same, and need not be 20. You can pick any length from 1 to 255, whatever seems most reasonable to you. (If you make a poor choice and it turns out later that you need a longer field, MySQL provides an ALTER TABLE statement.)

Animal sex can be represented in a variety of ways, for example, "m" and "f", or perhaps "male" and "female". It's simplest to use the single characters "m" and "f".

The use of the DATE data type for the birth and death columns is a fairly obvious choice.

Now that you have created a table, SHOW TABLES should produce some output:

mysql> SHOW TABLES;
+---------------------+
| Tables in menagerie |
+---------------------+
| pet                 |
+---------------------+

To verify that your table was created the way you expected, use a DESCRIBE statement:

mysql> DESCRIBE pet;
+---------+-------------+------+-----+---------+-------+
| Field   | Type        | Null | Key | Default | Extra |
+---------+-------------+------+-----+---------+-------+
| name    | varchar(20) | YES  |     | NULL    |       |
| owner   | varchar(20) | YES  |     | NULL    |       |
| species | varchar(20) | YES  |     | NULL    |       |
| sex     | char(1)     | YES  |     | NULL    |       |
| birth   | date        | YES  |     | NULL    |       |
| death   | date        | YES  |     | NULL    |       |
+---------+-------------+------+-----+---------+-------+

You can use DESCRIBE any time, for example, if you forget the names of the columns in your table or what types they are.

3.3.3 Loading Data into a Table

After creating your table, you need to populate it. The LOAD DATA and INSERT statements are useful for this.

Suppose your pet records can be described as shown below. (Observe that MySQL expects dates in YYYY-MM-DD format; this may be different than what you are used to.)

name owner species sex birth death
Fluffy Harold cat f 1993-02-04
Claws Gwen cat m 1994-03-17
Buffy Harold dog f 1989-05-13
Fang Benny dog m 1990-08-27
Bowser Diane dog m 1989-08-31 1995-07-29
Chirpy Gwen bird f 1998-09-11
Whistler Gwen bird 1997-12-09
Slim Benny snake m 1996-04-29

Because you are beginning with an empty table, an easy way to populate it is to create a text file containing a row for each of your animals, then load the contents of the file into the table with a single statement.

You could create a text file `pet.txt' containing one record per line, with values separated by tabs, and given in the order in which the columns were listed in the CREATE TABLE statement. For missing values (such as unknown sexes or death dates for animals that are still living), you can use NULL values. To represent these in your text file, use \N. For example, the record for Whistler the bird would look like this (where the whitespace between values is a single tab character):

Whistler Gwen bird \N 1997-12-09 \N

To load the text file `pet.txt' into the pet table, use this command:

mysql> LOAD DATA LOCAL INFILE "pet.txt" INTO TABLE pet;

You can specify the column value separator and end of line marker explicitly in the LOAD DATA statement if you wish, but the defaults are tab and linefeed. These are sufficient for the statement to read the file `pet.txt' properly.

When you want to add new records one at a time, the INSERT statement is useful. In its simplest form, you supply values for each column, in the order in which the columns were listed in the CREATE TABLE statement. Suppose Diane gets a new hamster named Puffball. You could add a new record using an INSERT statement like this:

mysql> INSERT INTO pet
    -> VALUES ('Puffball','Diane','hamster','f','1999-03-30',NULL);

Note that string and date values are specified as quoted strings here. Also, with INSERT, you can insert NULL directly to represent a missing value. You do not use \N like you do with LOAD DATA.

From this example, you should be able to see that there would be a lot more typing involved to load your records initially using several INSERT statements rather than a single LOAD DATA statement.

3.3.4 Retrieving Information from a Table

The SELECT statement is used to pull information from a table. The general form of the statement is:

SELECT what_to_select
FROM which_table
WHERE conditions_to_satisfy

what_to_select indicates what you want to see. This can be a list of columns, or * to indicate ``all columns.'' which_table indicates the table from which you want to retrieve data. The WHERE clause is optional. If it's present, conditions_to_satisfy specifies conditions that rows must satisfy to qualify for retrieval.

3.3.4.1 Selecting All Data

The simplest form of SELECT retrieves everything from a table:

mysql> SELECT * FROM pet;
+----------+--------+---------+------+------------+------------+
| name     | owner  | species | sex  | birth      | death      |
+----------+--------+---------+------+------------+------------+
| Fluffy   | Harold | cat     | f    | 1993-02-04 | NULL       |
| Claws    | Gwen   | cat     | m    | 1994-03-17 | NULL       |
| Buffy    | Harold | dog     | f    | 1989-05-13 | NULL       |
| Fang     | Benny  | dog     | m    | 1990-08-27 | NULL       |
| Bowser   | Diane  | dog     | m    | 1998-08-31 | 1995-07-29 |
| Chirpy   | Gwen   | bird    | f    | 1998-09-11 | NULL       |
| Whistler | Gwen   | bird    | NULL | 1997-12-09 | NULL       |
| Slim     | Benny  | snake   | m    | 1996-04-29 | NULL       |
| Puffball | Diane  | hamster | f    | 1999-03-30 | NULL       |
+----------+--------+---------+------+------------+------------+

This form of SELECT is useful if you want to review your entire table, for instance, after you've just loaded it with your initial dataset. As it happens, the output just shown reveals an error in your data file: Bowser appears to have been born after he died! Consulting your original pedigree papers, you find that the correct birth year is 1989, not 1998.

There are are least a couple of ways to fix this:

As shown above, it is easy to retrieve an entire table. But typically you don't want to do that, particularly when the table becomes large. Instead, you're usually more interested in answering a particular question, in which case you specify some constraints on the information you want. Let's look at some selection queries in terms of questions about your pets that they answer.

3.3.4.2 Selecting Particular Rows

You can select only particular rows from your table. For example, if you want to verify the change that you made to Bowser's birth date, select Bowser's record like this:

mysql> SELECT * FROM pet WHERE name = "Bowser";
+--------+-------+---------+------+------------+------------+
| name   | owner | species | sex  | birth      | death      |
+--------+-------+---------+------+------------+------------+
| Bowser | Diane | dog     | m    | 1989-08-31 | 1995-07-29 |
+--------+-------+---------+------+------------+------------+

The output confirms that the year is correctly recorded now as 1989, not 1998.

String comparisons are normally case insensitive, so you can specify the name as "bowser", "BOWSER", etc. The query result will be the same.

You can specify conditions on any column, not just name. For example, if you want to know which animals were born after 1998, test the birth column:

mysql> SELECT * FROM pet WHERE birth >= "1998-1-1";
+----------+-------+---------+------+------------+-------+
| name     | owner | species | sex  | birth      | death |
+----------+-------+---------+------+------------+-------+
| Chirpy   | Gwen  | bird    | f    | 1998-09-11 | NULL  |
| Puffball | Diane | hamster | f    | 1999-03-30 | NULL  |
+----------+-------+---------+------+------------+-------+

You can combine conditions, for example, to locate female dogs:

mysql> SELECT * FROM pet WHERE species = "dog" AND sex = "f";
+-------+--------+---------+------+------------+-------+
| name  | owner  | species | sex  | birth      | death |
+-------+--------+---------+------+------------+-------+
| Buffy | Harold | dog     | f    | 1989-05-13 | NULL  |
+-------+--------+---------+------+------------+-------+

The preceding query uses the AND logical operator. There is also an OR operator:

mysql> SELECT * FROM pet WHERE species = "snake" OR species = "bird";
+----------+-------+---------+------+------------+-------+
| name     | owner | species | sex  | birth      | death |
+----------+-------+---------+------+------------+-------+
| Chirpy   | Gwen  | bird    | f    | 1998-09-11 | NULL  |
| Whistler | Gwen  | bird    | NULL | 1997-12-09 | NULL  |
| Slim     | Benny | snake   | m    | 1996-04-29 | NULL  |
+----------+-------+---------+------+------------+-------+

AND and OR may be intermixed. If you do that, it's a good idea to use parentheses to indicate how conditions should be grouped:

mysql> SELECT * FROM pet WHERE (species = "cat" AND sex = "m")
    -> OR (species = "dog" AND sex = "f");
+-------+--------+---------+------+------------+-------+
| name  | owner  | species | sex  | birth      | death |
+-------+--------+---------+------+------------+-------+
| Claws | Gwen   | cat     | m    | 1994-03-17 | NULL  |
| Buffy | Harold | dog     | f    | 1989-05-13 | NULL  |
+-------+--------+---------+------+------------+-------+

3.3.4.3 Selecting Particular Columns

If you don't want to see entire rows from your table, just name the columns in which you're interested, separated by commas. For example, if you want to know when your animals were born, select the name and birth columns:

mysql> SELECT name, birth FROM pet;
+----------+------------+
| name     | birth      |
+----------+------------+
| Fluffy   | 1993-02-04 |
| Claws    | 1994-03-17 |
| Buffy    | 1989-05-13 |
| Fang     | 1990-08-27 |
| Bowser   | 1989-08-31 |
| Chirpy   | 1998-09-11 |
| Whistler | 1997-12-09 |
| Slim     | 1996-04-29 |
| Puffball | 1999-03-30 |
+----------+------------+

To find out who owns pets, use this query:

mysql> SELECT owner FROM pet;
+--------+
| owner  |
+--------+
| Harold |
| Gwen   |
| Harold |
| Benny  |
| Diane  |
| Gwen   |
| Gwen   |
| Benny  |
| Diane  |
+--------+

However, notice that the query simply retrieves the owner field from each record, and some of them appear more than once. To minimize the output, retrieve each unique output record just once by adding the keyword DISTINCT:

mysql> SELECT DISTINCT owner FROM pet;
+--------+
| owner  |
+--------+
| Benny  |
| Diane  |
| Gwen   |
| Harold |
+--------+

You can use a WHERE clause to combine row selection with column selection. For example, to get birth dates for dogs and cats only, use this query:

mysql> SELECT name, species, birth FROM pet
    -> WHERE species = "dog" OR species = "cat";
+--------+---------+------------+
| name   | species | birth      |
+--------+---------+------------+
| Fluffy | cat     | 1993-02-04 |
| Claws  | cat     | 1994-03-17 |
| Buffy  | dog     | 1989-05-13 |
| Fang   | dog     | 1990-08-27 |
| Bowser | dog     | 1989-08-31 |
+--------+---------+------------+

3.3.4.4 Sorting Rows

You may have noticed in the preceding examples that the result rows are displayed in no particular order. However, it's often easier to examine query output when the rows are sorted in some meaningful way. To sort a result, use an ORDER BY clause.

Here are animal birthdays, sorted by date:

mysql> SELECT name, birth FROM pet ORDER BY birth;
+----------+------------+
| name     | birth      |
+----------+------------+
| Buffy    | 1989-05-13 |
| Bowser   | 1989-08-31 |
| Fang     | 1990-08-27 |
| Fluffy   | 1993-02-04 |
| Claws    | 1994-03-17 |
| Slim     | 1996-04-29 |
| Whistler | 1997-12-09 |
| Chirpy   | 1998-09-11 |
| Puffball | 1999-03-30 |
+----------+------------+

To sort in reverse order, add the DESC (descending) keyword to the name of the column you are sorting by:

mysql> SELECT name, birth FROM pet ORDER BY birth DESC;
+----------+------------+
| name     | birth      |
+----------+------------+
| Puffball | 1999-03-30 |
| Chirpy   | 1998-09-11 |
| Whistler | 1997-12-09 |
| Slim     | 1996-04-29 |
| Claws    | 1994-03-17 |
| Fluffy   | 1993-02-04 |
| Fang     | 1990-08-27 |
| Bowser   | 1989-08-31 |
| Buffy    | 1989-05-13 |
+----------+------------+

You can sort on multiple columns. For example, to sort by type of animal, then by birth date within animal type with youngest animals first, use the following query:

mysql> SELECT name, species, birth FROM pet ORDER BY species, birth DESC;
+----------+---------+------------+
| name     | species | birth      |
+----------+---------+------------+
| Chirpy   | bird    | 1998-09-11 |
| Whistler | bird    | 1997-12-09 |
| Claws    | cat     | 1994-03-17 |
| Fluffy   | cat     | 1993-02-04 |
| Fang     | dog     | 1990-08-27 |
| Bowser   | dog     | 1989-08-31 |
| Buffy    | dog     | 1989-05-13 |
| Puffball | hamster | 1999-03-30 |
| Slim     | snake   | 1996-04-29 |
+----------+---------+------------+

Note that the DESC keyword applies only to the column name immediately preceding it (birth); species values are still sorted in ascending order.

3.3.4.5 Date Calculations

MySQL provides several functions that you can use to perform calculations on dates, for example, to calculate ages or extract parts of dates.

To determine how many years old each of your pets is, compute the difference in the year part of the current date and the birth date, then subtract one if the current date occurs earlier in the calendar year than the birth date. The following query shows, for each pet, the birth date, the current date, and the age in years.

mysql> SELECT name, birth, CURRENT_DATE,
    -> (YEAR(CURRENT_DATE)-YEAR(birth))
    -> - (RIGHT(CURRENT_DATE,5)<RIGHT(birth,5))
    -> AS age
    -> FROM pet;
+----------+------------+--------------+------+
| name     | birth      | CURRENT_DATE | age  |
+----------+------------+--------------+------+
| Fluffy   | 1993-02-04 | 2001-08-29   |    8 |
| Claws    | 1994-03-17 | 2001-08-29   |    7 |
| Buffy    | 1989-05-13 | 2001-08-29   |   12 |
| Fang     | 1990-08-27 | 2001-08-29   |   11 |
| Bowser   | 1989-08-31 | 2001-08-29   |   11 |
| Chirpy   | 1998-09-11 | 2001-08-29   |    2 |
| Whistler | 1997-12-09 | 2001-08-29   |    3 |
| Slim     | 1996-04-29 | 2001-08-29   |    5 |
| Puffball | 1999-03-30 | 2001-08-29   |    2 |
+----------+------------+--------------+------+

Here, YEAR() pulls out the year part of a date and RIGHT() pulls off the rightmost five characters that represent the MM-DD (calendar year) part of the date. The part of the expression that compares the MM-DD values evaluates to 1 or 0, which adjusts the year difference down a year if CURRENT_DATE occurs earlier in the year than birth. The full expression is somewhat ungainly, so an alias (age) is used to make the output column label more meaningful.

The query works, but the result could be scanned more easily if the rows were presented in some order. This can be done by adding an ORDER BY name clause to sort the output by name:

mysql> SELECT name, birth, CURRENT_DATE,
    -> (YEAR(CURRENT_DATE)-YEAR(birth))
    -> - (RIGHT(CURRENT_DATE,5)<RIGHT(birth,5))
    -> AS age
    -> FROM pet ORDER BY name;
+----------+------------+--------------+------+
| name     | birth      | CURRENT_DATE | age  |
+----------+------------+--------------+------+
| Bowser   | 1989-08-31 | 2001-08-29   |   11 |
| Buffy    | 1989-05-13 | 2001-08-29   |   12 |
| Chirpy   | 1998-09-11 | 2001-08-29   |    2 |
| Claws    | 1994-03-17 | 2001-08-29   |    7 |
| Fang     | 1990-08-27 | 2001-08-29   |   11 |
| Fluffy   | 1993-02-04 | 2001-08-29   |    8 |
| Puffball | 1999-03-30 | 2001-08-29   |    2 |
| Slim     | 1996-04-29 | 2001-08-29   |    5 |
| Whistler | 1997-12-09 | 2001-08-29   |    3 |
+----------+------------+--------------+------+

To sort the output by age rather than name, just use a different ORDER BY clause:

mysql> SELECT name, birth, CURRENT_DATE,
    -> (YEAR(CURRENT_DATE)-YEAR(birth))
    -> - (RIGHT(CURRENT_DATE,5)<RIGHT(birth,5))
    -> AS age
    -> FROM pet ORDER BY age;
+----------+------------+--------------+------+
| name     | birth      | CURRENT_DATE | age  |
+----------+------------+--------------+------+
| Chirpy   | 1998-09-11 | 2001-08-29   |    2 |
| Puffball | 1999-03-30 | 2001-08-29   |    2 |
| Whistler | 1997-12-09 | 2001-08-29   |    3 |
| Slim     | 1996-04-29 | 2001-08-29   |    5 |
| Claws    | 1994-03-17 | 2001-08-29   |    7 |
| Fluffy   | 1993-02-04 | 2001-08-29   |    8 |
| Fang     | 1990-08-27 | 2001-08-29   |   11 |
| Bowser   | 1989-08-31 | 2001-08-29   |   11 |
| Buffy    | 1989-05-13 | 2001-08-29   |   12 |
+----------+------------+--------------+------+

A similar query can be used to determine age at death for animals that have died. You determine which animals these are by checking whether or not the death value is NULL. Then, for those with non-NULL values, compute the difference between the death and birth values:

mysql> SELECT name, birth, death,
    -> (YEAR(death)-YEAR(birth)) - (RIGHT(death,5)<RIGHT(birth,5))
    -> AS age
    -> FROM pet WHERE death IS NOT NULL ORDER BY age;
+--------+------------+------------+------+
| name   | birth      | death      | age  |
+--------+------------+------------+------+
| Bowser | 1989-08-31 | 1995-07-29 |    5 |
+--------+------------+------------+------+

The query uses death IS NOT NULL rather than death != NULL because NULL is a special value. This is explained later. See section 3.3.4.6 Working with NULL Values.

What if you want to know which animals have birthdays next month? For this type of calculation, year and day are irrelevant; you simply want to extract the month part of the birth column. MySQL provides several date-part extraction functions, such as YEAR(), MONTH(), and DAYOFMONTH(). MONTH() is the appropriate function here. To see how it works, run a simple query that displays the value of both birth and MONTH(birth):

mysql> SELECT name, birth, MONTH(birth) FROM pet;
+----------+------------+--------------+
| name     | birth      | MONTH(birth) |
+----------+------------+--------------+
| Fluffy   | 1993-02-04 |            2 |
| Claws    | 1994-03-17 |            3 |
| Buffy    | 1989-05-13 |            5 |
| Fang     | 1990-08-27 |            8 |
| Bowser   | 1989-08-31 |            8 |
| Chirpy   | 1998-09-11 |            9 |
| Whistler | 1997-12-09 |           12 |
| Slim     | 1996-04-29 |            4 |
| Puffball | 1999-03-30 |            3 |
+----------+------------+--------------+

Finding animals with birthdays in the upcoming month is easy, too. Suppose the current month is April. Then the month value is 4 and you look for animals born in May (month 5) like this:

mysql> SELECT name, birth FROM pet WHERE MONTH(birth) = 5;
+-------+------------+
| name  | birth      |
+-------+------------+
| Buffy | 1989-05-13 |
+-------+------------+

There is a small complication if the current month is December, of course. You don't just add one to the month number (12) and look for animals born in month 13, because there is no such month. Instead, you look for animals born in January (month 1).

You can even write the query so that it works no matter what the current month is. That way you don't have to use a particular month number in the query. DATE_ADD() allows you to add a time interval to a given date. If you add a month to the value of NOW(), then extract the month part with MONTH(), the result produces the month in which to look for birthdays:

mysql> SELECT name, birth FROM pet
    -> WHERE MONTH(birth) = MONTH(DATE_ADD(NOW(), INTERVAL 1 MONTH));

A different way to accomplish the same task is to add 1 to get the next month after the current one (after using the modulo function (MOD) to wrap around the month value to 0 if it is currently 12):

mysql> SELECT name, birth FROM pet
    -> WHERE MONTH(birth) = MOD(MONTH(NOW()), 12) + 1;

Note that MONTH returns a number between 1 and 12. And MOD(something,12) returns a number between 0 and 11. So the addition has to be after the MOD(), otherwise we would go from November (11) to January (1).

3.3.4.6 Working with NULL Values

The NULL value can be surprising until you get used to it. Conceptually, NULL means missing value or unknown value and it is treated somewhat differently than other values. To test for NULL, you cannot use the arithmetic comparison operators such as =, <, or !=. To demonstrate this for yourself, try the following query:

mysql> SELECT 1 = NULL, 1 != NULL, 1 < NULL, 1 > NULL;
+----------+-----------+----------+----------+
| 1 = NULL | 1 != NULL | 1 < NULL | 1 > NULL |
+----------+-----------+----------+----------+
|     NULL |      NULL |     NULL |     NULL |
+----------+-----------+----------+----------+

Clearly you get no meaningful results from these comparisons. Use the IS NULL and IS NOT NULL operators instead:

mysql> SELECT 1 IS NULL, 1 IS NOT NULL;
+-----------+---------------+
| 1 IS NULL | 1 IS NOT NULL |
+-----------+---------------+
|         0 |             1 |
+-----------+---------------+

In MySQL, 0 or NULL means false and anything else means true. The default truth value from a boolean operation is 1.

This special treatment of NULL is why, in the previous section, it was necessary to determine which animals are no longer alive using death IS NOT NULL instead of death != NULL.

3.3.4.7 Pattern Matching

MySQL provides standard SQL pattern matching as well as a form of pattern matching based on extended regular expressions similar to those used by Unix utilities such as vi, grep, and sed.

SQL pattern matching allows you to use `_' to match any single character and `%' to match an arbitrary number of characters (including zero characters). In MySQL, SQL patterns are case insensitive by default. Some examples are shown below. Note that you do not use = or != when you use SQL patterns; use the LIKE or NOT LIKE comparison operators instead.

To find names beginning with `b':

mysql> SELECT * FROM pet WHERE name LIKE "b%";
+--------+--------+---------+------+------------+------------+
| name   | owner  | species | sex  | birth      | death      |
+--------+--------+---------+------+------------+------------+
| Buffy  | Harold | dog     | f    | 1989-05-13 | NULL       |
| Bowser | Diane  | dog     | m    | 1989-08-31 | 1995-07-29 |
+--------+--------+---------+------+------------+------------+

To find names ending with `fy':

mysql> SELECT * FROM pet WHERE name LIKE "%fy";
+--------+--------+---------+------+------------+-------+
| name   | owner  | species | sex  | birth      | death |
+--------+--------+---------+------+------------+-------+
| Fluffy | Harold | cat     | f    | 1993-02-04 | NULL  |
| Buffy  | Harold | dog     | f    | 1989-05-13 | NULL  |
+--------+--------+---------+------+------------+-------+

To find names containing a `w':

mysql> SELECT * FROM pet WHERE name LIKE "%w%";
+----------+-------+---------+------+------------+------------+
| name     | owner | species | sex  | birth      | death      |
+----------+-------+---------+------+------------+------------+
| Claws    | Gwen  | cat     | m    | 1994-03-17 | NULL       |
| Bowser   | Diane | dog     | m    | 1989-08-31 | 1995-07-29 |
| Whistler | Gwen  | bird    | NULL | 1997-12-09 | NULL       |
+----------+-------+---------+------+------------+------------+

To find names containing exactly five characters, use the `_' pattern character:

mysql> SELECT * FROM pet WHERE name LIKE "_____";
+-------+--------+---------+------+------------+-------+
| name  | owner  | species | sex  | birth      | death |
+-------+--------+---------+------+------------+-------+
| Claws | Gwen   | cat     | m    | 1994-03-17 | NULL  |
| Buffy | Harold | dog     | f    | 1989-05-13 | NULL  |
+-------+--------+---------+------+------------+-------+

The other type of pattern matching provided by MySQL uses extended regular expressions. When you test for a match for this type of pattern, use the REGEXP and NOT REGEXP operators (or RLIKE and NOT RLIKE, which are synonyms).

Some characteristics of extended regular expressions are:

To demonstrate how extended regular expressions work, the LIKE queries shown above are rewritten below to use REGEXP.

To find names beginning with `b', use `^' to match the beginning of the name:

mysql> SELECT * FROM pet WHERE name REGEXP "^b";
+--------+--------+---------+------+------------+------------+
| name   | owner  | species | sex  | birth      | death      |
+--------+--------+---------+------+------------+------------+
| Buffy  | Harold | dog     | f    | 1989-05-13 | NULL       |
| Bowser | Diane  | dog     | m    | 1989-08-31 | 1995-07-29 |
+--------+--------+---------+------+------------+------------+

Prior to MySQL Version 3.23.4, REGEXP is case sensitive, and the previous query will return no rows. To match either lowercase or uppercase `b', use this query instead:

mysql> SELECT * FROM pet WHERE name REGEXP "^[bB]";

From MySQL 3.23.4 on, to force a REGEXP comparison to be case sensitive, use the BINARY keyword to make one of the strings a binary string. This query will match only lowercase `b' at the beginning of a name:

mysql> SELECT * FROM pet WHERE name REGEXP BINARY "^b";

To find names ending with `fy', use `$' to match the end of the name:

mysql> SELECT * FROM pet WHERE name REGEXP "fy$";
+--------+--------+---------+------+------------+-------+
| name   | owner  | species | sex  | birth      | death |
+--------+--------+---------+------+------------+-------+
| Fluffy | Harold | cat     | f    | 1993-02-04 | NULL  |
| Buffy  | Harold | dog     | f    | 1989-05-13 | NULL  |
+--------+--------+---------+------+------------+-------+

To find names containing a lowercase or uppercase `w', use this query:

mysql> SELECT * FROM pet WHERE name REGEXP "w";
+----------+-------+---------+------+------------+------------+
| name     | owner | species | sex  | birth      | death      |
+----------+-------+---------+------+------------+------------+
| Claws    | Gwen  | cat     | m    | 1994-03-17 | NULL       |
| Bowser   | Diane | dog     | m    | 1989-08-31 | 1995-07-29 |
| Whistler | Gwen  | bird    | NULL | 1997-12-09 | NULL       |
+----------+-------+---------+------+------------+------------+

Because a regular expression pattern matches if it occurs anywhere in the value, it is not necessary in the previous query to put a wild card on either side of the pattern to get it to match the entire value like it would be if you used a SQL pattern.

To find names containing exactly five characters, use `^' and `$' to match the beginning and end of the name, and five instances of `.' in between:

mysql> SELECT * FROM pet WHERE name REGEXP "^.....$";
+-------+--------+---------+------+------------+-------+
| name  | owner  | species | sex  | birth      | death |
+-------+--------+---------+------+------------+-------+
| Claws | Gwen   | cat     | m    | 1994-03-17 | NULL  |
| Buffy | Harold | dog     | f    | 1989-05-13 | NULL  |
+-------+--------+---------+------+------------+-------+

You could also write the previous query using the `{n}' ``repeat-n-times'' operator:

mysql> SELECT * FROM pet WHERE name REGEXP "^.{5}$";
+-------+--------+---------+------+------------+-------+
| name  | owner  | species | sex  | birth      | death |
+-------+--------+---------+------+------------+-------+
| Claws | Gwen   | cat     | m    | 1994-03-17 | NULL  |
| Buffy | Harold | dog     | f    | 1989-05-13 | NULL  |
+-------+--------+---------+------+------------+-------+

3.3.4.8 Counting Rows

Databases are often used to answer the question, ``How often does a certain type of data occur in a table?'' For example, you might want to know how many pets you have, or how many pets each owner has, or you might want to perform various kinds of censuses on your animals.

Counting the total number of animals you have is the same question as ``How many rows are in the pet table?'' because there is one record per pet. The COUNT() function counts the number of non-NULL results, so the query to count your animals looks like this:

mysql> SELECT COUNT(*) FROM pet;
+----------+
| COUNT(*) |
+----------+
|        9 |
+----------+

Earlier, you retrieved the names of the people who owned pets. You can use COUNT() if you want to find out how many pets each owner has:

mysql> SELECT owner, COUNT(*) FROM pet GROUP BY owner;
+--------+----------+
| owner  | COUNT(*) |
+--------+----------+
| Benny  |        2 |
| Diane  |        2 |
| Gwen   |        3 |
| Harold |        2 |
+--------+----------+

Note the use of GROUP BY to group together all records for each owner. Without it, all you get is an error message:

mysql> SELECT owner, COUNT(owner) FROM pet;
ERROR 1140 at line 1: Mixing of GROUP columns (MIN(),MAX(),COUNT()...)
with no GROUP columns is illegal if there is no GROUP BY clause

COUNT() and GROUP BY are useful for characterizing your data in various ways. The following examples show different ways to perform animal census operations.

Number of animals per species:

mysql> SELECT species, COUNT(*) FROM pet GROUP BY species;
+---------+----------+
| species | COUNT(*) |
+---------+----------+
| bird    |        2 |
| cat     |        2 |
| dog     |        3 |
| hamster |        1 |
| snake   |        1 |
+---------+----------+

Number of animals per sex:

mysql> SELECT sex, COUNT(*) FROM pet GROUP BY sex;
+------+----------+
| sex  | COUNT(*) |
+------+----------+
| NULL |        1 |
| f    |        4 |
| m    |        4 |
+------+----------+

(In this output, NULL indicates sex unknown.)

Number of animals per combination of species and sex:

mysql> SELECT species, sex, COUNT(*) FROM pet GROUP BY species, sex;
+---------+------+----------+
| species | sex  | COUNT(*) |
+---------+------+----------+
| bird    | NULL |        1 |
| bird    | f    |        1 |
| cat     | f    |        1 |
| cat     | m    |        1 |
| dog     | f    |        1 |
| dog     | m    |        2 |
| hamster | f    |        1 |
| snake   | m    |        1 |
+---------+------+----------+

You need not retrieve an entire table when you use COUNT(). For example, the previous query, when performed just on dogs and cats, looks like this:

mysql> SELECT species, sex, COUNT(*) FROM pet
    -> WHERE species = "dog" OR species = "cat"
    -> GROUP BY species, sex;
+---------+------+----------+
| species | sex  | COUNT(*) |
+---------+------+----------+
| cat     | f    |        1 |
| cat     | m    |        1 |
| dog     | f    |        1 |
| dog     | m    |        2 |
+---------+------+----------+

Or, if you wanted the number of animals per sex only for known-sex animals:

mysql> SELECT species, sex, COUNT(*) FROM pet
    -> WHERE sex IS NOT NULL
    -> GROUP BY species, sex;
+---------+------+----------+
| species | sex  | COUNT(*) |
+---------+------+----------+
| bird    | f    |        1 |
| cat     | f    |        1 |
| cat     | m    |        1 |
| dog     | f    |        1 |
| dog     | m    |        2 |
| hamster | f    |        1 |
| snake   | m    |        1 |
+---------+------+----------+

3.3.4.9 Using More Than one Table

The pet table keeps track of which pets you have. If you want to record other information about them, such as events in their lives like visits to the vet or when litters are born, you need another table. What should this table look like? It needs:

Given these considerations, the CREATE TABLE statement for the event table might look like this:

mysql> CREATE TABLE event (name VARCHAR(20), date DATE,
    -> type VARCHAR(15), remark VARCHAR(255));

As with the pet table, it's easiest to load the initial records by creating a tab-delimited text file containing the information:

Fluffy 1995-05-15 litter 4 kittens, 3 female, 1 male
Buffy 1993-06-23 litter 5 puppies, 2 female, 3 male
Buffy 1994-06-19 litter 3 puppies, 3 female
Chirpy 1999-03-21 vet needed beak straightened
Slim 1997-08-03 vet broken rib
Bowser 1991-10-12 kennel
Fang 1991-10-12 kennel
Fang 1998-08-28 birthday Gave him a new chew toy
Claws 1998-03-17 birthday Gave him a new flea collar
Whistler 1998-12-09 birthday First birthday

Load the records like this:

mysql> LOAD DATA LOCAL INFILE "event.txt" INTO TABLE event;

Based on what you've learned from the queries you've run on the pet table, you should be able to perform retrievals on the records in the event table; the principles are the same. But when is the event table by itself insufficient to answer questions you might ask?

Suppose you want to find out the ages of each pet when they had their litters. The event table indicates when this occurred, but to calculate the age of the mother, you need her birth date. Because that is stored in the pet table, you need both tables for the query:

mysql> SELECT pet.name, (TO_DAYS(date) - TO_DAYS(birth))/365 AS age, remark
    -> FROM pet, event
    -> WHERE pet.name = event.name AND type = "litter";
+--------+------+-----------------------------+
| name   | age  | remark                      |
+--------+------+-----------------------------+
| Fluffy | 2.27 | 4 kittens, 3 female, 1 male |
| Buffy  | 4.12 | 5 puppies, 2 female, 3 male |
| Buffy  | 5.10 | 3 puppies, 3 female         |
+--------+------+-----------------------------+

There are several things to note about this query:

You need not have two different tables to perform a join. Sometimes it is useful to join a table to itself, if you want to compare records in a table to other records in that same table. For example, to find breeding pairs among your pets, you can join the pet table with itself to pair up males and females of like species:

mysql> SELECT p1.name, p1.sex, p2.name, p2.sex, p1.species
    -> FROM pet AS p1, pet AS p2
    -> WHERE p1.species = p2.species AND p1.sex = "f" AND p2.sex = "m";
+--------+------+--------+------+---------+
| name   | sex  | name   | sex  | species |
+--------+------+--------+------+---------+
| Fluffy | f    | Claws  | m    | cat     |
| Buffy  | f    | Fang   | m    | dog     |
| Buffy  | f    | Bowser | m    | dog     |
+--------+------+--------+------+---------+

In this query, we specify aliases for the table name in order to refer to the columns and keep straight which instance of the table each column reference is associated with.

3.4 Getting Information About Databases and Tables

What if you forget the name of a database or table, or what the structure of a given table is (for example, what its columns are called)? MySQL addresses this problem through several statements that provide information about the databases and tables it supports.

You have already seen SHOW DATABASES, which lists the databases managed by the server. To find out which database is currently selected, use the DATABASE() function:

mysql> SELECT DATABASE();
+------------+
| DATABASE() |
+------------+
| menagerie  |
+------------+

If you haven't selected any database yet, the result is blank.

To find out what tables the current database contains (for example, when you're not sure about the name of a table), use this command:

mysql> SHOW TABLES;
+---------------------+
| Tables in menagerie |
+---------------------+
| event               |
| pet                 |
+---------------------+

If you want to find out about the structure of a table, the DESCRIBE command is useful; it displays information about each of a table's columns:

mysql> DESCRIBE pet;
+---------+-------------+------+-----+---------+-------+
| Field   | Type        | Null | Key | Default | Extra |
+---------+-------------+------+-----+---------+-------+
| name    | varchar(20) | YES  |     | NULL    |       |
| owner   | varchar(20) | YES  |     | NULL    |       |
| species | varchar(20) | YES  |     | NULL    |       |
| sex     | char(1)     | YES  |     | NULL    |       |
| birth   | date        | YES  |     | NULL    |       |
| death   | date        | YES  |     | NULL    |       |
+---------+-------------+------+-----+---------+-------+

Field indicates the column name, Type is the data type for the column, Null indicates whether or not the column can contain NULL values, Key indicates whether or not the column is indexed, and Default specifies the column's default value.

If you have indexes on a table, SHOW INDEX FROM tbl_name produces information about them.

3.5 Examples of Common Queries

Here are examples of how to solve some common problems with MySQL.

Some of the examples use the table shop to hold the price of each article (item number) for certain traders (dealers). Supposing that each trader has a single fixed price per article, then (item, trader) is a primary key for the records.

Start the command line tool mysql and select a database:

mysql your-database-name

(In most MySQL installations, you can use the database-name 'test').

You can create the example table as:

CREATE TABLE shop (
 article INT(4) UNSIGNED ZEROFILL DEFAULT '0000' NOT NULL,
 dealer  CHAR(20)                 DEFAULT ''     NOT NULL,
 price   DOUBLE(16,2)             DEFAULT '0.00' NOT NULL,
 PRIMARY KEY(article, dealer));

INSERT INTO shop VALUES
(1,'A',3.45),(1,'B',3.99),(2,'A',10.99),(3,'B',1.45),(3,'C',1.69),
(3,'D',1.25),(4,'D',19.95);

Okay, so the example data is:

mysql> SELECT * FROM shop;

+---------+--------+-------+
| article | dealer | price |
+---------+--------+-------+
|    0001 | A      |  3.45 |
|    0001 | B      |  3.99 |
|    0002 | A      | 10.99 |
|    0003 | B      |  1.45 |
|    0003 | C      |  1.69 |
|    0003 | D      |  1.25 |
|    0004 | D      | 19.95 |
+---------+--------+-------+

3.5.1 The Maximum Value for a Column

``What's the highest item number?''

SELECT MAX(article) AS article FROM shop

+---------+
| article |
+---------+
|       4 |
+---------+

3.5.2 The Row Holding the Maximum of a Certain Column

``Find number, dealer, and price of the most expensive article.''

In ANSI SQL this is easily done with a sub-query:

SELECT article, dealer, price
FROM   shop
WHERE  price=(SELECT MAX(price) FROM shop)

In MySQL (which does not yet have sub-selects), just do it in two steps:

  1. Get the maximum price value from the table with a SELECT statement.
  2. Using this value compile the actual query:
    SELECT article, dealer, price
    FROM   shop
    WHERE  price=19.95
    

Another solution is to sort all rows descending by price and only get the first row using the MySQL specific LIMIT clause:

SELECT article, dealer, price
FROM   shop
ORDER BY price DESC
LIMIT 1

NOTE: If there are several most expensive articles (for example, each 19.95) the LIMIT solution shows only one of them!

3.5.3 Maximum of Column per Group

``What's the highest price per article?''

SELECT article, MAX(price) AS price
FROM   shop
GROUP BY article

+---------+-------+
| article | price |
+---------+-------+
|    0001 |  3.99 |
|    0002 | 10.99 |
|    0003 |  1.69 |
|    0004 | 19.95 |
+---------+-------+

3.5.4 The Rows Holding the Group-wise Maximum of a Certain Field

``For each article, find the dealer(s) with the most expensive price.''

In ANSI SQL, I'd do it with a sub-query like this:

SELECT article, dealer, price
FROM   shop s1
WHERE  price=(SELECT MAX(s2.price)
              FROM shop s2
              WHERE s1.article = s2.article);

In MySQL it's best do it in several steps:

  1. Get the list of (article,maxprice).
  2. For each article get the corresponding rows that have the stored maximum price.

This can easily be done with a temporary table:

CREATE TEMPORARY TABLE tmp (
        article INT(4) UNSIGNED ZEROFILL DEFAULT '0000' NOT NULL,
        price   DOUBLE(16,2)             DEFAULT '0.00' NOT NULL);

LOCK TABLES shop read;

INSERT INTO tmp SELECT article, MAX(price) FROM shop GROUP BY article;

SELECT shop.article, dealer, shop.price FROM shop, tmp
WHERE shop.article=tmp.article AND shop.price=tmp.price;

UNLOCK TABLES;

DROP TABLE tmp;

If you don't use a TEMPORARY table, you must also lock the 'tmp' table.

``Can it be done with a single query?''

Yes, but only by using a quite inefficient trick that I call the ``MAX-CONCAT trick'':

SELECT article,
       SUBSTRING( MAX( CONCAT(LPAD(price,6,'0'),dealer) ), 7) AS dealer,
  0.00+LEFT(      MAX( CONCAT(LPAD(price,6,'0'),dealer) ), 6) AS price
FROM   shop
GROUP BY article;

+---------+--------+-------+
| article | dealer | price |
+---------+--------+-------+
|    0001 | B      |  3.99 |
|    0002 | A      | 10.99 |
|    0003 | C      |  1.69 |
|    0004 | D      | 19.95 |
+---------+--------+-------+

The last example can, of course, be made a bit more efficient by doing the splitting of the concatenated column in the client.

3.5.5 Using user variables

You can use MySQL user variables to remember results without having to store them in a temporary variables in the client. See section 6.1.4 User Variables.

For example, to find the articles with the highest and lowest price you can do:

select @min_price:=min(price),@max_price:=max(price) from shop;
select * from shop where price=@min_price or price=@max_price;

+---------+--------+-------+
| article | dealer | price |
+---------+--------+-------+
|    0003 | D      |  1.25 |
|    0004 | D      | 19.95 |
+---------+--------+-------+

3.5.6 Using Foreign Keys

You don't need foreign keys to join 2 tables.

The only thing MySQL doesn't do is CHECK to make sure that the keys you use really exist in the table(s) you're referencing and it doesn't automatically delete rows from table with a foreign key definition. If you use your keys like normal, it'll work just fine:

CREATE TABLE persons (
    id SMALLINT UNSIGNED NOT NULL AUTO_INCREMENT,
    name CHAR(60) NOT NULL,
    PRIMARY KEY (id)
);

CREATE TABLE shirts (
    id SMALLINT UNSIGNED NOT NULL AUTO_INCREMENT,
    style ENUM('t-shirt', 'polo', 'dress') NOT NULL,
    color ENUM('red', 'blue', 'orange', 'white', 'black') NOT NULL,
    owner SMALLINT UNSIGNED NOT NULL REFERENCES persons,
    PRIMARY KEY (id)
);

INSERT INTO persons VALUES (NULL, 'Antonio Paz');

INSERT INTO shirts VALUES
(NULL, 'polo', 'blue', LAST_INSERT_ID()),
(NULL, 'dress', 'white', LAST_INSERT_ID()),
(NULL, 't-shirt', 'blue', LAST_INSERT_ID());

INSERT INTO persons VALUES (NULL, 'Lilliana Angelovska');

INSERT INTO shirts VALUES
(NULL, 'dress', 'orange', LAST_INSERT_ID()),
(NULL, 'polo', 'red', LAST_INSERT_ID()),
(NULL, 'dress', 'blue', LAST_INSERT_ID()),
(NULL, 't-shirt', 'white', LAST_INSERT_ID());

SELECT * FROM persons;
+----+---------------------+
| id | name                |
+----+---------------------+
|  1 | Antonio Paz         |
|  2 | Lilliana Angelovska |
+----+---------------------+

SELECT * FROM shirts;
+----+---------+--------+-------+
| id | style   | color  | owner |
+----+---------+--------+-------+
|  1 | polo    | blue   |     1 |
|  2 | dress   | white  |     1 |
|  3 | t-shirt | blue   |     1 |
|  4 | dress   | orange |     2 |
|  5 | polo    | red    |     2 |
|  6 | dress   | blue   |     2 |
|  7 | t-shirt | white  |     2 |
+----+---------+--------+-------+

SELECT s.* FROM persons p, shirts s
 WHERE p.name LIKE 'Lilliana%'
   AND s.owner = p.id
   AND s.color <> 'white';

+----+-------+--------+-------+
| id | style | color  | owner |
+----+-------+--------+-------+
|  4 | dress | orange |     2 |
|  5 | polo  | red    |     2 |
|  6 | dress | blue   |     2 |
+----+-------+--------+-------+

3.5.7 Searching on Two Keys

MySQL doesn't yet optimize when you search on two different keys combined with OR (Searching on one key with different OR parts is optimized quite good):

SELECT field1_index, field2_index FROM test_table WHERE field1_index = '1'
OR  field2_index = '1'

The reason is that we haven't yet had time to come up with an efficient way to handle this in the general case. (The AND handling is, in comparison, now completely general and works very well).

For the moment you can solve this very efficiently by using a TEMPORARY table. This type of optimization is also very good if you are using very complicated queries where the SQL server does the optimizations in the wrong order.

CREATE TEMPORARY TABLE tmp
SELECT field1_index, field2_index FROM test_table WHERE field1_index = '1';
INSERT INTO tmp
SELECT field1_index, field2_index FROM test_table WHERE field2_index = '1';
SELECT * from tmp;
DROP TABLE tmp;

The above way to solve this query is in effect an UNION of two queries.

3.5.8 Calculating visits per day

The following shows an idea of how you can use the bit group functions to calculate the number of days per month a user has visited a web page.

CREATE TABLE t1 (year YEAR(4), month INT(2) UNSIGNED ZEROFILL, day INT(2) UNSIGNED ZEROFILL);                                                            
INSERT INTO t1 VALUES(2000,1,1),(2000,1,20),(2000,1,30),(2000,2,2),(2000,2,23),(2000,2,23);

SELECT year,month,BIT_COUNT(BIT_OR(1<<day)) AS days FROM t1 GROUP BY year,month;

Which returns:

+------+-------+------+
| year | month | days |
+------+-------+------+
| 2000 |    01 |    3 |
| 2000 |    02 |    2 |
+------+-------+------+

The above calculates how many different days was used for a given year/month combination, with automatic removal of duplicate entries.

3.6 Using mysql in Batch Mode

In the previous sections, you used mysql interactively to enter queries and view the results. You can also run mysql in batch mode. To do this, put the commands you want to run in a file, then tell mysql to read its input from the file:

shell> mysql < batch-file

If you need to specify connection parameters on the command line, the command might look like this:

shell> mysql -h host -u user -p < batch-file
Enter password: ********

When you use mysql this way, you are creating a script file, then executing the script.

Why use a script? Here are a few reasons:

The default output format is different (more concise) when you run mysql in batch mode than when you use it interactively. For example, the output of SELECT DISTINCT species FROM pet looks like this when run interactively:

+---------+
| species |
+---------+
| bird    |
| cat     |
| dog     |
| hamster |
| snake   |
+---------+

But like this when run in batch mode:

species
bird
cat
dog
hamster
snake

If you want to get the interactive output format in batch mode, use mysql -t. To echo to the output the commands that are executed, use mysql -vvv.

3.7 Queries from Twin Project

At Analytikerna and Lentus, we have been doing the systems and field work for a big research project. This project is a collaboration between the Institute of Environmental Medicine at Karolinska Institutet Stockholm and the Section on Clinical Research in Aging and Psychology at the University of Southern California.

The project involves a screening part where all twins in Sweden older than 65 years are interviewed by telephone. Twins who meet certain criteria are passed on to the next stage. In this latter stage, twins who want to participate are visited by a doctor/nurse team. Some of the examinations include physical and neuropsychological examination, laboratory testing, neuroimaging, psychological status assessment, and family history collection. In addition, data are collected on medical and environmental risk factors.

More information about Twin studies can be found at:

http://www.imm.ki.se/TWIN/TWINUKW.HTM

The latter part of the project is administered with a Web interface written using Perl and MySQL.

Each night all data from the interviews are moved into a MySQL database.

3.7.1 Find all Non-distributed Twins

The following query is used to determine who goes into the second part of the project:

select
        concat(p1.id, p1.tvab) + 0 as tvid,
        concat(p1.christian_name, " ", p1.surname) as Name,
        p1.postal_code as Code,
        p1.city as City,
        pg.abrev as Area,
        if(td.participation = "Aborted", "A", " ") as A,
        p1.dead as dead1,
        l.event as event1,
        td.suspect as tsuspect1,
        id.suspect as isuspect1,
        td.severe as tsevere1,
        id.severe as isevere1,
        p2.dead as dead2,
        l2.event as event2,
        h2.nurse as nurse2,
        h2.doctor as doctor2,
        td2.suspect as tsuspect2,
        id2.suspect as isuspect2,
        td2.severe as tsevere2,
        id2.severe as isevere2,
        l.finish_date
from
        twin_project as tp
        /* For Twin 1 */
        left join twin_data as td on tp.id = td.id and tp.tvab = td.tvab
        left join informant_data as id on tp.id = id.id and tp.tvab = id.tvab
        left join harmony as h on tp.id = h.id and tp.tvab = h.tvab
        left join lentus as l on tp.id = l.id and tp.tvab = l.tvab
        /* For Twin 2 */
        left join twin_data as td2 on p2.id = td2.id and p2.tvab = td2.tvab
        left join informant_data as id2 on p2.id = id2.id and p2.tvab = id2.tvab
        left join harmony as h2 on p2.id = h2.id and p2.tvab = h2.tvab
        left join lentus as l2 on p2.id = l2.id and p2.tvab = l2.tvab,
        person_data as p1,
        person_data as p2,
        postal_groups as pg
where
        /* p1 gets main twin and p2 gets his/her twin. */
        /* ptvab is a field inverted from tvab */
        p1.id = tp.id and p1.tvab = tp.tvab and
        p2.id = p1.id and p2.ptvab = p1.tvab and
        /* Just the sceening survey */
        tp.survey_no = 5 and
        /* Skip if partner died before 65 but allow emigration (dead=9) */
        (p2.dead = 0 or p2.dead = 9 or
         (p2.dead = 1 and
          (p2.death_date = 0 or
           (((to_days(p2.death_date) - to_days(p2.birthday)) / 365)
            >= 65))))
        and
        (
        /* Twin is suspect */
        (td.future_contact = 'Yes' and td.suspect = 2) or
        /* Twin is suspect - Informant is Blessed */
        (td.future_contact = 'Yes' and td.suspect = 1 and id.suspect = 1) or
        /* No twin - Informant is Blessed */
        (ISNULL(td.suspect) and id.suspect = 1 and id.future_contact = 'Yes') or
        /* Twin broken off - Informant is Blessed */
        (td.participation = 'Aborted'
         and id.suspect = 1 and id.future_contact = 'Yes') or
        /* Twin broken off - No inform - Have partner */
        (td.participation = 'Aborted' and ISNULL(id.suspect) and p2.dead = 0))
        and
        l.event = 'Finished'
        /* Get at area code */
        and substring(p1.postal_code, 1, 2) = pg.code
        /* Not already distributed */
        and (h.nurse is NULL or h.nurse=00 or h.doctor=00)
        /* Has not refused or been aborted */
        and not (h.status = 'Refused' or h.status = 'Aborted'
        or h.status = 'Died' or h.status = 'Other')
order by
        tvid;

Some explanations:

concat(p1.id, p1.tvab) + 0 as tvid
We want to sort on the concatenated id and tvab in numerical order. Adding 0 to the result causes MySQL to treat the result as a number.
column id
This identifies a pair of twins. It is a key in all tables.
column tvab
This identifies a twin in a pair. It has a value of 1 or 2.
column ptvab
This is an inverse of tvab. When tvab is 1 this is 2, and vice versa. It exists to save typing and to make it easier for MySQL to optimize the query.

This query demonstrates, among other things, how to do lookups on a table from the same table with a join (p1 and p2). In the example, this is used to check whether a twin's partner died before the age of 65. If so, the row is not returned.

All of the above exist in all tables with twin-related information. We have a key on both id,tvab (all tables), and id,ptvab (person_data) to make queries faster.

On our production machine (A 200MHz UltraSPARC), this query returns about 150-200 rows and takes less than one second.

The current number of records in the tables used above:
Table Rows
person_data 71074
lentus 5291
twin_project 5286
twin_data 2012
informant_data 663
harmony 381
postal_groups 100

3.7.2 Show a Table on Twin Pair Status

Each interview ends with a status code called event. The query shown below is used to display a table over all twin pairs combined by event. This indicates in how many pairs both twins are finished, in how many pairs one twin is finished and the other refused, and so on.

select
        t1.event,
        t2.event,
        count(*)
from
        lentus as t1,
        lentus as t2,
        twin_project as tp
where
        /* We are looking at one pair at a time */
        t1.id = tp.id
        and t1.tvab=tp.tvab
        and t1.id = t2.id
        /* Just the sceening survey */
        and tp.survey_no = 5
        /* This makes each pair only appear once */
        and t1.tvab='1' and t2.tvab='2'
group by
        t1.event, t2.event;

3.8 Using MySQL with Apache

The Contrib section includes programs that let you authenticate your users from a MySQL database and also let you log your log files into a MySQL table. See section D Contributed Programs.

You can change the Apache logging format to be easily readable by MySQL by putting the following into the Apache configuration file:

LogFormat \
        "\"%h\",%{%Y%m%d%H%M%S}t,%>s,\"%b\",\"%{Content-Type}o\",  \
        \"%U\",\"%{Referer}i\",\"%{User-Agent}i\""

In MySQL you can do something like this:

LOAD DATA INFILE '/local/access_log' INTO TABLE table_name
FIELDS TERMINATED BY ',' OPTIONALLY ENCLOSED BY '"' ESCAPED BY '\\'

4 MySQL Database Administration

4.1 Configuring MySQL

4.1.1 mysqld Command-line Options

mysqld accepts the following command-line options:

--ansi
Use ANSI SQL syntax instead of MySQL syntax. See section 1.4.3 Running MySQL in ANSI Mode.
-b, --basedir=path
Path to installation directory. All paths are usually resolved relative to this.
--big-tables
Allow big result sets by saving all temporary sets on file. It solves most 'table full' errors, but also slows down the queries where in-memory tables would suffice. Since Version 3.23.2, MySQL is able to solve it automatically by using memory for small temporary tables and switching to disk tables where necessary.
--bind-address=IP
IP address to bind to.
--character-sets-dir=path
Directory where character sets are. See section 4.6.1 The Character Set Used for Data and Sorting.
--chroot=path
Chroot mysqld daemon during startup. Recommended security measure. It will somewhat limit LOAD DATA INFILE and SELECT ... INTO OUTFILE though.
--core-file
Write a core file if mysqld dies. For some systems you must also specify --core-file-size to safe_mysqld. See section 4.7.2 safe_mysqld, the wrapper around mysqld.
-h, --datadir=path
Path to the database root.
--default-character-set=charset
Set the default character set. See section 4.6.1 The Character Set Used for Data and Sorting.
--default-table-type=type
Set the default table type for tables. See section 7 MySQL Table Types.
--debug[...]=
If MySQL is configured with --with-debug, you can use this option to get a trace file of what mysqld is doing. See section G.1.2 Creating trace files.
--delay-key-write-for-all-tables
Don't flush key buffers between writes for any MyISAM table. See section 5.5.2 Tuning Server Parameters.
--enable-locking
Enable system locking. Note that if you use this option on a system which a not fully working lockd() (as on Linux) you will easily get mysqld to deadlock.
-T, --exit-info
This is a bit mask of different flags one can use for debugging the mysqld server; One should not use this option if one doesn't know exactly what it does!
--flush
Flush all changes to disk after each SQL command. Normally MySQL only does a write of all changes to disk after each SQL command and lets the operating system handle the syncing to disk. See section A.4.1 What To Do If MySQL Keeps Crashing.
-?, --help
Display short help and exit.
--init-file=file
Read SQL commands from this file at startup.
-L, --language=...
Client error messages in given language. May be given as a full path. See section 4.6.2 Non-English Error Messages.
-l, --log[=file]
Log connections and queries to file. See section 4.9.2 The General Query Log.
--log-isam[=file]
Log all ISAM/MyISAM changes to file (only used when debugging ISAM/MyISAM).
--log-slow-queries[=file]
Log all queries that have taken more than long_query_time seconds to execute to file. See section 4.9.5 The Slow Query Log.
--log-update[=file]
Log updates to file.# where # is a unique number if not given. See section 4.9.3 The Update Log.
--log-long-format
Log some extra information to update log. If you are using --log-slow-queries then queries that are not using indexes are logged to the slow query log.
--low-priority-updates
Table-modifying operations (INSERT/DELETE/UPDATE) will have lower priority than selects. It can also be done via {INSERT | REPLACE | UPDATE | DELETE} LOW_PRIORITY ... to lower the priority of only one query, or by SET OPTION SQL_LOW_PRIORITY_UPDATES=1 to change the priority in one thread. See section 5.3.2 Table Locking Issues.
--memlock
Lock the mysqld process in memory. This works only if your system supports the mlockall() system call (like Solaris). This may help if you have a problem where the operating system is causing mysqld to swap on disk.
--myisam-recover [=option[,option...]]] where option is any combination
of DEFAULT, BACKUP, FORCE or QUICK. You can also set this explicitely to "" if you want to disable this option. If this option is used, mysqld will on open check if the table is marked as crashed or if if the table wasn't closed properly. (The last option only works if you are running with --skip-locking). If this is the case mysqld will run check on the table. If the table was corrupted, mysqld will attempt to repair it. The following options affects how the repair works.
DEFAULT The same as not giving any option to --myisam-recover.
BACKUP If the data table was changed during recover, save a backup of the `table_name.MYD' data file as `table_name-datetime.BAK'.
FORCE Run recover even if we will loose more than one row from the .MYD file.
QUICK Don't check the rows in the table if there isn't any delete blocks.
Before a table is automatically repaired, MySQL will add a note about this in the error log. If you want to be able to recover from most things without user intervention, you should use the options BACKUP,FORCE. This will force a repair of a table even if some rows would be deleted, but it will keep the old data file as a backup so that you can later examine what happened.
--pid-file=path
Path to pid file used by safe_mysqld.
-P, --port=...
Port number to listen for TCP/IP connections.
-o, --old-protocol
Use the 3.20 protocol for compatibility with some very old clients. See section 2.5.3 Upgrading from Version 3.20 to Version 3.21.
--one-thread
Only use one thread (for debugging under Linux). See section G.1 Debugging a MySQL server.
-O, --set-variable var=option
Give a variable a value. --help lists variables. You can find a full description for all variables in the SHOW VARIABLES section in this manual. See section 4.5.5.4 SHOW VARIABLES. The tuning server parameters section includes information of how to optimize these. See section 5.5.2 Tuning Server Parameters.
--safe-mode
Skip some optimize stages. Implies --skip-delay-key-write.
--safe-show-database
Don't show databases for which the user doesn't have any privileges.
--safe-user-create
If this is enabled, a user can't create new users with the GRANT command, if the user doesn't have INSERT privilege to the mysql.user table or any column in this table.
--skip-concurrent-insert
Turn off the ability to select and insert at the same time on MyISAM tables. (This is only to be used if you think you have found a bug in this feature).
--skip-delay-key-write
Ignore the delay_key_write option for all tables. See section 5.5.2 Tuning Server Parameters.
--skip-grant-tables
This option causes the server not to use the privilege system at all. This gives everyone full access to all databases! (You can tell a running server to start using the grant tables again by executing mysqladmin flush-privileges or mysqladmin reload.)
--skip-host-cache
Never use host name cache for faster name-ip resolution, but query DNS server on every connect instead. See section 5.5.5 How MySQL uses DNS.
--skip-locking
Don't use system locking. To use isamchk or myisamchk you must shut down the server. See section 1.1.7 How Stable Is MySQL?. Note that in MySQL Version 3.23 you can use REPAIR and CHECK to repair/check MyISAM tables.
--skip-name-resolve
Hostnames are not resolved. All Host column values in the grant tables must be IP numbers or localhost. See section 5.5.5 How MySQL uses DNS.
--skip-networking
Don't listen for TCP/IP connections at all. All interaction with mysqld must be made via Unix sockets. This option is highly recommended for systems where only local requests are allowed. See section 5.5.5 How MySQL uses DNS.
--skip-new
Don't use new, possible wrong routines. Implies --skip-delay-key-write. This will also set default table type to ISAM. See section 7.3 ISAM Tables.
--skip-symlink
Don't delete or rename files that a symlinked file in the data directory points to.
--skip-safemalloc
If MySQL is configured with --with-debug=full, all programs will check the memory for overruns for every memory allocation and memory freeing. As this checking is very slow, you can avoid this, when you don't need memory checking, by using this option.
--skip-show-database
Don't allow 'SHOW DATABASE' commands, unless the user has process privilege.
--skip-stack-trace
Don't write stack traces. This option is useful when you are running mysqld under a debugger. See section G.1 Debugging a MySQL server.
--skip-thread-priority
Disable using thread priorities for faster response time.
--socket=path
Socket file to use for local connections instead of default /tmp/mysql.sock.
--sql-mode=option[,option[,option...]]
Option can be any combination of: REAL_AS_FLOAT, PIPES_AS_CONCAT, ANSI_QUOTES, IGNORE_SPACE, SERIALIZE, ONLY_FULL_GROUP_BY. It can also be empty ("") if you want to reset this. By specifying all of the above options is same as using --ansi. With this option one can turn on only needed SQL modes. See section 1.4.3 Running MySQL in ANSI Mode.
transaction-isolation= { READ-UNCOMMITTED | READ-COMMITTED | REPEATABLE-READ | SERIALIZABLE }
Sets the default transaction isolation level. See section 6.7.3 SET TRANSACTION Syntax.
-t, --tmpdir=path
Path for temporary files. It may be useful if your default /tmp directory resides on a partition too small to hold temporary tables.
-u, --user=user_name
Run mysqld daemon as user user_name. This option is mandatory when starting mysqld as root.
-V, --version
Output version information and exit.
-W, --warnings
Print out warnings like Aborted connection... to the .err file. See section A.2.9 Communication Errors / Aborted Connection.

4.1.2 my.cnf Option Files

MySQL can, since Version 3.22, read default startup options for the server and for clients from option files.

MySQL reads default options from the following files on Unix:

Filename Purpose
/etc/my.cnf Global options
DATADIR/my.cnf Server-specific options
defaults-extra-file The file specified with --defaults-extra-file=#
~/.my.cnf User-specific options

DATADIR is the MySQL data directory (typically `/usr/local/mysql/data' for a binary installation or `/usr/local/var' for a source installation). Note that this is the directory that was specified at configuration time, not the one specified with --datadir when mysqld starts up! (--datadir has no effect on where the server looks for option files, because it looks for them before it processes any command-line arguments.)

MySQL reads default options from the following files on Windows:

Filename Purpose
windows-system-directory\my.ini Global options
C:\my.cnf Global options
C:\mysql\data\my.cnf Server-specific options

Note that on Windows, you should specify all paths with / instead of \. If you use \, you need to specify this twice, as \ is the escape character in MySQL.

MySQL tries to read option files in the order listed above. If multiple option files exist, an option specified in a file read later takes precedence over the same option specified in a file read earlier. Options specified on the command line take precedence over options specified in any option file. Some options can be specified using environment variables. Options specified on the command line or in option files take precedence over environment variable values. See section H Environment Variables.

The following programs support option files: mysql, mysqladmin, mysqld, mysqldump, mysqlimport, mysql.server, myisamchk, and myisampack.

You can use option files to specify any long option that a program supports! Run the program with --help to get a list of available options.

An option file can contain lines of the following forms:

#comment
Comment lines start with `#' or `;'. Empty lines are ignored.
[group]
group is the name of the program or group for which you want to set options. After a group line, any option or set-variable lines apply to the named group until the end of the option file or another group line is given.
option
This is equivalent to --option on the command line.
option=value
This is equivalent to --option=value on the command line.
set-variable = variable=value
This is equivalent to --set-variable variable=value on the command line. This syntax must be used to set a mysqld variable.

The client group allows you to specify options that apply to all MySQL clients (not mysqld). This is the perfect group to use to specify the password you use to connect to the server. (But make sure the option file is readable and writable only by yourself.)

Note that for options and values, all leading and trailing blanks are automatically deleted. You may use the escape sequences `\b', `\t', `\n', `\r', `\\', and `\s' in your value string (`\s' == blank).

Here is a typical global option file:

[client]
port=3306
socket=/tmp/mysql.sock

[mysqld]
port=3306
socket=/tmp/mysql.sock
set-variable = key_buffer_size=16M
set-variable = max_allowed_packet=1M

[mysqldump]
quick

Here is typical user option file:

[client]
# The following password will be sent to all standard MySQL clients
password=my_password

[mysql]
no-auto-rehash
set-variable = connect_timeout=2

[mysqlhotcopy]
interactive-timeout

If you have a source distribution, you will find sample configuration files named `my-xxxx.cnf' in the `support-files' directory. If you have a binary distribution, look in the `DIR/support-files' directory, where DIR is the pathname to the MySQL installation directory (typically `/usr/local/mysql'). Currently there are sample configuration files for small, medium, large, and very large systems. You can copy `my-xxxx.cnf' to your home directory (rename the copy to `.my.cnf') to experiment with this.

All MySQL clients that support option files support the following options:

--no-defaults Don't read any option files.
--print-defaults Print the program name and all options that it will get.
--defaults-file=full-path-to-default-file Only use the given configuration file.
--defaults-extra-file=full-path-to-default-file Read this configuration file after the global configuration file but before the user configuration file.

Note that the above options must be first on the command line to work! --print-defaults may however be used directly after the --defaults-xxx-file commands.

Note for developers: Option file handling is implemented simply by processing all matching options (that is, options in the appropriate group) before any command-line arguments. This works nicely for programs that use the last instance of an option that is specified multiple times. If you have an old program that handles multiply-specified options this way but doesn't read option files, you need add only two lines to give it that capability. Check the source code of any of the standard MySQL clients to see how to do this.

In shell scripts you can use the `my_print_defaults' command to parse the config files:


shell> my_print_defaults client mysql
--port=3306
--socket=/tmp/mysql.sock
--no-auto-rehash

The above output contains all options for the groups 'client' and 'mysql'.

4.1.3 Installing Many Servers on the Same Machine

In some cases you may want to have many different mysqld daemons (servers) running on the same machine. You may for example want to run a new version of MySQL for testing together with an old version that is in production. Another case is when you want to give different users access to different mysqld servers that they manage themselves.

One way to get a new server running is by starting it with a different socket and port as follows:

shell> MYSQL_UNIX_PORT=/tmp/mysqld-new.sock
shell> MYSQL_TCP_PORT=3307
shell> export MYSQL_UNIX_PORT MYSQL_TCP_PORT
shell> scripts/mysql_install_db
shell> bin/safe_mysqld &

The environment variables appendix includes a list of other environment variables you can use to affect mysqld. See section H Environment Variables.

The above is the quick and dirty way that one commonly uses for testing. The nice thing with this is that all connections you do in the above shell will automatically be directed to the new running server!

If you need to do this more permanently, you should create an option file for each server. See section 4.1.2 my.cnf Option Files. In your startup script that is executed at boot time (mysql.server?) you should specify for both servers:

safe_mysqld --default-file=path-to-option-file

At least the following options should be different per server:

port=#
socket=path
pid-file=path

The following options should be different, if they are used:

log=path
log-bin=path
log-update=path
log-isam=path
bdb-logdir=path

If you want more performance, you can also specify the following differently:

tmpdir=path
bdb-tmpdir=path

See section 4.1.1 mysqld Command-line Options.

If you are installing binary MySQL versions (.tar files) and start them with ./bin/safe_mysqld then in most cases the only option you need to add/change is the socket and port argument to safe_mysqld.

4.1.4 Running Multiple MySQL Servers on the Same Machine

There are circumstances when you might want to run multiple servers on the same machine. For example, you might want to test a new MySQL release while leaving your existing production setup undisturbed. Or you might be an Internet service provider that wants to provide independent MySQL installations for different customers.

If you want to run multiple servers, the easiest way is to compile the servers with different TCP/IP ports and socket files so they are not both listening to the same TCP/IP port or socket file. See section 4.7.3 mysqld_multi, program for managing multiple MySQL servers.

Assume an existing server is configured for the default port number and socket file. Then configure the new server with a configure command something like this:

shell> ./configure  --with-tcp-port=port_number \
             --with-unix-socket-path=file_name \
             --prefix=/usr/local/mysql-3.22.9

Here port_number and file_name should be different than the default port number and socket file pathname, and the --prefix value should specify an installation directory different than the one under which the existing MySQL installation is located.

You can check the socket used by any currently executing MySQL server with this command:

shell> mysqladmin -h hostname --port=port_number variables

Note that if you specify ``localhost'' as a hostname, mysqladmin will default to using Unix sockets instead of TCP/IP.

If you have a MySQL server running on the port you used, you will get a list of some of the most important configurable variables in MySQL, including the socket name.

You don't have to recompile a new MySQL server just to start with a different port and socket. You can change the port and socket to be used by specifying them at run time as options to safe_mysqld:

shell> /path/to/safe_mysqld --socket=file_name --port=port_number

mysqld_multi can also take safe_mysqld (or mysqld) as an argument and pass the options from a configuration file to safe_mysqld and further to mysqld.

If you run the new server on the same database directory as another server with logging enabled, you should also specify the name of the log files to safe_mysqld with --log, --log-update, or --log-slow-queries. Otherwise, both servers may be trying to write to the same log file.

WARNING: Normally you should never have two servers that update data in the same database! If your OS doesn't support fault-free system locking, this may lead to unpleasant surprises!

If you want to use another database directory for the second server, you can use the --datadir=path option to safe_mysqld.

NOTE also that starting several MySQL servers (mysqlds) in different machines and letting them access one data directory over NFS is generally a BAD IDEA! The problem is that the NFS will become the bottleneck with the speed. It is not meant for such use. And last but not least, you would still have to come up with a solution how to make sure that two or more mysqlds are not interfering with each other. At the moment there is no platform that would 100% reliable do the file locking (lockd daemon usually) in every situation. Yet there would be one more possible risk with NFS; it would make the work even more complicated for lockd daemon to handle. So make it easy for your self and forget about the idea. The working solution is to have one computer with an operating system that efficiently handles threads and have several CPUs in it.

When you want to connect to a MySQL server that is running with a different port than the port that is compiled into your client, you can use one of the following methods:

4.2 General Security Issues and the MySQL Access Privilege System

MySQL has an advanced but non-standard security/privilege system. This section describes how it works.

4.2.1 General Security Guidelines

Anyone using MySQL on a computer connected to the Internet should read this section to avoid the most common security mistakes.

In discussing security, we emphasize the necessity of fully protecting the entire server host (not simply the MySQL server) against all types of applicable attacks: eavesdropping, altering, playback, and denial of service. We do not cover all aspects of availability and fault tolerance here.

MySQL uses security based on Access Control Lists (ACLs) for all connections, queries, and other operations that a user may attempt to perform. There is also some support for SSL-encrypted connections between MySQL clients and servers. Many of the concepts discussed here are not specific to MySQL at all; the same general ideas apply to almost all applications.

When running MySQL, follow these guidelines whenever possible:

4.2.2 How to Make MySQL Secure Against Crackers

When you connect to a MySQL server, you normally should use a password. The password is not transmitted in clear text over the connection, however the encryption algorithm is not very strong, and with some effort a clever attacker can crack the password if he is able to sniff the traffic between the client and the server. If the connection between the client and the server goes through an untrusted network, you should use an SSH tunnel to encrypt the communication.

All other information is transferred as text that can be read by anyone who is able to watch the connection. If you are concerned about this, you can use the compressed protocol (in MySQL Version 3.22 and above) to make things much harder. To make things even more secure you should use ssh. You can find an open source ssh client at http://www.openssh.org, and a commercial ssh client at http://www.ssh.com. With this, you can get an encrypted TCP/IP connection between a MySQL server and a MySQL client.

To make a MySQL system secure, you should strongly consider the following suggestions:

4.2.3 Startup Options for mysqld Concerning Security

The following mysqld options affect security:

--safe-show-database
With this option, SHOW DATABASES returns only those databases for which the user has some kind of privilege.
--safe-user-create
If this is enabled, an user can't create new users with the GRANT command, if the user doesn't have INSERT privilege to the mysql.user table. If you want to give a user access to just create new users with those privileges that the user has right to grant, you should give the user the following privilege:
GRANT INSERT(user) on mysql.user to 'user''hostname';
This will ensure that the user can't change any privilege columns directly, but has to use the GRANT command to give privileges to other users.
--skip-grant-tables
This option causes the server not to use the privilege system at all. This gives everyone full access to all databases! (You can tell a running server to start using the grant tables again by executing mysqladmin flush-privileges or mysqladmin reload.)
--skip-name-resolve
Hostnames are not resolved. All Host column values in the grant tables must be IP numbers or localhost.
--skip-networking
Don't allow TCP/IP connections over the network. All connections to mysqld must be made via Unix sockets. This option is unsuitable for systems that use MIT-pthreads, because the MIT-pthreads package doesn't support Unix sockets.
--skip-show-database
With this option, the SHOW DATABASES statement doesn't return anything.

4.2.4 What the Privilege System Does

The primary function of the MySQL privilege system is to authenticate a user connecting from a given host, and to associate that user with privileges on a database such as select, insert, update and delete.

Additional functionality includes the ability to have an anonymous user and to grant privileges for MySQL-specific functions such as LOAD DATA INFILE and administrative operations.

4.2.5 How the Privilege System Works

The MySQL privilege system ensures that all users may do exactly the things that they are supposed to be allowed to do. When you connect to a MySQL server, your identity is determined by the host from which you connect and the user name you specify. The system grants privileges according to your identity and what you want to do.

MySQL considers both your hostname and user name in identifying you because there is little reason to assume that a given user name belongs to the same person everywhere on the Internet. For example, the user bill who connects from whitehouse.gov need not be the same person as the user bill who connects from microsoft.com. MySQL handles this by allowing you to distinguish users on different hosts that happen to have the same name: you can grant bill one set of privileges for connections from whitehouse.gov, and a different set of privileges for connections from microsoft.com.

MySQL access control involves two stages:

The server uses the user, db, and host tables in the mysql database at both stages of access control. The fields in these grant tables are shown below:

Table name user db host
Scope fields Host Host Host
User Db Db
Password User
Privilege fields Select_priv Select_priv Select_priv
Insert_priv Insert_priv Insert_priv
Update_priv Update_priv Update_priv
Delete_priv Delete_priv Delete_priv
Index_priv Index_priv Index_priv
Alter_priv Alter_priv Alter_priv
Create_priv Create_priv Create_priv
Drop_priv Drop_priv Drop_priv
Grant_priv Grant_priv Grant_priv
References_priv
Reload_priv
Shutdown_priv
Process_priv
File_priv

For the second stage of access control (request verification), the server may, if the request involves tables, additionally consult the tables_priv and columns_priv tables. The fields in these tables are shown below:

Table name tables_priv columns_priv
Scope fields Host Host
Db Db
User User
Table_name Table_name
Column_name
Privilege fields Table_priv Column_priv
Column_priv
Other fields Timestamp Timestamp
Grantor

Each grant table contains scope fields and privilege fields.

Scope fields determine the scope of each entry in the tables, that is, the context in which the entry applies. For example, a user table entry with Host and User values of 'thomas.loc.gov' and 'bob' would be used for authenticating connections made to the server by bob from the host thomas.loc.gov. Similarly, a db table entry with Host, User, and Db fields of 'thomas.loc.gov', 'bob' and 'reports' would be used when bob connects from the host thomas.loc.gov to access the reports database. The tables_priv and columns_priv tables contain scope fields indicating tables or table/column combinations to which each entry applies.

For access-checking purposes, comparisons of Host values are case insensitive. User, Password, Db, and Table_name values are case sensitive. Column_name values are case insensitive in MySQL Version 3.22.12 or later.

Privilege fields indicate the privileges granted by a table entry, that is, what operations can be performed. The server combines the information in the various grant tables to form a complete description of a user's privileges. The rules used to do this are described in section 4.2.9 Access Control, Stage 2: Request Verification.

Scope fields are strings, declared as shown below; the default value for each is the empty string:

Field name Type
Host CHAR(60)
User CHAR(16)
Password CHAR(16)
Db CHAR(64) (CHAR(60) for the tables_priv and columns_priv tables)
Table_name CHAR(60)
Column_name CHAR(60)

In the user, db and host tables, all privilege fields are declared as ENUM('N','Y') -- each can have a value of 'N' or 'Y', and the default value is 'N'.

In the tables_priv and columns_priv tables, the privilege fields are declared as SET fields:

Table name Field name Possible set elements
tables_priv Table_priv 'Select', 'Insert', 'Update', 'Delete', 'Create', 'Drop', 'Grant', 'References', 'Index', 'Alter'
tables_priv Column_priv 'Select', 'Insert', 'Update', 'References'
columns_priv Column_priv 'Select', 'Insert', 'Update', 'References'

Briefly, the server uses the grant tables like this:

Note that administrative privileges (reload, shutdown, etc.) are specified only in the user table. This is because administrative operations are operations on the server itself and are not database-specific, so there is no reason to list such privileges in the other grant tables. In fact, only the user table need be consulted to determine whether or not you can perform an administrative operation.

The file privilege is specified only in the user table, too. It is not an administrative privilege as such, but your ability to read or write files on the server host is independent of the database you are accessing.

The mysqld server reads the contents of the grant tables once, when it starts up. Changes to the grant tables take effect as indicated in section 4.3.3 When Privilege Changes Take Effect.

When you modify the contents of the grant tables, it is a good idea to make sure that your changes set up privileges the way you want. For help in diagnosing problems, see section 4.2.10 Causes of Access denied Errors. For advice on security issues, see section 4.2.2 How to Make MySQL Secure Against Crackers.

A useful diagnostic tool is the mysqlaccess script, which Yves Carlier has provided for the MySQL distribution. Invoke mysqlaccess with the --help option to find out how it works. Note that mysqlaccess checks access using only the user, db and host tables. It does not check table- or column-level privileges.

4.2.6 Privileges Provided by MySQL

Information about user privileges is stored in the user, db, host, tables_priv, and columns_priv tables in the mysql database (that is, in the database named mysql). The MySQL server reads the contents of these tables when it starts up and under the circumstances indicated in section 4.3.3 When Privilege Changes Take Effect.

The names used in this manual to refer to the privileges provided by MySQL are shown below, along with the table column name associated with each privilege in the grant tables and the context in which the privilege applies:

Privilege Column Context
select Select_priv tables
insert Insert_priv tables
update Update_priv tables
delete Delete_priv tables
index Index_priv tables
alter Alter_priv tables
create Create_priv databases, tables, or indexes
drop Drop_priv databases or tables
grant Grant_priv databases or tables
references References_priv databases or tables
reload Reload_priv server administration
shutdown Shutdown_priv server administration
process Process_priv server administration
file File_priv file access on server

The select, insert, update, and delete privileges allow you to perform operations on rows in existing tables in a database.

SELECT statements require the select privilege only if they actually retrieve rows from a table. You can execute certain SELECT statements even without permission to access any of the databases on the server. For example, you could use the mysql client as a simple calculator:

mysql> SELECT 1+1;
mysql> SELECT PI()*2;

The index privilege allows you to create or drop (remove) indexes.

The alter privilege allows you to use ALTER TABLE.

The create and drop privileges allow you to create new databases and tables, or to drop (remove) existing databases and tables.

Note that if you grant the drop privilege for the mysql database to a user, that user can drop the database in which the MySQL access privileges are stored!

The grant privilege allows you to give to other users those privileges you yourself possess.

The file privilege gives you permission to read and write files on the server using the LOAD DATA INFILE and SELECT ... INTO OUTFILE statements. Any user to whom this privilege is granted can read or write any file that the MySQL server can read or write.

The remaining privileges are used for administrative operations, which are performed using the mysqladmin program. The table below shows which mysqladmin commands each administrative privilege allows you to execute:

Privilege Commands permitted to privilege holders
reload reload, refresh, flush-privileges, flush-hosts, flush-logs, and flush-tables
shutdown shutdown
process processlist, kill

The reload command tells the server to re-read the grant tables. The refresh command flushes all tables and opens and closes the log files. flush-privileges is a synonym for reload. The other flush-* commands perform functions similar to refresh but are more limited in scope, and may be preferable in some instances. For example, if you want to flush just the log files, flush-logs is a better choice than refresh.

The shutdown command shuts down the server.

The processlist command displays information about the threads executing within the server. The kill command kills server threads. You can always display or kill your own threads, but you need the process privilege to display or kill threads initiated by other users. See section 4.5.4 KILL Syntax.

It is a good idea in general to grant privileges only to those users who need them, but you should exercise particular caution in granting certain privileges:

There are some things that you cannot do with the MySQL privilege system:

4.2.7 Connecting to the MySQL Server

MySQL client programs generally require that you specify connection parameters when you want to access a MySQL server: the host you want to connect to, your user name, and your password. For example, the mysql client can be started like this (optional arguments are enclosed between `[' and `]'):

shell> mysql [-h host_name] [-u user_name] [-pyour_pass]

Alternate forms of the -h, -u, and -p options are --host=host_name, --user=user_name, and --password=your_pass. Note that there is no space between -p or --password= and the password following it.

NOTE: Specifying a password on the command line is not secure! Any user on your system may then find out your password by typing a command like: ps auxww. See section 4.1.2 my.cnf Option Files.

mysql uses default values for connection parameters that are missing from the command line:

Thus, for a Unix user joe, the following commands are equivalent:

shell> mysql -h localhost -u joe
shell> mysql -h localhost
shell> mysql -u joe
shell> mysql

Other MySQL clients behave similarly.

On Unix systems, you can specify different default values to be used when you make a connection, so that you need not enter them on the command line each time you invoke a client program. This can be done in a couple of ways:

4.2.8 Access Control, Stage 1: Connection Verification

When you attempt to connect to a MySQL server, the server accepts or rejects the connection based on your identity and whether or not you can verify your identity by supplying the correct password. If not, the server denies access to you completely. Otherwise, the server accepts the connection, then enters Stage 2 and waits for requests.

Your identity is based on two pieces of information:

Identity checking is performed using the three user table scope fields (Host, User, and Password). The server accepts the connection only if a user table entry matches your hostname and user name, and you supply the correct password.

Values in the user table scope fields may be specified as follows:

Non-blank Password values represent encrypted passwords. MySQL does not store passwords in plaintext form for anyone to see. Rather, the password supplied by a user who is attempting to connect is encrypted (using the PASSWORD() function). The encrypted password is then used when the client/server is checking if the password is correct (This is done without the encrypted password ever traveling over the connection.) Note that from MySQL's point of view the encrypted password is the REAL password, so you should not give anyone access to it! In particular, don't give normal users read access to the tables in the mysql database!

The examples below show how various combinations of Host and User values in user table entries apply to incoming connections:

Host value User value Connections matched by entry
'thomas.loc.gov' 'fred' fred, connecting from thomas.loc.gov
'thomas.loc.gov' '' Any user, connecting from thomas.loc.gov
'%' 'fred' fred, connecting from any host
'%' '' Any user, connecting from any host
'%.loc.gov' 'fred' fred, connecting from any host in the loc.gov domain
'x.y.%' 'fred' fred, connecting from x.y.net, x.y.com,x.y.edu, etc. (this is probably not useful)
'144.155.166.177' 'fred' fred, connecting from the host with IP address 144.155.166.177
'144.155.166.%' 'fred' fred, connecting from any host in the 144.155.166 class C subnet
'144.155.166.0/255.255.255.0' 'fred' Same as previous example

Because you can use IP wild-card values in the Host field (for example, '144.155.166.%' to match every host on a subnet), there is the possibility that someone might try to exploit this capability by naming a host 144.155.166.somewhere.com. To foil such attempts, MySQL disallows matching on hostnames that start with digits and a dot. Thus, if you have a host named something like 1.2.foo.com, its name will never match the Host column of the grant tables. Only an IP number can match an IP wild-card value.

An incoming connection may be matched by more than one entry in the user table. For example, a connection from thomas.loc.gov by fred would be matched by several of the entries just shown above. How does the server choose which entry to use if more than one matches? The server resolves this question by sorting the user table after reading it at startup time, then looking through the entries in sorted order when a user attempts to connect. The first matching entry is the one that is used.

user table sorting works as follows. Suppose the user table looks like this:

+-----------+----------+-
| Host      | User     | ...
+-----------+----------+-
| %         | root     | ...
| %         | jeffrey  | ...
| localhost | root     | ...
| localhost |          | ...
+-----------+----------+-

When the server reads in the table, it orders the entries with the most-specific Host values first ('%' in the Host column means ``any host'' and is least specific). Entries with the same Host value are ordered with the most-specific User values first (a blank User value means ``any user'' and is least specific). The resulting sorted user table looks like this:

+-----------+----------+-
| Host      | User     | ...
+-----------+----------+-
| localhost | root     | ...
| localhost |          | ...
| %         | jeffrey  | ...
| %         | root     | ...
+-----------+----------+-

When a connection is attempted, the server looks through the sorted entries and uses the first match found. For a connection from localhost by jeffrey, the entries with 'localhost' in the Host column match first. Of those, the entry with the blank user name matches both the connecting hostname and user name. (The '%'/'jeffrey' entry would have matched, too, but it is not the first match in the table.)

Here is another example. Suppose the user table looks like this:

+----------------+----------+-
| Host           | User     | ...
+----------------+----------+-
| %              | jeffrey  | ...
| thomas.loc.gov |          | ...
+----------------+----------+-

The sorted table looks like this:

+----------------+----------+-
| Host           | User     | ...
+----------------+----------+-
| thomas.loc.gov |          | ...
| %              | jeffrey  | ...
+----------------+----------+-

A connection from thomas.loc.gov by jeffrey is matched by the first entry, whereas a connection from whitehouse.gov by jeffrey is matched by the second.

A common misconception is to think that for a given user name, all entries that explicitly name that user will be used first when the server attempts to find a match for the connection. This is simply not true. The previous example illustrates this, where a connection from thomas.loc.gov by jeffrey is first matched not by the entry containing 'jeffrey' as the User field value, but by the entry with no user name!

If you have problems connecting to the server, print out the user table and sort it by hand to see where the first match is being made.

4.2.9 Access Control, Stage 2: Request Verification

Once you establish a connection, the server enters Stage 2. For each request that comes in on the connection, the server checks whether you have sufficient privileges to perform it, based on the type of operation you wish to perform. This is where the privilege fields in the grant tables come into play. These privileges can come from any of the user, db, host, tables_priv, or columns_priv tables. The grant tables are manipulated with GRANT and REVOKE commands. See section 4.3.1 GRANT and REVOKE Syntax. (You may find it helpful to refer to section 4.2.5 How the Privilege System Works, which lists the fields present in each of the grant tables.)

The user table grants privileges that are assigned to you on a global basis and that apply no matter what the current database is. For example, if the user table grants you the delete privilege, you can delete rows from any database on the server host! In other words, user table privileges are superuser privileges. It is wise to grant privileges in the user table only to superusers such as server or database administrators. For other users, you should leave the privileges in the user table set to 'N' and grant privileges on a database-specific basis only, using the db and host tables.

The db and host tables grant database-specific privileges. Values in the scope fields may be specified as follows:

The db and host tables are read in and sorted when the server starts up (at the same time that it reads the user table). The db table is sorted on the Host, Db, and User scope fields, and the host table is sorted on the Host and Db scope fields. As with the user table, sorting puts the most-specific values first and least-specific values last, and when the server looks for matching entries, it uses the first match that it finds.

The tables_priv and columns_priv tables grant table- and column-specific privileges. Values in the scope fields may be specified as follows:

The tables_priv and columns_priv tables are sorted on the Host, Db, and User fields. This is similar to db table sorting, although the sorting is simpler because only the Host field may contain wild cards.

The request verification process is described below. (If you are familiar with the access-checking source code, you will notice that the description here differs slightly from the algorithm used in the code. The description is equivalent to what the code actually does; it differs only to make the explanation simpler.)

For administrative requests (shutdown, reload, etc.), the server checks only the user table entry, because that is the only table that specifies administrative privileges. Access is granted if the entry allows the requested operation and denied otherwise. For example, if you want to execute mysqladmin shutdown but your user table entry doesn't grant the shutdown privilege to you, access is denied without even checking the db or host tables. (They contain no Shutdown_priv column, so there is no need to do so.)

For database-related requests (insert, update, etc.), the server first checks the user's global (superuser) privileges by looking in the user table entry. If the entry allows the requested operation, access is granted. If the global privileges in the user table are insufficient, the server determines the user's database-specific privileges by checking the db and host tables:

  1. The server looks in the db table for a match on the Host, Db, and User fields. The Host and User fields are matched to the connecting user's hostname and MySQL user name. The Db field is matched to the database the user wants to access. If there is no entry for the Host and User, access is denied.
  2. If there is a matching db table entry and its Host field is not blank, that entry defines the user's database-specific privileges.
  3. If the matching db table entry's Host field is blank, it signifies that the host table enumerates which hosts should be allowed access to the database. In this case, a further lookup is done in the host table to find a match on the Host and Db fields. If no host table entry matches, access is denied. If there is a match, the user's database-specific privileges are computed as the intersection (not the union!) of the privileges in the db and host table entries, that is, the privileges that are 'Y' in both entries. (This way you can grant general privileges in the db table entry and then selectively restrict them on a host-by-host basis using the host table entries.)

After determining the database-specific privileges granted by the db and host table entries, the server adds them to the global privileges granted by the user table. If the result allows the requested operation, access is granted. Otherwise, the server checks the user's table and column privileges in the tables_priv and columns_priv tables and adds those to the user's privileges. Access is allowed or denied based on the result.

Expressed in boolean terms, the preceding description of how a user's privileges are calculated may be summarized like this:

global privileges
OR (database privileges AND host privileges)
OR table privileges
OR column privileges

It may not be apparent why, if the global user entry privileges are initially found to be insufficient for the requested operation, the server adds those privileges to the database-, table-, and column-specific privileges later. The reason is that a request might require more than one type of privilege. For example, if you execute an INSERT ... SELECT statement, you need both insert and select privileges. Your privileges might be such that the user table entry grants one privilege and the db table entry grants the other. In this case, you have the necessary privileges to perform the request, but the server cannot tell that from either table by itself; the privileges granted by the entries in both tables must be combined.

The host table can be used to maintain a list of secure servers.

At TcX, the host table contains a list of all machines on the local network. These are granted all privileges.

You can also use the host table to indicate hosts that are not secure. Suppose you have a machine public.your.domain that is located in a public area that you do not consider secure. You can allow access to all hosts on your network except that machine by using host table entries like this:

+--------------------+----+-
| Host               | Db | ...
+--------------------+----+-
| public.your.domain | %  | ... (all privileges set to 'N')
| %.your.domain      | %  | ... (all privileges set to 'Y')
+--------------------+----+-

Naturally, you should always test your entries in the grant tables (for example, using mysqlaccess) to make sure your access privileges are actually set up the way you think they are.

4.2.10 Causes of Access denied Errors

If you encounter Access denied errors when you try to connect to the MySQL server, the list below indicates some courses of action you can take to correct the problem:

4.3 MySQL User Account Management

4.3.1 GRANT and REVOKE Syntax

GRANT priv_type [(column_list)] [, priv_type [(column_list)] ...]
    ON {tbl_name | * | *.* | db_name.*}
    TO user_name [IDENTIFIED BY 'password']
        [, user_name [IDENTIFIED BY 'password'] ...]
    [WITH GRANT OPTION]

REVOKE priv_type [(column_list)] [, priv_type [(column_list)] ...]
    ON {tbl_name | * | *.* | db_name.*}
    FROM user_name [, user_name ...]

GRANT is implemented in MySQL Version 3.22.11 or later. For earlier MySQL versions, the GRANT statement does nothing.

The GRANT and REVOKE commands allow system administrators to create users and grant and revoke rights to MySQL users at four privilege levels:

Global level
Global privileges apply to all databases on a given server. These privileges are stored in the mysql.user table.
Database level
Database privileges apply to all tables in a given database. These privileges are stored in the mysql.db and mysql.host tables.
Table level
Table privileges apply to all columns in a given table. These privileges are stored in the mysql.tables_priv table.
Column level
Column privileges apply to single columns in a given table. These privileges are stored in the mysql.columns_priv table.

If you give a grant for a users that doesn't exists, that user is created. For examples of how GRANT works, see section 4.3.5 Adding New Users to MySQL.

For the GRANT and REVOKE statements, priv_type may be specified as any of the following:

ALL PRIVILEGES      FILE                RELOAD
ALTER               INDEX               SELECT
CREATE              INSERT              SHUTDOWN
DELETE              PROCESS             UPDATE
DROP                REFERENCES          USAGE

ALL is a synonym for ALL PRIVILEGES. REFERENCES is not yet implemented. USAGE is currently a synonym for ``no privileges.'' It can be used when you want to create a user that has no privileges.

To revoke the grant privilege from a user, use a priv_type value of GRANT OPTION:

REVOKE GRANT OPTION ON ... FROM ...;

The only priv_type values you can specify for a table are SELECT, INSERT, UPDATE, DELETE, CREATE, DROP, GRANT, INDEX, and ALTER.

The only priv_type values you can specify for a column (that is, when you use a column_list clause) are SELECT, INSERT, and UPDATE.

You can set global privileges by using ON *.* syntax. You can set database privileges by using ON db_name.* syntax. If you specify ON * and you have a current database, you will set the privileges for that database. (WARNING: If you specify ON * and you don't have a current database, you will affect the global privileges!)

In order to accommodate granting rights to users from arbitrary hosts, MySQL supports specifying the user_name value in the form user@host. If you want to specify a user string containing special characters (such as `-'), or a host string containing special characters or wild-card characters (such as `%'), you can quote the user or host name (for example, 'test-user'@'test-hostname').

You can specify wild cards in the hostname. For example, user@"%.loc.gov" applies to user for any host in the loc.gov domain, and user@"144.155.166.%" applies to user for any host in the 144.155.166 class C subnet.

The simple form user is a synonym for user@"%". NOTE: If you allow anonymous users to connect to the MySQL server (which is the default), you should also add all local users as user@localhost because otherwise the anonymous user entry for the local host in the mysql.user table will be used when the user tries to log into the MySQL server from the local machine! Anonymous users are defined by inserting entries with User='' into the mysql.user table. You can verify if this applies to you by executing this query:

mysql> SELECT Host,User FROM mysql.user WHERE User='';

For the moment, GRANT only supports host, table, database, and column names up to 60 characters long. A user name can be up to 16 characters.

The privileges for a table or column are formed from the logical OR of the privileges at each of the four privilege levels. For example, if the mysql.user table specifies that a user has a global select privilege, this can't be denied by an entry at the database, table, or column level.

The privileges for a column can be calculated as follows:

global privileges
OR (database privileges AND host privileges)
OR table privileges
OR column privileges

In most cases, you grant rights to a user at only one of the privilege levels, so life isn't normally as complicated as above. The details of the privilege-checking procedure are presented in section 4.2 General Security Issues and the MySQL Access Privilege System.

If you grant privileges for a user/hostname combination that does not exist in the mysql.user table, an entry is added and remains there until deleted with a DELETE command. In other words, GRANT may create user table entries, but REVOKE will not remove them; you must do that explicitly using DELETE.

In MySQL Version 3.22.12 or later, if a new user is created or if you have global grant privileges, the user's password will be set to the password specified by the IDENTIFIED BY clause, if one is given. If the user already had a password, it is replaced by the new one.

WARNING: If you create a new user but do not specify an IDENTIFIED BY clause, the user has no password. This is insecure.

Passwords can also be set with the SET PASSWORD command. See section 5.5.6 SET Syntax.

If you grant privileges for a database, an entry in the mysql.db table is created if needed. When all privileges for the database have been removed with REVOKE, this entry is deleted.

If a user doesn't have any privileges on a table, the table is not displayed when the user requests a list of tables (for example, with a SHOW TABLES statement).

The WITH GRANT OPTION clause gives the user the ability to give to other users any privileges the user has at the specified privilege level. You should be careful to whom you give the grant privilege, as two users with different privileges may be able to join privileges!

You cannot grant another user a privilege you don't have yourself; the grant privilege allows you to give away only those privileges you possess.

Be aware that when you grant a user the grant privilege at a particular privilege level, any privileges the user already possesses (or is given in the future!) at that level are also grantable by that user. Suppose you grant a user the insert privilege on a database. If you then grant the select privilege on the database and specify WITH GRANT OPTION, the user can give away not only the select privilege, but also insert. If you then grant the update privilege to the user on the database, the user can give away the insert, select and update.

You should not grant alter privileges to a normal user. If you do that, the user can try to subvert the privilege system by renaming tables!

Note that if you are using table or column privileges for even one user, the server examines table and column privileges for all users and this will slow down MySQL a bit.

When mysqld starts, all privileges are read into memory. Database, table, and column privileges take effect at once, and user-level privileges take effect the next time the user connects. Modifications to the grant tables that you perform using GRANT or REVOKE are noticed by the server immediately. If you modify the grant tables manually (using INSERT, UPDATE, etc.), you should execute a FLUSH PRIVILEGES statement or run mysqladmin flush-privileges to tell the server to reload the grant tables. See section 4.3.3 When Privilege Changes Take Effect.

The biggest differences between the ANSI SQL and MySQL versions of GRANT are:

4.3.2 MySQL User Names and Passwords

There are several distinctions between the way user names and passwords are used by MySQL and the way they are used by Unix or Windows:

MySQL users and they privileges are normally created with the GRANT command. See section 4.3.1 GRANT and REVOKE Syntax.

When you login to a MySQL server with a command line client you should specify the password with --password=your-password. See section 4.2.7 Connecting to the MySQL Server.

mysql --user=monty --password=guess database_name

If you want the client to prompt for a password, you should use --password without any argument

mysql --user=monty --password database_name

or the short form:

mysql -u monty -p database_name

Note that in the last example the password is NOT 'database_name'.

If you want to use the -p option to supply a password you should do like this:

mysql -u monty -pguess database_name

On some system the library call that MySQL uses to prompt for a password will automatically cut the password to 8 characters. Internally MySQL doesn't have any limit for the length of the password.

4.3.3 When Privilege Changes Take Effect

When mysqld starts, all grant table contents are read into memory and become effective at that point.

Modifications to the grant tables that you perform using GRANT, REVOKE, or SET PASSWORD are noticed by the server immediately.

If you modify the grant tables manually (using INSERT, UPDATE, etc.), you should execute a FLUSH PRIVILEGES statement or run mysqladmin flush-privileges or mysqladmin reload to tell the server to reload the grant tables. Otherwise your changes will have no effect until you restart the server. If you change the grant tables manually but forget to reload the privileges, you will be wondering why your changes don't seem to make any difference!

When the server notices that the grant tables have been changed, existing client connections are affected as follows:

Global privilege changes and password changes take effect the next time the client connects.

4.3.4 Setting Up the Initial MySQL Privileges

After installing MySQL, you set up the initial access privileges by running scripts/mysql_install_db. See section 2.3.1 Quick Installation Overview. The mysql_install_db script starts up the mysqld server, then initializes the grant tables to contain the following set of privileges:

NOTE: The default privileges are different for Windows. See section 2.6.2.3 Running MySQL on Windows.

Because your installation is initially wide open, one of the first things you should do is specify a password for the MySQL root user. You can do this as follows (note that you specify the password using the PASSWORD() function):

shell> mysql -u root mysql
mysql> UPDATE user SET Password=PASSWORD('new_password')
           WHERE user='root';
mysql> FLUSH PRIVILEGES;

You can, in MySQL Version 3.22 and above, use the SET PASSWORD statement:

shell> mysql -u root mysql
mysql> SET PASSWORD FOR root=PASSWORD('new_password');

Another way to set the password is by using the mysqladmin command:

shell> mysqladmin -u root password new_password

Only users with write/update access to the mysql database can change the password for others users. All normal users (not anonymous ones) can only change their own password with either of the above commands or with SET PASSWORD=PASSWORD('new password').

Note that if you update the password in the user table directly using the first method, you must tell the server to re-read the grant tables (with FLUSH PRIVILEGES), because the change will go unnoticed otherwise.

Once the root password has been set, thereafter you must supply that password when you connect to the server as root.

You may wish to leave the root password blank so that you don't need to specify it while you perform additional setup or testing. However, be sure to set it before using your installation for any real production work.

See the scripts/mysql_install_db script to see how it sets up the default privileges. You can use this as a basis to see how to add other users.

If you want the initial privileges to be different than those just described above, you can modify mysql_install_db before you run it.

To re-create the grant tables completely, remove all the `.frm', `.MYI', and `.MYD' files in the directory containing the mysql database. (This is the directory named `mysql' under the database directory, which is listed when you run mysqld --help.) Then run the mysql_install_db script, possibly after editing it first to have the privileges you want.

NOTE: For MySQL versions older than Version 3.22.10, you should NOT delete the `.frm' files. If you accidentally do this, you should copy them back from your MySQL distribution before running mysql_install_db.

4.3.5 Adding New Users to MySQL

You can add users two different ways: by using GRANT statements or by manipulating the MySQL grant tables directly. The preferred method is to use GRANT statements, because they are more concise and less error-prone. See section 4.3.1 GRANT and REVOKE Syntax.

There is also a lot of contributed programs like phpmyadmin that can be used to create and administrate users. See section D Contributed Programs.

The examples below show how to use the mysql client to set up new users. These examples assume that privileges are set up according to the defaults described in the previous section. This means that to make changes, you must be on the same machine where mysqld is running, you must connect as the MySQL root user, and the root user must have the insert privilege for the mysql database and the reload administrative privilege. Also, if you have changed the root user password, you must specify it for the mysql commands below.

You can add new users by issuing GRANT statements:

shell> mysql --user=root mysql
mysql> GRANT ALL PRIVILEGES ON *.* TO monty@localhost
           IDENTIFIED BY 'some_pass' WITH GRANT OPTION;
mysql> GRANT ALL PRIVILEGES ON *.* TO monty@"%"
           IDENTIFIED BY 'some_pass' WITH GRANT OPTION;
mysql> GRANT RELOAD,PROCESS ON *.* TO admin@localhost;
mysql> GRANT USAGE ON *.* TO dummy@localhost;

These GRANT statements set up three new users:

monty
A full superuser who can connect to the server from anywhere, but who must use a password 'some_pass' to do so. Note that we must issue GRANT statements for both monty@localhost and monty@"%". If we don't add the entry with localhost, the anonymous user entry for localhost that is created by mysql_install_db will take precedence when we connect from the local host, because it has a more specific Host field value and thus comes earlier in the user table sort order.
admin
A user who can connect from localhost without a password and who is granted the reload and process administrative privileges. This allows the user to execute the mysqladmin reload, mysqladmin refresh, and mysqladmin flush-* commands, as well as mysqladmin processlist . No database-related privileges are granted. (They can be granted later by issuing additional GRANT statements.)
dummy
A user who can connect without a password, but only from the local host. The global privileges are all set to 'N' -- the USAGE privilege type allows you to create a user with no privileges. It is assumed that you will grant database-specific privileges later.

You can also add the same user access information directly by issuing INSERT statements and then telling the server to reload the grant tables:

shell> mysql --user=root mysql
mysql> INSERT INTO user VALUES('localhost','monty',PASSWORD('some_pass'),
                'Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y');
mysql> INSERT INTO user VALUES('%','monty',PASSWORD('some_pass'),
                'Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y');
mysql> INSERT INTO user SET Host='localhost',User='admin',
                 Reload_priv='Y', Process_priv='Y';
mysql> INSERT INTO user (Host,User,Password)
                        VALUES('localhost','dummy','');
mysql> FLUSH PRIVILEGES;

Depending on your MySQL version, you may have to use a different number of 'Y' values above (versions prior to Version 3.22.11 had fewer privilege columns). For the admin user, the more readable extended INSERT syntax that is available starting with Version 3.22.11 is used.

Note that to set up a superuser, you need only create a user table entry with the privilege fields set to 'Y'. No db or host table entries are necessary.

The privilege columns in the user table were not set explicitly in the last INSERT statement (for the dummy user), so those columns are assigned the default value of 'N'. This is the same thing that GRANT USAGE does.

The following example adds a user custom who can connect from hosts localhost, server.domain, and whitehouse.gov. He wants to access the bankaccount database only from localhost, the expenses database only from whitehouse.gov, and the customer database from all three hosts. He wants to use the password stupid from all three hosts.

To set up this user's privileges using GRANT statements, run these commands:

shell> mysql --user=root mysql
mysql> GRANT SELECT,INSERT,UPDATE,DELETE,CREATE,DROP
           ON bankaccount.*
           TO custom@localhost
           IDENTIFIED BY 'stupid';
mysql> GRANT SELECT,INSERT,UPDATE,DELETE,CREATE,DROP
           ON expenses.*
           TO custom@whitehouse.gov
           IDENTIFIED BY 'stupid';
mysql> GRANT SELECT,INSERT,UPDATE,DELETE,CREATE,DROP
           ON customer.*
           TO custom@'%'
           IDENTIFIED BY 'stupid';

The reason that we do to grant statements for the user 'custom' is that we want the give the user access to MySQL both from the local machine with Unix sockets and from the remote machine 'whitehouse.gov' over TCP/IP.

To set up the user's privileges by modifying the grant tables directly, run these commands (note the FLUSH PRIVILEGES at the end):

shell> mysql --user=root mysql
mysql> INSERT INTO user (Host,User,Password)
       VALUES('localhost','custom',PASSWORD('stupid'));
mysql> INSERT INTO user (Host,User,Password)
       VALUES('server.domain','custom',PASSWORD('stupid'));
mysql> INSERT INTO user (Host,User,Password)
       VALUES('whitehouse.gov','custom',PASSWORD('stupid'));
mysql> INSERT INTO db
       (Host,Db,User,Select_priv,Insert_priv,Update_priv,Delete_priv,
        Create_priv,Drop_priv)
       VALUES
       ('localhost','bankaccount','custom','Y','Y','Y','Y','Y','Y');
mysql> INSERT INTO db
       (Host,Db,User,Select_priv,Insert_priv,Update_priv,Delete_priv,
        Create_priv,Drop_priv)
       VALUES
       ('whitehouse.gov','expenses','custom','Y','Y','Y','Y','Y','Y');
mysql> INSERT INTO db
       (Host,Db,User,Select_priv,Insert_priv,Update_priv,Delete_priv,
        Create_priv,Drop_priv)
       VALUES('%','customer','custom','Y','Y','Y','Y','Y','Y');
mysql> FLUSH PRIVILEGES;

The first three INSERT statements add user table entries that allow user custom to connect from the various hosts with the given password, but grant no permissions to him (all privileges are set to the default value of 'N'). The next three INSERT statements add db table entries that grant privileges to custom for the bankaccount, expenses, and customer databases, but only when accessed from the proper hosts. As usual, when the grant tables are modified directly, the server must be told to reload them (with FLUSH PRIVILEGES) so that the privilege changes take effect.

If you want to give a specific user access from any machine in a given domain, you can issue a GRANT statement like the following:

mysql> GRANT ...
           ON *.*
           TO myusername@"%.mydomainname.com"
           IDENTIFIED BY 'mypassword';

To do the same thing by modifying the grant tables directly, do this:

mysql> INSERT INTO user VALUES ('%.mydomainname.com', 'myusername',
           PASSWORD('mypassword'),...);
mysql> FLUSH PRIVILEGES;

You can also use xmysqladmin, mysql_webadmin, and even xmysql to insert, change, and update values in the grant tables. You can find these utilities in the Contrib directory of the MySQL Website.

4.3.6 Setting Up Passwords

In most cases you should use GRANT to set up your users/passwords, so the following only applies for advanced users. See section 4.3.1 GRANT and REVOKE Syntax.

The examples in the preceding sections illustrate an important principle: when you store a non-empty password using INSERT or UPDATE statements, you must use the PASSWORD() function to encrypt it. This is because the user table stores passwords in encrypted form, not as plaintext. If you forget that fact, you are likely to attempt to set passwords like this:

shell> mysql -u root mysql
mysql> INSERT INTO user (Host,User,Password)
       VALUES('%','jeffrey','biscuit');
mysql> FLUSH PRIVILEGES;

The result is that the plaintext value 'biscuit' is stored as the password in the user table. When the user jeffrey attempts to connect to the server using this password, the mysql client encrypts it with PASSWORD(), generates an authentification vector based on encrypted password and a random number, obtained from server, and sends the result to the server. The server uses the password value in the user table (that is not encrypted value 'biscuit') to perform the same calculations, and compares results. The comparison fails and the server rejects the connection:

shell> mysql -u jeffrey -pbiscuit test
Access denied

Passwords must be encrypted when they are inserted in the user table, so the INSERT statement should have been specified like this instead:

mysql> INSERT INTO user (Host,User,Password)
       VALUES('%','jeffrey',PASSWORD('biscuit'));

You must also use the PASSWORD() function when you use SET PASSWORD statements:

mysql> SET PASSWORD FOR jeffrey@"%" = PASSWORD('biscuit');

If you set passwords using the GRANT ... IDENTIFIED BY statement or the mysqladmin password command, the PASSWORD() function is unnecessary. They both take care of encrypting the password for you, so you would specify a password of 'biscuit' like this:

mysql> GRANT USAGE ON *.* TO jeffrey@"%" IDENTIFIED BY 'biscuit';

or

shell> mysqladmin -u jeffrey password biscuit

NOTE: PASSWORD() does not perform password encryption in the same way that Unix passwords are encrypted. You should not assume that if your Unix password and your MySQL password are the same, that PASSWORD() will result in the same encrypted value as is stored in the Unix password file. See section 4.3.2 MySQL User Names and Passwords.

4.3.7 Keeping Your Password Secure

It is inadvisable to specify your password in a way that exposes it to discovery by other users. The methods you can use to specify your password when you run client programs are listed below, along with an assessment of the risks of each method:

All in all, the safest methods are to have the client program prompt for the password or to specify the password in a properly protected `.my.cnf' file.

4.4 Disaster Prevention and Recovery

4.4.1 Database Backups

Because MySQL tables are stored as files, it is easy to do a backup. To get a consistent backup, do a LOCK TABLES on the relevant tables followed by FLUSH TABLES for the tables. See section 6.7.2 LOCK TABLES/UNLOCK TABLES Syntax. See section 4.5.3 FLUSH Syntax. You only need a read lock; this allows other threads to continue to query the tables while you are making a copy of the files in the database directory. The FLUSH TABLE is needed to ensure that the all active index pages is written to disk before you start the backup.

If you want to make a SQL level backup of a table, you can use SELECT INTO OUTFILE or BACKUP TABLE. See section 6.4.1 SELECT Syntax. See section 4.4.2 BACKUP TABLE Syntax.

Another way to back up a database is to use the mysqldump program or the mysqlhotcopy script. See section 4.8.5 mysqldump, Dumping Table Structure and Data. See section 4.8.6 mysqlhotcopy, Copying MySQL Databases and Tables.

  1. Do a full backup of your databases:
    shell> mysqldump --tab=/path/to/some/dir --opt --full
    
    or
    
    shell> mysqlhotcopy database /path/to/some/dir
    
    You can also simply copy all table files (`*.frm', `*.MYD', and `*.MYI' files) as long as the server isn't updating anything. The script mysqlhotcopy does use this method.
  2. Stop mysqld if it's running, then start it with the --log-update[=file_name] option. See section 4.9.3 The Update Log. The update log file(s) provide you with the information you need to replicate changes to the database that are made subsequent to the point at which you executed mysqldump.

If you have to restore something, try to recover your tables using REPAIR TABLE or myisamchk -r first. That should work in 99.9% of all cases. If myisamchk fails, try the following procedure: (This will only work if you have started MySQL with --log-update. See section 4.9.3 The Update Log.):

  1. Restore the original mysqldump backup.
  2. Execute the following command to re-run the updates in the binary log:
    shell> mysqlbinlog hostname-bin.[0-9]* | mysql
    
    If you are using the update log you can use:
    shell> ls -1 -t -r hostname.[0-9]* | xargs cat | mysql
    

ls is used to get all the update log files in the right order.

You can also do selective backups with SELECT * INTO OUTFILE 'file_name' FROM tbl_name and restore with LOAD DATA INFILE 'file_name' REPLACE ... To avoid duplicate records, you need a PRIMARY KEY or a UNIQUE key in the table. The REPLACE keyword causes old records to be replaced with new ones when a new record duplicates an old record on a unique key value.

If you get performance problems in making backups on your system, you can solve this by setting up replication and do the backups on the slave instead of on the master. See section 4.10.1 Introduction.

If you are using a Veritas file system, you can do:

  1. Execute in a client (perl ?) FLUSH TABLES WITH READ LOCK
  2. Fork a shell or execute in another client mount vxfs snapshot.
  3. Execute in the first client UNLOCK TABLES
  4. Copy files from snapshot
  5. Unmount snapshot

4.4.2 BACKUP TABLE Syntax

BACKUP TABLE tbl_name[,tbl_name...] TO '/path/to/backup/directory'

Make a copy of all the table files to the backup directory that are the minimum needed to restore it. Currenlty only works for MyISAM tables. For MyISAM table, copies .frm (definition) and .MYD (data) files. The index file can be rebuilt from those two.

Before using this command, please see See section 4.4.1 Database Backups.

During the backup, read lock will be held for each table, one at time, as they are being backed up. If you want to backup several tables as a snapshot, you must first issue LOCK TABLES obtaining a read lock for each table in the group.

The command returns a table with the following columns:

Column Value
Table Table name
Op Always ``backup''
Msg_type One of status, error, info or warning.
Msg_text The message.

Note that BACKUP TABLE is only available in MySQL version 3.23.25 and later.

4.4.3 RESTORE TABLE Syntax

RESTORE TABLE tbl_name[,tbl_name...] FROM '/path/to/backup/directory'

Restores the table(s) from the backup that was made with BACKUP TABLE. Existing tables will not be overwritten - if you try to restore over an existing table, you will get an error. Restore will take longer than BACKUP due to the need to rebuilt the index. The more keys you have, the longer it is going to take. Just as BACKUP TABLE, currently only works of MyISAM tables.

The command returns a table with the following columns:

Column Value
Table Table name
Op Always ``restore''
Msg_type One of status, error, info or warning.
Msg_text The message.

4.4.4 CHECK TABLE Syntax

CHECK TABLE tbl_name[,tbl_name...] [option [option...]]

option = QUICK | FAST | MEDIUM | EXTENDED | CHANGED

CHECK TABLE only works on MyISAM tables. On MyISAM tables it's the same thing as running myisamchk -m table_name on the table.

If you don't specify any option MEDIUM is used.

Checks the table(s) for errors. For MyISAM tables the key statistics is updated. The command returns a table with the following columns:

Column Value
Table Table name.
Op Always ``check''.
Msg_type One of status, error, info, or warning.
Msg_text The message.

Note that you can get many rows of information for each checked table. The last row will be of Msg_type status and should normally be OK. If you don't get OK, or Not checked you should normally run a repair of the table. See section 4.4.6 Using myisamchk for Table Maintenance and Crash Recovery. Not checked means that the table the given TYPE told MySQL that there wasn't any need to check the table.

The different check types stand for the following:

Type Meaning
QUICK Don't scan the rows to check for wrong links.
FAST Only check tables which haven't been closed properly.
CHANGED Only check tables which have been changed since last check or haven't been closed properly.
MEDIUM Scan rows to verify that deleted links are ok. This also calculates a key checksum for the rows and verifies this with a calcualted checksum for the keys.
EXTENDED Do a full key lookup for all keys for each row. This ensures that the table is 100 % consistent, but will take a long time!

For dynamic sized MyISAM tables a started check will always do a MEDIUM check. For static size rows we skip the row scan for QUICK and FAST as the rows are very seldom corrupted.

You can combine check options as in:

CHECK TABLE test_table FAST QUICK;

Which only would do a quick check on the table if it wasn't closed properly.

NOTE: that in some case CHECK TABLE will change the table! This happens if the table is marked as 'corrupted' or 'not closed properly' but CHECK TABLE didn't find any problems in the table. In this case CHECK TABLE will mark the table as ok.

If a table is corrupted, then it's most likely that the problem is in the indexes and not in the data part. All of the above check types checks the indexes throughly and should thus find most errors.

If you just want to check a table that you assume is ok, you should use no check options or the QUICK option. The later should be used when you are in a hurry and can take the very small risk that QUICK didn't find an error in the data file (In most cases MySQL should find, under normal usage, any error in the data file. If this happens then the table will be marked as 'corrupted', in which case the table can't be used until it's repaired).

FAST and CHANGED are mostly intended to be used from a script (for example to be executed from cron) if you want to check your table from time to time. In most cases you FAST is to be prefered over CHANGED. (The only case when it isn't is when you suspect a bug you have found a bug in the MyISAM code.).

EXTENDED is only to be used after you have run a normal check but still get strange errors from a table when MySQL tries to update a row or find a row by key (this is VERY unlikely to happen if a normal check has succeeded!).

Some things reported by check table, can't be corrected automatically:

4.4.5 REPAIR TABLE Syntax

REPAIR TABLE tbl_name[,tbl_name...] [QUICK] [EXTENDED]

REPAIR TABLE only works on MyISAM tables and is the same as running myisamchk -r table_name on the table.

Normally you should never have to run this command, but if disaster strikes you are very likely to get back all your data from a MyISAM table with REPAIR TABLE. If your tables get corrupted a lot you should try to find the reason for this! See section A.4.1 What To Do If MySQL Keeps Crashing. See section 7.1.3 MyISAM table problems..

REPAIR TABLE repairs a possible corrupted table. The command returns a table with the following columns:

Column Value
Table Table name
Op Always ``repair''
Msg_type One of status, error, info or warning.
Msg_text The message.

Note that you can get many rows of information for each repaired table. The last one row will be of Msg_type status and should normally be OK. If you don't get OK, you should try repairing the table with myisamchk -o, as REPAIR TABLE does not yet implement all the options of myisamchk. In the near future, we will make it more flexible.

If QUICK is given then MySQL will try to do a REPAIR of only the index tree.

If you use EXTENDED then MySQL will create the index row by row instead of creating one index at a time with sorting; This may be better than sorting on fixed-length keys if you have long char() keys that compress very good.

4.4.6 Using myisamchk for Table Maintenance and Crash Recovery

Starting with MySQL Version 3.23.13, you can check MyISAM tables with the CHECK TABLE command. See section 4.4.4 CHECK TABLE Syntax. You can repair tables with the REPAIR TABLE command. See section 4.4.5 REPAIR TABLE Syntax.

To check/repair MyISAM tables (.MYI and .MYD) you should use the myisamchk utility. To check/repair ISAM tables (.ISM and .ISD) you should use the isamchk utility. See section 7 MySQL Table Types.

In the following text we will talk about myisamchk, but everything also applies to the old isamchk.

You can use the myisamchk utility to get information about your database tables, check and repair them, or optimize them. The following sections describe how to invoke myisamchk (including a description of its options), how to set up a table maintenance schedule, and how to use myisamchk to perform its various functions.

You can, in most cases, also use the command OPTIMIZE TABLES to optimize and repair tables, but this is not as fast or reliable (in case of real fatal errors) as myisamchk. On the other hand, OPTIMIZE TABLE is easier to use and you don't have to worry about flushing tables. See section 4.5.1 OPTIMIZE TABLE Syntax.

Even that the repair in myisamchk is quite secure, it's always a good idea to make a backup BEFORE doing a repair (or anything that could make a lot of changes to a table)

4.4.6.1 myisamchk Invocation Syntax

myisamchk is invoked like this:

shell> myisamchk [options] tbl_name

The options specify what you want myisamchk to do. They are described below. (You can also get a list of options by invoking myisamchk --help.) With no options, myisamchk simply checks your table. To get more information or to tell myisamchk to take corrective action, specify options as described below and in the following sections.

tbl_name is the database table you want to check/repair. If you run myisamchk somewhere other than in the database directory, you must specify the path to the file, because myisamchk has no idea where your database is located. Actually, myisamchk doesn't care whether or not the files you are working on are located in a database directory; you can copy the files that correspond to a database table into another location and perform recovery operations on them there.

You can name several tables on the myisamchk command line if you wish. You can also specify a name as an index file name (with the `.MYI' suffix), which allows you to specify all tables in a directory by using the pattern `*.MYI'. For example, if you are in a database directory, you can check all the tables in the directory like this:

shell> myisamchk *.MYI

If you are not in the database directory, you can check all the tables there by specifying the path to the directory:

shell> myisamchk /path/to/database_dir/*.MYI

You can even check all tables in all databases by specifying a wild card with the path to the MySQL data directory:

shell> myisamchk /path/to/datadir/*/*.MYI

The recommended way to quickly check all tables is:

myisamchk --silent --fast /path/to/datadir/*/*.MYI
isamchk --silent /path/to/datadir/*/*.ISM

If you want to check all tables and repair all tables that are corrupted, you can use the following line:

myisamchk --silent --force --fast --update-state -O key_buffer=64M -O sort_buffer=64M -O read_buffer=1M -O write_buffer=1M /path/to/datadir/*/*.MYI
isamchk --silent --force -O key_buffer=64M -O sort_buffer=64M -O read_buffer=1M -O write_buffer=1M /path/to/datadir/*/*.ISM

The above assumes that you have more than 64 M free.

Note that if you get an error like:

myisamchk: warning: 1 clients is using or hasn't closed the table properly

This means that you are trying to check a table that has been updated by the another program (like the mysqld server) that hasn't yet closed the file or that has died without closing the file properly.

If you mysqld is running, you must force a sync/close of all tables with FLUSH TABLES and ensure that no one is using the tables while you are running myisamchk. In MySQL Version 3.23 the easiest way to avoid this problem is to use CHECK TABLE instead of myisamchk to check tables.

4.4.6.2 General Options for myisamchk

myisamchk supports the following options.

-# or --debug=debug_options
Output debug log. The debug_options string often is 'd:t:o,filename'.
-? or --help
Display a help message and exit.
-O var=option, --set-variable var=option
Set the value of a variable. The possible variables and their default values for myisamchk can be examined with myisamchk --help:
key_buffer_size 523264
read_buffer_size 262136
write_buffer_size 262136
sort_buffer_size 2097144
sort_key_blocks 16
decode_bits 9
sort_buffer_size is used when the keys are repaired by sorting keys, which is the normal case when you use --recover. key_buffer_size is used when you are checking the table with --extended-check or when the keys are repaired by inserting key row by row in to the table (like when doing normal inserts). Repairing through the key buffer is used in the following cases: Reparing through the key buffer takes much less disk space than using sorting, but is also much slower. If you want a faster repair, set the above variables to about 1/4 of your available memory. You can set both variables to big values, as only one of the above buffers will be used at a time.
-s or --silent
Silent mode. Write output only when errors occur. You can use -s twice (-ss) to make myisamchk very silent.
-v or --verbose
Verbose mode. Print more information. This can be used with -d and -e. Use -v multiple times (-vv, -vvv) for more verbosity!
-V or --version
Print the myisamchk version and exit.
-w or, --wait
Instead of giving an error if the table is locked, wait until the table is unlocked before continuing. Note that if you are running mysqld on the table with --skip-locking, the table can only be locked by another myisamchk command.

4.4.6.3 Check Options for myisamchk

-c or --check
Check table for errors. This is the default operation if you are not giving myisamchk any options that override this.
-e or --extend-check
Check the table VERY thoroughly (which is quite slow if you have many indexes). This option should only be used in extreme cases. Normally, myisamchk or myisamchk --medium-check should, in most cases, be able to find out if there are any errors in the table. If you are using --extended-check and have much memory, you should increase the value of key_buffer_size a lot!
-F or --fast
Check only tables that haven't been closed properly.
-C or --check-only-changed
Check only tables that have changed since the last check.
-f or --force
Restart myisamchk with -r (repair) on the table, if myisamchk finds any errors in the table.
-i or --information
Print informational statistics about the table that is checked.
-m or --medium-check
Faster than extended-check, but only finds 99.99% of all errors. Should, however, be good enough for most cases.
-U or --update-state
Store in the `.MYI' file when the table was checked and if the table crashed. This should be used to get full benefit of the --check-only-changed option, but you shouldn't use this option if the mysqld server is using the table and you are running mysqld with --skip-locking.
-T or --read-only
Don't mark table as checked. This is useful if you use myisamchk to check a table that is in use by some other application that doesn't use locking (like mysqld --skip-locking).

4.4.6.4 Repair Options for myisamchk

The following options are used if you start myisamchk with -r or -o:

-D # or --data-file-length=#
Max length of data file (when re-creating data file when it's 'full').
-e or --extend-check
Try to recover every possible row from the data file. Normally this will also find a lot of garbage rows. Don't use this option if you are not totally desperate.
-f or --force
Overwrite old temporary files (table_name.TMD) instead of aborting.
-k # or keys-used=#
If you are using ISAM, tells the ISAM table handler to update only the first # indexes. If you are using MyISAM, tells which keys to use, where each binary bit stands for one key (first key is bit 0). This can be used to get faster inserts! Deactivated indexes can be reactivated by using myisamchk -r. keys.
-l or --no-symlinks
Do not follow symbolic links. Normally myisamchk repairs the table a symlink points at. This option doesn't exist in MySQL 4.0, as MySQL 4.0 will not remove symlinks during repair.
-r or --recover
Can fix almost anything except unique keys that aren't unique (which is an extremely unlikely error with ISAM/MyISAM tables). If you want to recover a table, this is the option to try first. Only if myisamchk reports that the table can't be recovered by -r, you should then try -o. (Note that in the unlikely case that -r fails, the data file is still intact.) If you have lots of memory, you should increase the size of sort_buffer_size!
-o or --safe-recover
Uses an old recovery method (reads through all rows in order and updates all index trees based on the found rows); this is a magnitude slower than -r, but can handle a couple of very unlikely cases that -r cannot handle. This recovery method also uses much less disk space than -r. Normally one should always first repair with -r, and only if this fails use -o. If you have lots of memory, you should increase the size of key_buffer_size!
-n or --sort-recover
Force myisamchk to use sorting to resolve the keys even if the temporary files should be very big. This will not have any effect if you have fulltext keys in the table.
--character-sets-dir=...
Directory where character sets are stored.
--set-character-set=name
Change the character set used by the index
.t or --tmpdir=path
Path for storing temporary files. If this is not set, myisamchk will use the environment variable TMPDIR for this.
-q or --quick
Faster repair by not modifying the data file. One can give a second -q to force myisamchk to modify the original datafile in case of duplicate keys
-u or --unpack
Unpack file packed with myisampack.

4.4.6.5 Other Options for myisamchk

Other actions that myisamchk can do, besides repair and check tables:

-a or --analyze
Analyze the distribution of keys. This improves join performance by enabling the join optimizer to better choose in which order it should join the tables and which keys it should use: myisamchk --describe --verbose table_name' or using SHOW KEYS in MySQL.
-d or --description
Prints some information about table.
-A or --set-auto-increment[=value]
Force auto_increment to start at this or higher value. If no value is given, then sets the next auto_increment value to the highest used value for the auto key + 1.
-S or --sort-index
Sort the index tree blocks in high-low order. This will optimize seeks and will make table scanning by key faster.
-R or --sort-records=#
Sorts records according to an index. This makes your data much more localized and may speed up ranged SELECT and ORDER BY operations on this index. (It may be VERY slow to do a sort the first time!) To find out a table's index numbers, use SHOW INDEX, which shows a table's indexes in the same order that myisamchk sees them. Indexes are numbered beginning with 1.

4.4.6.6 myisamchk Memory Usage

Memory allocation is important when you run myisamchk. myisamchk uses no more memory than you specify with the -O options. If you are going to use myisamchk on very large files, you should first decide how much memory you want it to use. The default is to use only about 3M to fix things. By using larger values, you can get myisamchk to operate faster. For example, if you have more than 32M RAM, you could use options such as these (in addition to any other options you might specify):

shell> myisamchk -O sort=16M -O key=16M -O read=1M -O write=1M ...

Using -O sort=16M should probably be enough for most cases.

Be aware that myisamchk uses temporary files in TMPDIR. If TMPDIR points to a memory file system, you may easily get out of memory errors. If this happens, set TMPDIR to point at some directory with more space and restart myisamchk.

When repairing, myisamchk will also need a lot of disk space:

If you have a problem with disk space during repair, you can try to use --safe-recover instead of --recover.

4.4.6.7 Using myisamchk for Crash Recovery

If you run mysqld with --skip-locking (which is the default on some systems, like Linux), you can't reliably use myisamchk to check a table when mysqld is using the same table. If you can be sure that no one is accessing the tables through mysqld while you run myisamchk, you only have to do mysqladmin flush-tables before you start checking the tables. If you can't guarantee the above, then you must take down mysqld while you check the tables. If you run myisamchk while mysqld is updating the tables, you may get a warning that a table is corrupt even if it isn't.

If you are not using --skip-locking, you can use myisamchk to check tables at any time. While you do this, all clients that try to update the table will wait until myisamchk is ready before continuing.

If you use myisamchk to repair or optimize tables, you MUST always ensure that the mysqld server is not using the table (this also applies if you are using --skip-locking). If you don't take down mysqld you should at least do a mysqladmin flush-tables before you run myisamchk.

This chapter describes how to check for and deal with data corruption in MySQL databases. If your tables get corrupted a lot you should try to find the reason for this! See section A.4.1 What To Do If MySQL Keeps Crashing.

The MyISAM table section contains reason for why a table could be corrupted. See section 7.1.3 MyISAM table problems..

When performing crash recovery, it is important to understand that each table tbl_name in a database corresponds to three files in the database directory:

File Purpose
`tbl_name.frm' Table definition (form) file
`tbl_name.MYD' Data file
`tbl_name.MYI' Index file

Each of these three file types is subject to corruption in various ways, but problems occur most often in data files and index files.

myisamchk works by creating a copy of the `.MYD' (data) file row by row. It ends the repair stage by removing the old `.MYD' file and renaming the new file to the original file name. If you use --quick, myisamchk does not create a temporary `.MYD' file, but instead assumes that the `.MYD' file is correct and only generates a new index file without touching the `.MYD' file. This is safe, because myisamchk automatically detects if the `.MYD' file is corrupt and aborts the repair in this case. You can also give two --quick options to myisamchk. In this case, myisamchk does not abort on some errors (like duplicate key) but instead tries to resolve them by modifying the `.MYD' file. Normally the use of two --quick options is useful only if you have too little free disk space to perform a normal repair. In this case you should at least make a backup before running myisamchk.

4.4.6.8 How to Check Tables for Errors

To check a MyISAM table, use the following commands:

myisamchk tbl_name
This finds 99.99% of all errors. What it can't find is corruption that involves ONLY the data file (which is very unusual). If you want to check a table, you should normally run myisamchk without options or with either the -s or --silent option.
myisamchk -m tbl_name
This finds 99.999% of all errors. It checks first all index entries for errors and then it reads through all rows. It calculates a checksum for all keys in the rows and verifies that they checksum matches the checksum for the keys in the index tree.
myisamchk -e tbl_name
This does a complete and thorough check of all data (-e means ``extended check''). It does a check-read of every key for each row to verify that they indeed point to the correct row. This may take a LONG time on a big table with many keys. myisamchk will normally stop after the first error it finds. If you want to obtain more information, you can add the --verbose (-v) option. This causes myisamchk to keep going, up through a maximum of 20 errors. In normal usage, a simple myisamchk (with no arguments other than the table name) is sufficient.
myisamchk -e -i tbl_name
Like the previous command, but the -i option tells myisamchk to print some informational statistics, too.

4.4.6.9 How to Repair Tables

In the following section we only talk about using myisamchk on MyISAM tables (extensions .MYI and .MYD). If you are using ISAM tables (extensions .ISM and .ISD), you should use isamchk instead.

Starting with MySQL Version 3.23.14, you can repair MyISAM tables with the REPAIR TABLE command. See section 4.4.5 REPAIR TABLE Syntax.

The symptoms of a corrupted table include queries that abort unexpectedly and observable errors such as these:

In the other cases, you must repair your tables. myisamchk can usually detect and fix most things that go wrong.

The repair process involves up to four stages, described below. Before you begin, you should cd to the database directory and check the permissions of the table files. Make sure they are readable by the Unix user that mysqld runs as (and to you, because you need to access the files you are checking). If it turns out you need to modify files, they must also be writable by you.

If you are using MySQL Version 3.23.16 and above, you can (and should) use the CHECK and REPAIR commands to check and repair MyISAM tables. See section 4.4.4 CHECK TABLE Syntax. See section 4.4.5 REPAIR TABLE Syntax.

The manual section about table maintenance includes the options to isamchk/myisamchk. See section 4.4.6 Using myisamchk for Table Maintenance and Crash Recovery.

The following section is for the cases where the above command fails or if you want to use the extended features that isamchk/myisamchk provides.

If you are going to repair a table from the command line, you must first take down the mysqld server. Note that when you do mysqladmin shutdown on a remote server, the mysqld server will still be alive for a while after mysqladmin returns, until all queries are stopped and all keys have been flushed to disk.

Stage 1: Checking your tables

Run myisamchk *.MYI or myisamchk -e *.MYI if you have more time. Use the -s (silent) option to suppress unnecessary information.

If the mysqld server is done you should use the --update option to tell myisamchk to mark the table as 'checked'.

You have to repair only those tables for which myisamchk announces an error. For such tables, proceed to Stage 2.

If you get weird errors when checking (such as out of memory errors), or if myisamchk crashes, go to Stage 3.

Stage 2: Easy safe repair

NOTE: If you want repairing to go much faster, you should add: -O sort_buffer=# -O key_buffer=# (where # is about 1/4 of the available memory) to all isamchk/myisamchk commands.

First, try myisamchk -r -q tbl_name (-r -q means ``quick recovery mode''). This will attempt to repair the index file without touching the data file. If the data file contains everything that it should and the delete links point at the correct locations within the data file, this should work, and the table is fixed. Start repairing the next table. Otherwise, use the following procedure:

  1. Make a backup of the data file before continuing.
  2. Use myisamchk -r tbl_name (-r means ``recovery mode''). This will remove incorrect records and deleted records from the data file and reconstruct the index file.
  3. If the preceding step fails, use myisamchk --safe-recover tbl_name. Safe recovery mode uses an old recovery method that handles a few cases that regular recovery mode doesn't (but is slower).

If you get weird errors when repairing (such as out of memory errors), or if myisamchk crashes, go to Stage 3.

Stage 3: Difficult repair

You should only reach this stage if the first 16K block in the index file is destroyed or contains incorrect information, or if the index file is missing. In this case, it's necessary to create a new index file. Do so as follows:

  1. Move the data file to some safe place.
  2. Use the table description file to create new (empty) data and index files:
    shell> mysql db_name
    mysql> SET AUTOCOMMIT=1;
    mysql> TRUNCATE TABLE table_name;
    mysql> quit
    
    If your SQL version doesn't have TRUNCATE TABLE, use DELETE FROM table_name instead.
  3. Copy the old data file back onto the newly created data file. (Don't just move the old file back onto the new file; you want to retain a copy in case something goes wrong.)

Go back to Stage 2. myisamchk -r -q should work now. (This shouldn't be an endless loop.)

Stage 4: Very difficult repair

You should reach this stage only if the description file has also crashed. That should never happen, because the description file isn't changed after the table is created:

  1. Restore the description file from a backup and go back to Stage 3. You can also restore the index file and go back to Stage 2. In the latter case, you should start with myisamchk -r.
  2. If you don't have a backup but know exactly how the table was created, create a copy of the table in another database. Remove the new data file, then move the description and index files from the other database to your crashed database. This gives you new description and index files, but leaves the data file alone. Go back to Stage 2 and attempt to reconstruct the index file.

4.4.6.10 Table Optimization

To coalesce fragmented records and eliminate wasted space resulting from deleting or updating records, run myisamchk in recovery mode:

shell> myisamchk -r tbl_name

You can optimize a table in the same way using the SQL OPTIMIZE TABLE statement. OPTIMIZE TABLE does a repair of the table, a key analyzes and also sorts the index tree to give faster key lookups. There is also no possibility of unwanted interaction between a utility and the server, because the server does all the work when you use OPTIMIZE TABLE. See section 4.5.1 OPTIMIZE TABLE Syntax.

myisamchk also has a number of other options you can use to improve the performance of a table:

-S, --sort-index
-R index_num, --sort-records=index_num
-a, --analyze

For a full description of the option. See section 4.4.6.1 myisamchk Invocation Syntax.

4.4.7 Setting Up a Table Maintenance Regimen

Starting with MySQL Version 3.23.13, you can check MyISAM tables with the CHECK TABLE command. See section 4.4.4 CHECK TABLE Syntax. You can repair tables with the REPAIR TABLE command. See section 4.4.5 REPAIR TABLE Syntax.

It is a good idea to perform table checks on a regular basis rather than waiting for problems to occur. For maintenance purposes, you can use myisamchk -s to check tables. The -s option (short for --silent) causes myisamchk to run in silent mode, printing messages only when errors occur.

It's also a good idea to check tables when the server starts up. For example, whenever the machine has done a reboot in the middle of an update, you usually need to check all the tables that could have been affected. (This is an ``expected crashed table''.) You could add a test to safe_mysqld that runs myisamchk to check all tables that have been modified during the last 24 hours if there is an old `.pid' (process ID) file left after a reboot. (The `.pid' file is created by mysqld when it starts up and removed when it terminates normally. The presence of a `.pid' file at system startup time indicates that mysqld terminated abnormally.)

An even better test would be to check any table whose last-modified time is more recent than that of the `.pid' file.

You should also check your tables regularly during normal system operation. At MySQL AB, we run a cron job to check all our important tables once a week, using a line like this in a `crontab' file:

35 0 * * 0 /path/to/myisamchk --fast --silent /path/to/datadir/*/*.MYI

This prints out information about crashed tables so we can examine and repair them when needed.

As we haven't had any unexpectedly crashed tables (tables that become corrupted for reasons other than hardware trouble) for a couple of years now (this is really true), once a week is more than enough for us.

We recommend that to start with, you execute myisamchk -s each night on all tables that have been updated during the last 24 hours, until you come to trust MySQL as much as we do.

Normally you don't need to maintain MySQL tables that much. If you are changing tables with dynamic size rows (tables with VARCHAR, BLOB or TEXT columns) or have tables with many deleted rows you may want to from time to time (once a month?) defragment/reclaim space from the tables.

You can do this by using OPTIMIZE TABLE on the tables in question or if you can take the mysqld server down for a while do:

isamchk -r --silent --sort-index -O sort_buffer_size=16M */*.ISM
myisamchk -r --silent --sort-index  -O sort_buffer_size=16M */*.MYI

4.4.8 Getting Information About a Table

To get a description of a table or statistics about it, use the commands shown below. We explain some of the information in more detail later:

myisamchk -d tbl_name
Runs myisamchk in ``describe mode'' to produce a description of your table. If you start the MySQL server using the --skip-locking option, myisamchk may report an error for a table that is updated while it runs. However, because myisamchk doesn't change the table in describe mode, there isn't any risk of destroying data.
myisamchk -d -v tbl_name
To produce more information about what myisamchk is doing, add -v to tell it to run in verbose mode.
myisamchk -eis tbl_name
Shows only the most important information from a table. It is slow because it must read the whole table.
myisamchk -eiv tbl_name
This is like -eis, but tells you what is being done.

Example of myisamchk -d output:

MyISAM file:     company.MYI
Record format:   Fixed length
Data records:    1403698  Deleted blocks:         0
Recordlength:    226

table description:
Key Start Len Index   Type
1   2     8   unique  double
2   15    10  multip. text packed stripped
3   219   8   multip. double
4   63    10  multip. text packed stripped
5   167   2   multip. unsigned short
6   177   4   multip. unsigned long
7   155   4   multip. text
8   138   4   multip. unsigned long
9   177   4   multip. unsigned long
    193   1           text

Example of myisamchk -d -v output:

MyISAM file:         company
Record format:       Fixed length
File-version:        1
Creation time:       1999-10-30 12:12:51
Recover time:        1999-10-31 19:13:01
Status:              checked
Data records:           1403698  Deleted blocks:              0
Datafile parts:         1403698  Deleted data:                0
Datafilepointer (bytes):      3  Keyfile pointer (bytes):     3
Max datafile length: 3791650815  Max keyfile length: 4294967294
Recordlength:               226

table description:
Key Start Len Index   Type                  Rec/key     Root Blocksize
1   2     8   unique  double                      1 15845376      1024
2   15    10  multip. text packed stripped        2 25062400      1024
3   219   8   multip. double                     73 40907776      1024
4   63    10  multip. text packed stripped        5 48097280      1024
5   167   2   multip. unsigned short           4840 55200768      1024
6   177   4   multip. unsigned long            1346 65145856      1024
7   155   4   multip. text                     4995 75090944      1024
8   138   4   multip. unsigned long              87 85036032      1024
9   177   4   multip. unsigned long             178 96481280      1024
    193   1           text

Example of myisamchk -eis output:

Checking MyISAM file: company
Key:  1:  Keyblocks used:  97%  Packed:    0%  Max levels:  4
Key:  2:  Keyblocks used:  98%  Packed:   50%  Max levels:  4
Key:  3:  Keyblocks used:  97%  Packed:    0%  Max levels:  4
Key:  4:  Keyblocks used:  99%  Packed:   60%  Max levels:  3
Key:  5:  Keyblocks used:  99%  Packed:    0%  Max levels:  3
Key:  6:  Keyblocks used:  99%  Packed:    0%  Max levels:  3
Key:  7:  Keyblocks used:  99%  Packed:    0%  Max levels:  3
Key:  8:  Keyblocks used:  99%  Packed:    0%  Max levels:  3
Key:  9:  Keyblocks used:  98%  Packed:    0%  Max levels:  4
Total:    Keyblocks used:  98%  Packed:   17%

Records:          1403698    M.recordlength:     226   Packed:             0%
Recordspace used:     100%   Empty space:          0%  Blocks/Record:   1.00
Record blocks:    1403698    Delete blocks:        0
Recorddata:     317235748    Deleted data:         0
Lost space:             0    Linkdata:             0

User time 1626.51, System time 232.36
Maximum resident set size 0, Integral resident set size 0
Non physical pagefaults 0, Physical pagefaults 627, Swaps 0
Blocks in 0 out 0, Messages in 0 out 0, Signals 0
Voluntary context switches 639, Involuntary context switches 28966

Example of myisamchk -eiv output:

Checking MyISAM file: company
Data records: 1403698   Deleted blocks:       0
- check file-size
- check delete-chain
block_size 1024:
index  1:
index  2:
index  3:
index  4:
index  5:
index  6:
index  7:
index  8:
index  9:
No recordlinks
- check index reference
- check data record references index: 1
Key:  1:  Keyblocks used:  97%  Packed:    0%  Max levels:  4
- check data record references index: 2
Key:  2:  Keyblocks used:  98%  Packed:   50%  Max levels:  4
- check data record references index: 3
Key:  3:  Keyblocks used:  97%  Packed:    0%  Max levels:  4
- check data record references index: 4
Key:  4:  Keyblocks used:  99%  Packed:   60%  Max levels:  3
- check data record references index: 5
Key:  5:  Keyblocks used:  99%  Packed:    0%  Max levels:  3
- check data record references index: 6
Key:  6:  Keyblocks used:  99%  Packed:    0%  Max levels:  3
- check data record references index: 7
Key:  7:  Keyblocks used:  99%  Packed:    0%  Max levels:  3
- check data record references index: 8
Key:  8:  Keyblocks used:  99%  Packed:    0%  Max levels:  3
- check data record references index: 9
Key:  9:  Keyblocks used:  98%  Packed:    0%  Max levels:  4
Total:    Keyblocks used:   9%  Packed:   17%

- check records and index references
[LOTS OF ROW NUMBERS DELETED]

Records:          1403698    M.recordlength:     226   Packed:             0%
Recordspace used:     100%   Empty space:          0%  Blocks/Record:   1.00
Record blocks:    1403698    Delete blocks:        0
Recorddata:     317235748    Deleted data:         0
Lost space:             0    Linkdata:             0

User time 1639.63, System time 251.61
Maximum resident set size 0, Integral resident set size 0
Non physical pagefaults 0, Physical pagefaults 10580, Swaps 0
Blocks in 4 out 0, Messages in 0 out 0, Signals 0
Voluntary context switches 10604, Involuntary context switches 122798

Here are the sizes of the data and index files for the table used in the preceding examples:

-rw-rw-r--   1 monty    tcx     317235748 Jan 12 17:30 company.MYD
-rw-rw-r--   1 davida   tcx      96482304 Jan 12 18:35 company.MYM

Explanations for the types of information myisamchk produces are given below. The ``keyfile'' is the index file. ``Record'' and ``row'' are synonymous:

ISAM file
Name of the ISAM (index) file.
Isam-version
Version of ISAM format. Currently always 2.
Creation time
When the data file was created.
Recover time
When the index/data file was last reconstructed.
Data records
How many records are in the table.
Deleted blocks
How many deleted blocks still have reserved space. You can optimize your table to minimize this space. See section 4.4.6.10 Table Optimization.
Datafile: Parts
For dynamic record format, this indicates how many data blocks there are. For an optimized table without fragmented records, this is the same as Data records.
Deleted data
How many bytes of non-reclaimed deleted data there are. You can optimize your table to minimize this space. See section 4.4.6.10 Table Optimization.
Datafile pointer
The size of the data file pointer, in bytes. It is usually 2, 3, 4, or 5 bytes. Most tables manage with 2 bytes, but this cannot be controlled from MySQL yet. For fixed tables, this is a record address. For dynamic tables, this is a byte address.
Keyfile pointer
The size of the index file pointer, in bytes. It is usually 1, 2, or 3 bytes. Most tables manage with 2 bytes, but this is calculated automatically by MySQL. It is always a block address.
Max datafile length
How long the table's data file (.MYD file) can become, in bytes.
Max keyfile length
How long the table's key file (.MYI file) can become, in bytes.
Recordlength
How much space each record takes, in bytes.
Record format
The format used to store table rows. The examples shown above use Fixed length. Other possible values are Compressed and Packed.
table description
A list of all keys in the table. For each key, some low-level information is presented:
Key
This key's number.
Start
Where in the record this index part starts.
Len
How long this index part is. For packed numbers, this should always be the full length of the column. For strings, it may be shorter than the full length of the indexed column, because you can index a prefix of a string column.
Index
unique or multip. (multiple). Indicates whether or not one value can exist multiple times in this index.
Type
What data-type this index part has. This is an ISAM data-type with the options packed, stripped or empty.
Root
Address of the root index block.
Blocksize
The size of each index block. By default this is 1024, but the value may be changed at compile time.
Rec/key
This is a statistical value used by the optimizer. It tells how many records there are per value for this key. A unique key always has a value of 1. This may be updated after a table is loaded (or greatly changed) with myisamchk -a. If this is not updated at all, a default value of 30 is given.
In the first example above, the 9th key is a multi-part key with two parts.
Keyblocks used
What percentage of the keyblocks are used. Because the table used in the examples had just been reorganized with myisamchk, the values are very high (very near the theoretical maximum).
Packed
MySQL tries to pack keys with a common suffix. This can only be used for CHAR/VARCHAR/DECIMAL keys. For long strings like names, this can significantly reduce the space used. In the third example above, the 4th key is 10 characters long and a 60% reduction in space is achieved.
Max levels
How deep the B-tree for this key is. Large tables with long keys get high values.
Records
How many rows are in the table.
M.recordlength
The average record length. For tables with fixed-length records, this is the exact record length.
Packed
MySQL strips spaces from the end of strings. The Packed value indicates the percentage of savings achieved by doing this.
Recordspace used
What percentage of the data file is used.
Empty space
What percentage of the data file is unused.
Blocks/Record
Average number of blocks per record (that is, how many links a fragmented record is composed of). This is always 1 for fixed-format tables. This value should stay as close to 1.0 as possible. If it gets too big, you can reorganize the table with myisamchk. See section 4.4.6.10 Table Optimization.
Recordblocks
How many blocks (links) are used. For fixed format, this is the same as the number of records.
Deleteblocks
How many blocks (links) are deleted.
Recorddata
How many bytes in the data file are used.
Deleted data
How many bytes in the data file are deleted (unused).
Lost space
If a record is updated to a shorter length, some space is lost. This is the sum of all such losses, in bytes.
Linkdata
When the dynamic table format is used, record fragments are linked with pointers (4 to 7 bytes each). Linkdata is the sum of the amount of storage used by all such pointers.

If a table has been compressed with myisampack, myisamchk -d prints additional information about each table column. See section 4.7.4 myisampack, The MySQL Compressed Read-only Table Generator, for an example of this information and a description of what it means.

4.5 Database Administration Language Reference

4.5.1 OPTIMIZE TABLE Syntax

OPTIMIZE TABLE tbl_name[,tbl_name]...

OPTIMIZE TABLE should be used if you have deleted a large part of a table or if you have made many changes to a table with variable-length rows (tables that have VARCHAR, BLOB, or TEXT columns). Deleted records are maintained in a linked list and subsequent INSERT operations reuse old record positions. You can use OPTIMIZE TABLE to reclaim the unused space and to defragment the data file.

For the moment OPTIMIZE TABLE only works on MyISAM and BDB tables. For BDB tables, OPTIMIZE TABLE is currently mapped to ANALYZE TABLE. See section 4.5.2 ANALYZE TABLE Syntax.

You can get optimize table to work on other table types by starting mysqld with --skip-new or --safe-mode, but in this case OPTIMIZE TABLE is just mapped to ALTER TABLE.

OPTIMIZE TABLE works the following way:

OPTIMIZE TABLE for MyISAM tables is equvialent of running myisamchk --quick --check-changed-tables --sort-index --analyze on the table.

Note that the table is locked during the time OPTIMIZE TABLE is running!

4.5.2 ANALYZE TABLE Syntax

ANALYZE TABLE tbl_name[,tbl_name...]

Analyze and store the key distribution for the table. During the analyze the table is locked with a read lock. This works on MyISAM and BDB tables.

This is equivalent to running myisamchk -a on the table.

MySQL uses the stored key distribution to decide in which order tables should be joined when one does a join on something else than a constant.

The command returns a table with the following columns:

Column Value
Table Table name
Op Always ``analyze''
Msg_type One of status, error, info or warning.
Msg_text The message.

You can check the stored key distribution with the SHOW INDEX command. See section 4.5.5.1 Retrieving information about Database, Tables, Columns, and Indexes.

If the table hasn't changed since the last ANALYZE TABLE command, the table will not be analyzed again.

4.5.3 FLUSH Syntax

FLUSH flush_option [,flush_option]

You should use the FLUSH command if you want to clear some of the internal caches MySQL uses. To execute FLUSH, you must have the RELOAD privilege.

flush_option can be any of the following:

HOSTS Empties the host cache tables. You should flush the host tables if some of your hosts change IP number or if you get the error message Host ... is blocked. When more than max_connect_errors errors occur in a row for a given host while connection to the MySQL server, MySQL assumes something is wrong and blocks the host from further connection requests. Flushing the host tables allows the host to attempt to connect again. See section A.2.4 Host '...' is blocked Error.) You can start mysqld with -O max_connection_errors=999999999 to avoid this error message.
LOGS Closes and reopens all log files. If you have specified the update log file or a binary log file without an extension, the extension number of the log file will be incremented by one relative to the previous file. If you have used an extension in the file name, MySQL will close and reopen the update log file. See section 4.9.3 The Update Log. This is the same thing as sending the SIGHUP signal to the mysqld server.
PRIVILEGES Reloads the privileges from the grant tables in the mysql database.
TABLES Closes all open tables and force all tables in use to be closed.
[TABLE | TABLES] table_name [,table_name...] Flushes only the given tables.
TABLES WITH READ LOCK Closes all open tables and locks all tables for all databases with a read until one executes UNLOCK TABLES. This is very convenient way to get backups if you have a file system, like Veritas,that can take snapshots in time.
STATUS Resets most status variables to zero. This is something one should only use when debugging a query.

You can also access each of the commands shown above with the mysqladmin utility, using the flush-hosts, flush-logs, reload, or flush-tables commands.

Take also a look at the RESET command used with replication. See section 4.10.6 SQL Commands Related to Replication.

4.5.4 KILL Syntax

KILL thread_id

Each connection to mysqld runs in a separate thread. You can see which threads are running with the SHOW PROCESSLIST command and kill a thread with the KILL thread_id command.

If you have the process privilege, you can see and kill all threads. Otherwise, you can see and kill only your own threads.

You can also use the mysqladmin processlist and mysqladmin kill commands to examine and kill threads.

When you do a KILL, a thread specific kill flag is set for the thread.

In most cases it may take some time for the thread to die as the kill flag is only checked at specific intervals.

4.5.5 SHOW Syntax

   SHOW DATABASES [LIKE wild]
or SHOW [OPEN] TABLES [FROM db_name] [LIKE wild]
or SHOW [FULL] COLUMNS FROM tbl_name [FROM db_name] [LIKE wild]
or SHOW INDEX FROM tbl_name [FROM db_name]
or SHOW TABLE STATUS [FROM db_name] [LIKE wild]
or SHOW STATUS [LIKE wild]
or SHOW VARIABLES [LIKE wild]
or SHOW LOGS
or SHOW [FULL] PROCESSLIST
or SHOW GRANTS FOR user
or SHOW CREATE TABLE table_name
or SHOW MASTER STATUS
or SHOW MASTER LOGS
or SHOW SLAVE STATUS

SHOW provides information about databases, tables, columns, or status information about the server. If the LIKE wild part is used, the wild string can be a string that uses the SQL `%' and `_' wild-card characters.

4.5.5.1 Retrieving information about Database, Tables, Columns, and Indexes

You can use db_name.tbl_name as an alternative to the tbl_name FROM db_name syntax. These two statements are equivalent:

mysql> SHOW INDEX FROM mytable FROM mydb;
mysql> SHOW INDEX FROM mydb.mytable;

SHOW DATABASES lists the databases on the MySQL server host. You can also get this list using the mysqlshow command.

SHOW TABLES lists the tables in a given database. You can also get this list using the mysqlshow db_name command.

NOTE: If a user doesn't have any privileges for a table, the table will not show up in the output from SHOW TABLES or mysqlshow db_name.

SHOW OPEN TABLES lists the tables that are currently open in the table cache. See section 5.4.6 How MySQL Opens and Closes Tables. The Comment field tells how many times the table is cached and in_use.

SHOW COLUMNS lists the columns in a given table. If you specify the FULL option, you will also get the privileges you have for each column. If the column types are different than you expect them to be based on a CREATE TABLE statement, note that MySQL sometimes changes column types. See section 6.5.3.1 Silent Column Specification Changes.

The DESCRIBE statement provides information similar to SHOW COLUMNS. See section 6.6.2 DESCRIBE Syntax (Get Information About Columns).

SHOW FIELDS is a synonym for SHOW COLUMNS, and SHOW KEYS is a synonym for SHOW INDEX. You can also list a table's columns or indexes with mysqlshow db_name tbl_name or mysqlshow -k db_name tbl_name.

SHOW INDEX returns the index information in a format that closely resembles the SQLStatistics call in ODBC. The following columns are returned:

Column Meaning
Table Name of the table.
Non_unique 0 if the index can't contain duplicates.
Key_name Name of the index.
Seq_in_index Column sequence number in index, starting with 1.
Column_name Column name.
Collation How the column is sorted in the index. In MySQL, this can have values `A' (Ascending) or NULL (Not sorted).
Cardinality Number of unique values in the index. This is updated by running isamchk -a.
Sub_part Number of indexed characters if the column is only partly indexed. NULL if the entire key is indexed.
Comment Various remarks. For now, it tells whether index is FULLTEXT or not.

Note that as the Cardinality is counted based on statistics stored as integers, it's not necessarily accurate for small tables.

4.5.5.2 SHOW TABLE STATUS

SHOW TABLE STATUS [FROM db_name] [LIKE wild]

SHOW TABLE STATUS (new in Version 3.23) works likes SHOW STATUS, but provides a lot of information about each table. You can also get this list using the mysqlshow --status db_name command. The following columns are returned:

Column Meaning
Name Name of the table.
Type Type of table. See section 7 MySQL Table Types.
Row_format The row storage format (Fixed, Dynamic, or Compressed).
Rows Number of rows.
Avg_row_length Average row length.
Data_length Length of the data file.
Max_data_length Max length of the data file.
Index_length Length of the index file.
Data_free Number of allocated but not used bytes.
Auto_increment Next autoincrement value.
Create_time When the table was created.
Update_time When the data file was last updated.
Check_time When the table was last checked.
Create_options Extra options used with CREATE TABLE.
Comment The comment used when creating the table (or some information why MySQL couldn't access the table information).

InnoDB tables will report the free space in the tablespace in the table comment.

4.5.5.3 SHOW STATUS

SHOW STATUS provides server status information (like mysqladmin extended-status). The output resembles that shown below, though the format and numbers probably differ:

+--------------------------+------------+
| Variable_name            | Value      |
+--------------------------+------------+
| Aborted_clients          | 0          |
| Aborted_connects         | 0          |
| Bytes_received           | 155372598  |
| Bytes_sent               | 1176560426 |
| Connections              | 30023      |
| Created_tmp_disk_tables  | 0          |
| Created_tmp_tables       | 8340       |
| Created_tmp_files        | 60         |
| Delayed_insert_threads   | 0          |
| Delayed_writes           | 0          |
| Delayed_errors           | 0          |
| Flush_commands           | 1          |
| Handler_delete           | 462604     |
| Handler_read_first       | 105881     |
| Handler_read_key         | 27820558   |
| Handler_read_next        | 390681754  |
| Handler_read_prev        | 6022500    |
| Handler_read_rnd         | 30546748   |
| Handler_read_rnd_next    | 246216530  |
| Handler_update           | 16945404   |
| Handler_write            | 60356676   |
| Key_blocks_used          | 14955      |
| Key_read_requests        | 96854827   |
| Key_reads                | 162040     |
| Key_write_requests       | 7589728    |
| Key_writes               | 3813196    |
| Max_used_connections     | 0          |
| Not_flushed_key_blocks   | 0          |
| Not_flushed_delayed_rows | 0          |
| Open_tables              | 1          |
| Open_files               | 2          |
| Open_streams             | 0          |
| Opened_tables            | 44600      |
| Questions                | 2026873    |
| Select_full_join         | 0          |
| Select_full_range_join   | 0          |
| Select_range             | 99646      |
| Select_range_check       | 0          |
| Select_scan              | 30802      |
| Slave_running            | OFF        |
| Slave_open_temp_tables   | 0          |
| Slow_launch_threads      | 0          |
| Slow_queries             | 0          |
| Sort_merge_passes        | 30         |
| Sort_range               | 500        |
| Sort_rows                | 30296250   |
| Sort_scan                | 4650       |
| Table_locks_immediate    | 1920382    |
| Table_locks_waited       | 0          |
| Threads_cached           | 0          |
| Threads_created          | 30022      |
| Threads_connected        | 1          |
| Threads_running          | 1          |
| Uptime                   | 80380      |
+--------------------------+------------+

The status variables listed above have the following meaning:

Variable Meaning
Aborted_clients Number of connections aborted because the client died without closing the connection properly. See section A.2.9 Communication Errors / Aborted Connection.
Aborted_connects Number of tries to connect to the MySQL server that failed. See section A.2.9 Communication Errors / Aborted Connection.
Bytes_received Number of bytes received from all clients.
Bytes_sent Number of bytes sent to all clients.
Connections Number of connection attempts to the MySQL server.
Created_tmp_disk_tables Number of implicit temporary tables on disk created while executing statements.
Created_tmp_tables Number of implicit temporary tables in memory created while executing statements.
Created_tmp_files How many temporary files mysqld have created.
Delayed_insert_threads Number of delayed insert handler threads in use.
Delayed_writes Number of rows written with INSERT DELAYED.
Delayed_errors Number of rows written with INSERT DELAYED for which some error occurred (probably duplicate key).
Flush_commands Number of executed FLUSH commands.
Handler_delete Number of times a row was deleted from a table.
Handler_read_first Number of times the first entry was read from an index. If this is high, it suggests that the server is doing a lot of full index scans, for example, SELECT col1 FROM foo, assuming that col1 is indexed.
Handler_read_key Number of requests to read a row based on a key. If this is high, it is a good indication that your queries and tables are properly indexed.
Handler_read_next Number of requests to read next row in key order. This will be incremented if you are querying an index column with a range constraint. This also will be incremented if you are doing an index scan.
Handler_read_rnd Number of requests to read a row based on a fixed position. This will be high if you are doing a lot of queries that require sorting of the result.
Handler_read_rnd_next Number of requests to read the next row in the datafile. This will be high if you are doing a lot of table scans. Generally this suggests that your tables are not properly indexed or that your queries are not written to take advantage of the indexes you have.
Handler_update Number of requests to update a row in a table.
Handler_write Number of requests to insert a row in a table.
Key_blocks_used The number of used blocks in the key cache.
Key_read_requests The number of requests to read a key block from the cache.
Key_reads The number of physical reads of a key block from disk.
Key_write_requests The number of requests to write a key block to the cache.
Key_writes The number of physical writes of a key block to disk.
Max_used_connections The maximum number of connections in use simultaneously.
Not_flushed_key_blocks Keys blocks in the key cache that has changed but hasn't yet been flushed to disk.
Not_flushed_delayed_rows Number of rows waiting to be written in INSERT DELAY queues.
Open_tables Number of tables that are open.
Open_files Number of files that are open.
Open_streams Number of streams that are open (used mainly for logging).
Opened_tables Number of tables that have been opened.
Select_full_join Number of joins without keys (Should be 0).
Select_full_range_join Number of joins where we used a range search on reference table.
Select_range Number of joins where we used ranges on the first table. (It's normally not critical even if this is big.)
Select_scan Number of joins where we scanned the first table.
Select_range_check Number of joins without keys where we check for key usage after each row (Should be 0).
Questions Number of queries sent to the server.
Slave_open_temp_tables Number of temporary tables currently open by the slave thread
Slow_launch_threads Number of threads that have taken more than slow_launch_time to connect.
Slow_queries Number of queries that have taken more than long_query_time. See section 4.9.5 The Slow Query Log.
Sort_merge_passes Number of merges the sort has to do. If this value is large you should consider increasing sort_buffer.
Sort_range Number of sorts that where done with ranges.
Sort_rows Number of sorted rows.
Sort_scan Number of sorts that where done by scanning the table.
Table_locks_immediate Number of times a table lock was acquired immediately. Available after 3.23.33.
Table_locks_waited Number of times a table lock could not be acquired immediately and a wait was needed. If this is high, and you have performance problems, you should first optimize your queries, and then either split your table(s) or use replication. Available after 3.23.33.
Threads_cached Number of threads in the thread cache.
Threads_connected Number of currently open connections.
Threads_created Number of threads created to handle connections.
Threads_running Number of threads that are not sleeping.
Uptime How many seconds the server has been up.

Some comments about the above:

4.5.5.4 SHOW VARIABLES

SHOW VARIABLES [LIKE wild]

SHOW VARIABLES shows the values of some MySQL system variables. You can also get this information using the mysqladmin variables command. If the default values are unsuitable, you can set most of these variables using command-line options when mysqld starts up. See section 4.1.1 mysqld Command-line Options.

The output resembles that shown below, though the format and numbers may differ somewhat:

+-------------------------+---------------------------+
| Variable_name           | Value                     |
+-------------------------+---------------------------+
| ansi_mode               | OFF                       |
| back_log                | 50                        |
| basedir                 | /my/monty/                |
| bdb_cache_size          | 16777216                  |
| bdb_log_buffer_size     | 32768                     |
| bdb_home                | /my/monty/data/           |
| bdb_max_lock            | 10000                     |
| bdb_logdir              |                           |
| bdb_shared_data         | OFF                       |
| bdb_tmpdir              | /tmp/                     |
| binlog_cache_size       | 32768                     |
| concurrent_insert       | ON                        |
| connect_timeout         | 5                         |
| datadir                 | /my/monty/data/           |
| delay_key_write         | ON                        |
| delayed_insert_limit    | 100                       |
| delayed_insert_timeout  | 300                       |
| delayed_queue_size      | 1000                      |
| flush                   | OFF                       |
| flush_time              | 0                         |
| have_bdb                | YES                       |
| have_innodb             | YES                       |
| have_raid               | YES                       |
| have_ssl                | NO                        |
| init_file               |                           |
| interactive_timeout     | 28800                     |
| join_buffer_size        | 131072                    |
| key_buffer_size         | 16776192                  |
| language                | /my/monty/share/english/  |
| large_files_support     | ON                        |
| log                     | OFF                       |
| log_update              | OFF                       |
| log_bin                 | OFF                       |
| log_slave_updates       | OFF                       |
| long_query_time         | 10                        |
| low_priority_updates    | OFF                       |
| lower_case_table_names  | 0                         |
| max_allowed_packet      | 1048576                   |
| max_binlog_cache_size   | 4294967295                |
| max_connections         | 100                       |
| max_connect_errors      | 10                        |
| max_delayed_threads     | 20                        |
| max_heap_table_size     | 16777216                  |
| max_join_size           | 4294967295                |
| max_sort_length         | 1024                      |
| max_tmp_tables          | 32                        |
| max_write_lock_count    | 4294967295                |
| myisam_recover_options  | DEFAULT                   |
| myisam_sort_buffer_size | 8388608                   |
| net_buffer_length       | 16384                     |
| net_read_timeout        | 30                        |
| net_retry_count         | 10                        |
| net_write_timeout       | 60                        |
| open_files_limit        | 0                         |
| pid_file                | /my/monty/data/donna.pid  |
| port                    | 3306                      |
| protocol_version        | 10                        |
| record_buffer           | 131072                    |
| query_buffer_size       | 0                         |
| safe_show_database      | OFF                       |
| server_id               | 0                         |
| skip_locking            | ON                        |
| skip_networking         | OFF                       |
| skip_show_database      | OFF                       |
| slow_launch_time        | 2                         |
| socket                  | /tmp/mysql.sock           |
| sort_buffer             | 2097116                   |
| table_cache             | 64                        |
| table_type              | MYISAM                    |
| thread_cache_size       | 4                         |
| thread_stack            | 65536                     |
| tmp_table_size          | 1048576                   |
| tmpdir                  | /tmp/                     |
| version                 | 3.23.29a-gamma-debug      |
| wait_timeout            | 28800                     |
+-------------------------+---------------------------+

Each option is described below. Values for buffer sizes, lengths, and stack sizes are given in bytes. You can specify values with a suffix of `K' or `M' to indicate kilobytes or megabytes. For example, 16M indicates 16 megabytes. The case of suffix letters does not matter; 16M and 16m are equivalent:

ansi_mode.
Is ON if mysqld was started with --ansi. See section 1.4.3 Running MySQL in ANSI Mode.
back_log
The number of outstanding connection requests MySQL can have. This comes into play when the main MySQL thread gets VERY many connection requests in a very short time. It then takes some time (although very little) for the main thread to check the connection and start a new thread. The back_log value indicates how many requests can be stacked during this short time before MySQL momentarily stops answering new requests. You need to increase this only if you expect a large number of connections in a short period of time. In other words, this value is the size of the listen queue for incoming TCP/IP connections. Your operating system has its own limit on the size of this queue. The manual page for the Unix listen(2) system call should have more details. Check your OS documentation for the maximum value for this variable. Attempting to set back_log higher than your operating system limit will be ineffective.
basedir
The value of the --basedir option.
bdb_cache_size
The buffer that is allocated to cache index and rows for BDB tables. If you don't use BDB tables, you should start mysqld with --skip-bdb to not waste memory for this cache.
bdb_log_buffer_size
The buffer that is allocated to cache index and rows for BDB tables. If you don't use BDB tables, you should set this to 0 or start mysqld with --skip-bdb to not waste memory for this cache.
bdb_home
The value of the --bdb-home option.
bdb_max_lock
The maximum number of locks (1000 by default) you can have active on a BDB table. You should increase this if you get errors of type bdb: Lock table is out of available locks or Got error 12 from ... when you have do long transactions or when mysqld has to examine a lot of rows to calculate the query.
bdb_logdir
The value of the --bdb-logdir option.
bdb_shared_data
Is ON if you are using --bdb-shared-data.
bdb_tmpdir
The value of the --bdb-tmpdir option.
binlog_cache_size. The size of the cache to hold the SQL
statements for the binary log during a transaction. If you often use big, multi-statement transactions you can increase this to get more performance. See section 6.7.1 BEGIN/COMMIT/ROLLBACK Syntax.
character_set
The default character set.
character_sets
The supported character sets.
concurrent_inserts
If ON (the default), MySQL will allow you to use INSERT on MyISAM tables at the same time as you run SELECT queries on them. You can turn this option off by starting mysqld with --safe or --skip-new.
connect_timeout
The number of seconds the mysqld server is waiting for a connect packet before responding with Bad handshake.
datadir
The value of the --datadir option.
delay_key_write
If enabled (is on by default), MySQL will honor the delay_key_write option CREATE TABLE. This means that the key buffer for tables with this option will not get flushed on every index update, but only when a table is closed. This will speed up writes on keys a lot, but you should add automatic checking of all tables with myisamchk --fast --force if you use this. Note that if you start mysqld with the --delay-key-write-for-all-tables option this means that all tables will be treated as if they were created with the delay_key_write option. You can clear this flag by starting mysqld with --skip-new or --safe-mode.
delayed_insert_limit
After inserting delayed_insert_limit rows, the INSERT DELAYED handler will check if there are any SELECT statements pending. If so, it allows these to execute before continuing.
delayed_insert_timeout
How long a INSERT DELAYED thread should wait for INSERT statements before terminating.
delayed_queue_size
What size queue (in rows) should be allocated for handling INSERT DELAYED. If the queue becomes full, any client that does INSERT DELAYED will wait until there is room in the queue again.
flush
This is ON if you have started MySQL with the --flush option.
flush_time
If this is set to a non-zero value, then every flush_time seconds all tables will be closed (to free up resources and sync things to disk). We only recommend this option on Win95, Win98, or on systems where you have very little resources.
have_bdb
YES if mysqld supports Berkeley DB tables. DISABLED if --skip-bdb is used.
have_innodb
YES if mysqld supports InnoDB tables. DISABLED if --skip-innodb is used.
have_raid
YES if mysqld supports the RAID option.
have_ssl
YES if mysqld supports SSL (encryption) on the client/server protocol.
init_file
The name of the file specified with the --init-file option when you start the server. This is a file of SQL statements you want the server to execute when it starts.
interactive_timeout
The number of seconds the server waits for activity on an interactive connection before closing it. An interactive client is defined as a client that uses the CLIENT_INTERACTIVE option to mysql_real_connect(). See also wait_timeout.
join_buffer_size
The size of the buffer that is used for full joins (joins that do not use indexes). The buffer is allocated one time for each full join between two tables. Increase this value to get a faster full join when adding indexes is not possible. (Normally the best way to get fast joins is to add indexes.)
key_buffer_size
Index blocks are buffered and are shared by all threads. key_buffer_size is the size of the buffer used for index blocks. Increase this to get better index handling (for all reads and multiple writes) to as much as you can afford; 64M on a 256M machine that mainly runs MySQL is quite common. If you, however, make this too big (more than 50% of your total memory?) your system may start to page and become REALLY slow. Remember that because MySQL does not cache data read, that you will have to leave some room for the OS filesystem cache. You can check the performance of the key buffer by doing show status and examine the variables Key_read_requests, Key_reads, Key_write_requests, and Key_writes. The Key_reads/Key_read_request ratio should normally be < 0.01. The Key_write/Key_write_requests is usually near 1 if you are using mostly updates/deletes but may be much smaller if you tend to do updates that affect many at the same time or if you are using delay_key_write. See section 4.5.5 SHOW Syntax. To get even more speed when writing many rows at the same time, use LOCK TABLES. See section 6.7.2 LOCK TABLES/UNLOCK TABLES Syntax.
language
The language used for error messages.
large_file_support
If mysqld was compiled with options for big file support.
locked_in_memory
If mysqld was locked in memory with --memlock
log
If logging of all queries is enabled.
log_update
If the update log is enabled.
log_bin
If the binary log is enabled.
log_slave_updates
If the updates from the slave should be logged.
long_query_time
If a query takes longer than this (in seconds), the Slow_queries counter will be incremented. If you are using --log-slow-queries, the query will be logged to the slow query logfile. See section 4.9.5 The Slow Query Log.
lower_case_table_names
If set to 1 table names are stored in lowercase on disk and table names will be case-insensitive. See section 6.1.3 Case Sensitivity in Names.
max_allowed_packet
The maximum size of one packet. The message buffer is initialized to net_buffer_length bytes, but can grow up to max_allowed_packet bytes when needed. This value by default is small, to catch big (possibly wrong) packets. You must increase this value if you are using big BLOB columns. It should be as big as the biggest BLOB you want to use. The current protocol limits max_allowed_packet to 16M.
max_binlog_cache_size
If a multi-statement transaction requires more than this amount of memory, one will get the error "Multi-statement transaction required more than 'max_binlog_cache_size' bytes of storage".
max_binlog_size
Available after 3.23.33. If a write to the binary (replication) log exceeds the given value, rotate the logs. You cannot set it to less than 1024 bytes, or more than 1 GB. Default is 1 GB.
max_connections
The number of simultaneous clients allowed. Increasing this value increases the number of file descriptors that mysqld requires. See below for comments on file descriptor limits. See section A.2.5 Too many connections Error.
max_connect_errors
If there is more than this number of interrupted connections from a host this host will be blocked from further connections. You can unblock a host with the command FLUSH HOSTS.
max_delayed_threads
Don't start more than this number of threads to handle INSERT DELAYED statements. If you try to insert data into a new table after all INSERT DELAYED threads are in use, the row will be inserted as if the DELAYED attribute wasn't specified.
max_heap_table_size
Don't allow creation of heap tables bigger than this.
max_join_size
Joins that are probably going to read more than max_join_size records return an error. Set this value if your users tend to perform joins that lack a WHERE clause, that take a long time, and that return millions of rows.
max_sort_length
The number of bytes to use when sorting BLOB or TEXT values (only the first max_sort_length bytes of each value are used; the rest are ignored).
max_user_connections
The maximum number of active connections for a single user (0 = no limit).
max_tmp_tables
(This option doesn't yet do anything.) Maximum number of temporary tables a client can keep open at the same time.
max_write_lock_count
After this many write locks, allow some read locks to run in between.
myisam_recover_options
The value of the --myisam-recover option.
myisam_sort_buffer_size
The buffer that is allocated when sorting the index when doing a REPAIR or when creating indexes with CREATE INDEX or ALTER TABLE.
myisam_max_extra_sort_file_size.
If the creating of the temporary file for fast index creation would be this much bigger than using the key cache, then prefer the key cache method. This is mainly used to force long character keys in large tables to use the slower key cache method to create the index. NOTE that this parameter is given in megabytes!
myisam_max_sort_file_size
The maximum size of the temporary file MySQL is allowed to use while recreating the index (during REPAIR, ALTER TABLE or LOAD DATA INFILE. If the file size would be bigger than this, the index will be created through the key cache (which is slower). NOTE that this parameter is given in megabytes!
net_buffer_length
The communication buffer is reset to this size between queries. This should not normally be changed, but if you have very little memory, you can set it to the expected size of a query. (That is, the expected length of SQL statements sent by clients. If statements exceed this length, the buffer is automatically enlarged, up to max_allowed_packet bytes.)
net_read_timeout
Number of seconds to wait for more data from a connection before aborting the read. Note that when we don't expect data from a connection, the timeout is defined by write_timeout. See also slave_read_timeout.
net_retry_count
If a read on a communication port is interrupted, retry this many times before giving up. This value should be quite high on FreeBSD as internal interrupts are sent to all threads.
net_write_timeout
Number of seconds to wait for a block to be written to a connection before aborting the write.
open_files_limit
If this is not 0, then mysqld will use this value to reserve file descriptors to use with setrlimit(). If this value is 0 then mysqld will reserve max_connections*5 or max_connections + table_cache*2 (whichever is larger) number of files. You should try increasing this if mysqld gives you the error 'Too many open files'.
pid_file
The value of the --pid-file option.
port
The value of the --port option.
protocol_version
The protocol version used by the MySQL server.
record_buffer
Each thread that does a sequential scan allocates a buffer of this size for each table it scans. If you do many sequential scans, you may want to increase this value.
record_rnd_buffer
When reading rows in sorted order after a sort, the rows are read through this buffer to avoid a disk seeks. If not set, then it's set to the value of record_buffer.
query_buffer_size
The initial allocation of the query buffer. If most of your queries are long (like when inserting blobs), you should increase this!
safe_show_databases
Don't show databases for which the user doesn't have any database or table privileges. This can improve security if you're concerned about people being able to see what databases other users have. See also skip_show_databases.
server_id
The value of the --server-id option.
skip_locking
Is OFF if mysqld uses external locking.
skip_networking
Is ON if we only allow local (socket) connections.
skip_show_databases
This prevents people from doing SHOW DATABASES if they don't have the PROCESS_PRIV privilege. This can improve security if you're concerned about people being able to see what databases other users have. See also safe_show_databases.
slave_read_timeout
Number of seconds to wait for more data from a master/slave connection before aborting the read.
slow_launch_time
If creating the thread takes longer than this value (in seconds), the Slow_launch_threads counter will be incremented.
socket
The Unix socket used by the server.
sort_buffer
Each thread that needs to do a sort allocates a buffer of this size. Increase this value for faster ORDER BY or GROUP BY operations. See section A.4.4 Where MySQL Stores Temporary Files.
table_cache
The number of open tables for all threads. Increasing this value increases the number of file descriptors that mysqld requires. MySQL needs two file descriptors for each unique open table. See below for comments on file descriptor limits. You can check if you need to increase the table cache by checking the Opened_tables variable. See section 4.5.5 SHOW Syntax. If this variable is big and you don't do FLUSH TABLES a lot (which just forces all tables to be closed and reopenend), then you should increase the value of this variable. Make sure that your operating system can handle the number of open file descriptors implied by the table_cache setting. If table_cache is set too high, MySQL may run out of file descriptors and refuse connections, fail to perform queries, and be very unreliable. For information about how the table cache works, see section 5.4.6 How MySQL Opens and Closes Tables.
table_type
The default table type
thread_cache_size
How many threads we should keep in a cache for reuse. When a client disconnects, the client's threads are put in the cache if there aren't more than thread_cache_size threads from before. All new threads are first taken from the cache, and only when the cache is empty is a new thread created. This variable can be increased to improve performance if you have a lot of new connections. (Normally this doesn't give a notable performance improvement if you have a good thread implementation.) By examing the difference between the Connections and Threads_created you can see how efficient the current thread cache is for you.
thread_concurrency
On Solaris, mysqld will call thr_setconcurrency() with this value. thr_setconcurrency() permits the application to give the threads system a hint for the desired number of threads that should be run at the same time.
thread_stack
The stack size for each thread. Many of the limits detected by the crash-me test are dependent on this value. The default is large enough for normal operation. See section 5.1.4 The MySQL Benchmark Suite.
timezone
The timezone for the server.
tmp_table_size
If an in-memory temporary table exceeds this size, MySQL will automatically convert it to an on-disk MyISAM table. Increase the value of tmp_table_size if you do many advanced GROUP BY queries and you have lots of memory.
tmpdir
The directory used for temporary files and temporary tables.
version
The version number for the server.
wait_timeout
The number of seconds the server waits for activity on a connection before closing it. See also interactive_timeout.

The manual section that describes tuning MySQL contains some information of how to tune the above variables. See section 5.5.2 Tuning Server Parameters.

4.5.5.5 SHOW LOGS

SHOW LOGS shows you status information about existing log files. It currently only displays information about Berkeley DB log files.

4.5.5.6 SHOW PROCESSLIST

SHOW PROCESSLIST shows you which threads are running. You can also get this information using the mysqladmin processlist command. If you have the process privilege, you can see all threads. Otherwise, you can see only your own threads. See section 4.5.4 KILL Syntax. If you don't use the FULL option, then only the first 100 characters of each query will be shown.

This command is very useful if you get the 'too many connections' error message and want to find out what's going on. MySQL reserves one extra connection for a client with the Process_priv privilege to ensure that you should always be able to login and check the system (assuming you are not giving this privilege to all your users).

4.5.5.7 SHOW GRANTS

SHOW GRANTS FOR user lists the grant commands that must be issued to duplicate the grants for a user.

mysql> SHOW GRANTS FOR root@localhost;
+---------------------------------------------------------------------+
| Grants for root@localhost                                           |
+---------------------------------------------------------------------+
| GRANT ALL PRIVILEGES ON *.* TO 'root'@'localhost' WITH GRANT OPTION |
+---------------------------------------------------------------------+

4.5.5.8 SHOW CREATE TABLE

Shows a CREATE TABLE statement that will create the given table:

mysql> show create table t\G
*************************** 1. row ***************************
       Table: t
Create Table: CREATE TABLE t (
  id int(11) default NULL auto_increment,
  s char(60) default NULL,
  PRIMARY KEY (id)
) TYPE=MyISAM

SHOW CREATE TABLE will quote table and column names according to SQL_QUOTE_SHOW_CREATE option. section 5.5.6 SET Syntax.

4.6 MySQL Localization and International Usage

4.6.1 The Character Set Used for Data and Sorting

By default, MySQL uses the ISO-8859-1 (Latin1) character set with sorting according to Swedish/Finnish. This is the character set suitable in the USA and western Europe.

All standard MySQL binaries are compiled with --with-extra-charsets=complex. This will add code to all standard programs to be able to handle latin1 and all multi-byte character sets within the binary. Other character sets will be loaded from a character-set definition file when needed.

The character set determines what characters are allowed in names and how things are sorted by the ORDER BY and GROUP BY clauses of the SELECT statement.

You can change the character set with the --default-character-set option when you start the server. The character sets available depend on the --with-charset=charset and --with-extra-charset= list-of-charset | complex | all options to configure, and the character set configuration files listed in `SHAREDIR/charsets/Index'. See section 2.3.3 Typical configure Options.

If you change the character set when running MySQL (which may also change the sort order), you must run myisamchk -r -q on all tables. Otherwise your indexes may not be ordered correctly.

When a client connects to a MySQL server, the server sends the default character set in use to the client. The client will switch to use this character set for this connection.

One should use mysql_real_escape_string() when escaping strings for a SQL query. mysql_real_escape_string() is identical to the old mysql_escape_string() function, except that it takes the MYSQL connection handle as the first parameter.

If the client is compiled with different paths than where the server is installed and the user who configured MySQL didn't included all character sets in the MySQL binary, one must specify for the client where it can find the additional character sets it will need if the server runs with a different character set than the client.

One can specify this by putting in a MySQL option file:

[client]
character-sets-dir=/usr/local/mysql/share/mysql/charsets

where the path points to where the dynamic MySQL character sets are stored.

One can force the client to use specific character set by specifying:

[client]
default-character-set=character-set-name

but normally this is never needed.

4.6.2 Non-English Error Messages

mysqld can issue error messages in the following languages: Czech, Danish, Dutch, English (the default), Estonian, French, German, Greek, Hungarian, Italian, Japanese, Korean, Norwegian, Norwegian-ny, Polish, Portuguese, Romanian, Russian, Slovak, Spanish, and Swedish.

To start mysqld with a particular language, use either the --language=lang or -L lang options. For example:

shell> mysqld --language=swedish

or:

shell> mysqld --language=/usr/local/share/swedish

Note that all language names are specified in lowercase.

The language files are located (by default) in `mysql_base_dir/share/LANGUAGE/'.

To update the error message file, you should edit the `errmsg.txt' file and execute the following command to generate the `errmsg.sys' file:

shell> comp_err errmsg.txt errmsg.sys

If you upgrade to a newer version of MySQL, remember to repeat your changes with the new `errmsg.txt' file.

4.6.3 Adding a New Character Set

To add another character set to MySQL, use the following procedure.

Decide if the set is simple or complex. If the character set does not need to use special string collating routines for sorting and does not need multi-byte character support, it is simple. If it needs either of those features, it is complex.

For example, latin1 and danish are simple charactersets while big5 or czech are complex character sets.

In the following section, we have assumed that you name your character set MYSET.

For a simple character set do the following:

  1. Add MYSET to the end of the `sql/share/charsets/Index' file Assign an unique number to it.
  2. Create the file `sql/share/charsets/MYSET.conf'. (You can use `sql/share/charsets/latin1.conf' as a base for this). The syntax for the file very simple: See section 4.6.4 The character definition arrays.
  3. Add the character set name to the CHARSETS_AVAILABLE and COMPILED_CHARSETS lists in configure.in.
  4. Reconfigure, recompile, and test.

For a complex character set do the following:

  1. Create the file `strings/ctype-MYSET.c' in the MySQL source distribution.
  2. Add MYSET to the end of the `sql/share/charsets/Index' file. Assign an unique number to it.
  3. Look at one of the existing `ctype-*.c' files to see what needs to be defined, for example `strings/ctype-big5.c'. Note that the arrays in your file must have names like ctype_MYSET, to_lower_MYSET, and so on. This corresponds to the arrays in the simple character set. See section 4.6.4 The character definition arrays. For a complex character set
  4. Near the top of the file, place a special comment like this:
    /*
     * This comment is parsed by configure to create ctype.c,
     * so don't change it unless you know what you are doing.
     *
     * .configure. number_MYSET=MYNUMBER
     * .configure. strxfrm_multiply_MYSET=N
     * .configure. mbmaxlen_MYSET=N
     */
    
    The configure program uses this comment to include the character set into the MySQL library automatically. The strxfrm_multiply and mbmaxlen lines will be explained in the following sections. Only include them if you the string collating functions or the multi-byte character set functions, respectively.
  5. You should then create some of the following functions: See section 4.6.5 String Collating Support.
  6. Add the character set name to the CHARSETS_AVAILABLE and COMPILED_CHARSETS lists in configure.in.
  7. Reconfigure, recompile, and test.

The file `sql/share/charsets/README' includes some more instructions.

If you want to have the character set included in the MySQL distribution, mail a patch to internals@lists.mysql.com.

4.6.4 The character definition arrays

to_lower[] and to_upper[] are simple arrays that hold the lowercase and uppercase characters corresponding to each member of the character set. For example:

to_lower['A'] should contain 'a'
to_upper['a'] should contain 'A'

sort_order[] is a map indicating how characters should be ordered for comparison and sorting purposes. For many character sets, this is the same as to_upper[] (which means sorting will be case insensitive). MySQL will sort characters based on the value of sort_order[character]. For more complicated sorting rules, see the discussion of string collating below. See section 4.6.5 String Collating Support.

ctype[] is an array of bit values, with one element for one character. (Note that to_lower[], to_upper[], and sort_order[] are indexed by character value, but ctype[] is indexed by character value + 1. This is an old legacy to be able to handle EOF.)

You can find the following bitmask definitions in `m_ctype.h':

#define _U      01      /* Uppercase */
#define _L      02      /* Lowercase */
#define _N      04      /* Numeral (digit) */
#define _S      010     /* Spacing character */
#define _P      020     /* Punctuation */
#define _C      040     /* Control character */
#define _B      0100    /* Blank */
#define _X      0200    /* heXadecimal digit */

The ctype[] entry for each character should be the union of the applicable bitmask values that describe the character. For example, 'A' is an uppercase character (_U) as well as a hexadecimal digit (_X), so ctype['A'+1] should contain the value:

_U + _X = 01 + 0200 = 0201

4.6.5 String Collating Support

If the sorting rules for your language are too complex to be handled with the simple sort_order[] table, you need to use the string collating functions.

Right now the best documentation on this is the character sets that are already implemented. Look at the big5, czech, gbk, sjis, and tis160 character sets for examples.

You must specify the strxfrm_multiply_MYSET=N value in the special comment at the top of the file. N should be set to the maximum ratio the strings may grow during my_strxfrm_MYSET (it must be a positive integer).

4.6.6 Multi-byte Character Support

If your want to add support for a new character set that includes multi-byte characters, you need to use the multi-byte character functions.

Right now the best documentation on this is the character sets that are already implemented. Look at the euc_kr, gb2312, gbk, sjis and ujis character sets for examples. These are implemented in the ctype-'charset'.c files in the `strings' directory.

You must specify the mbmaxlen_MYSET=N value in the special comment at the top of the source file. N should be set to the size in bytes of the largest character in the set.

4.7 MySQL Server-Side Scripts and Utilities

4.7.1 Overview of the Server-Side Scripts and Utilities

All MySQL clients that communicate with the server using the mysqlclient library use the following environment variables:

Name Description
MYSQL_UNIX_PORT The default socket; used for connections to localhost
MYSQL_TCP_PORT The default TCP/IP port
MYSQL_PWD The default password
MYSQL_DEBUG Debug-trace options when debugging
TMPDIR The directory where temporary tables/files are created

Use of MYSQL_PWD is insecure. See section 4.2.7 Connecting to the MySQL Server.

The `mysql' client uses the file named in the MYSQL_HISTFILE environment variable to save the command-line history. The default value for the history file is `$HOME/.mysql_history', where $HOME is the value of the HOME environment variable. See section H Environment Variables.

All MySQL programs take many different options. However, every MySQL program provides a --help option that you can use to get a full description of the program's different options. For example, try mysql --help.

You can override default options for all standard client programs with an option file. section 4.1.2 my.cnf Option Files.

The list below briefly describes the MySQL programs:

myisamchk
Utility to describe, check, optimize, and repair MySQL tables. Because myisamchk has many functions, it is described in its own chapter. See section 4 MySQL Database Administration.
make_binary_distribution
Makes a binary release of a compiled MySQL. This could be sent by FTP to `/pub/mysql/Incoming' on support.mysql.com for the convenience of other MySQL users.
msql2mysql
A shell script that converts mSQL programs to MySQL. It doesn't handle all cases, but it gives a good start when converting.
mysqlaccess
A script that checks the access privileges for a host, user, and database combination.
mysqladmin
Utility for performing administrative operations, such as creating or dropping databases, reloading the grant tables, flushing tables to disk, and reopening log files. mysqladmin can also be used to retrieve version, process, and status information from the server. See section 4.8.3 mysqladmin, Administrating a MySQL Server.
mysqlbug
The MySQL bug report script. This script should always be used when filing a bug report to the MySQL list.
mysqld
The SQL daemon. This should always be running.
mysqldump
Dumps a MySQL database into a file as SQL statements or as tab-separated text files. Enhanced freeware originally by Igor Romanenko. See section 4.8.5 mysqldump, Dumping Table Structure and Data.
mysqlimport
Imports text files into their respective tables using LOAD DATA INFILE. See section 4.8.7 mysqlimport, Importing Data from Text Files.
mysqlshow
Displays information about databases, tables, columns, and indexes.
mysql_install_db
Creates the MySQL grant tables with default privileges. This is usually executed only once, when first installing MySQL on a system.
replace
A utility program that is used by msql2mysql, but that has more general applicability as well. replace changes strings in place in files or on the standard input. Uses a finite state machine to match longer strings first. Can be used to swap strings. For example, this command swaps a and b in the given files:
shell> replace a b b a -- file1 file2 ...

4.7.2 safe_mysqld, the wrapper around mysqld

safe_mysqld is the recommended way to start a mysqld daemon on Unix. safe_mysqld adds some safety features such as restarting the server when an error occurs and logging run-time information to a log file.

If you don't use --mysqld=# or --mysqld-version=# safe_mysqld will use an executable named mysqld-max if it exists. If not, safe_mysqld will start mysqld. This makes it very easy to test to use mysqld-max instead of mysqld; Just copy mysqld-max to where you have mysqld and it will be used.

Normally one should never edit the safe_mysqld script, but instead put the options to safe_mysqld in the [safe_mysqld] section in the my.cnf file. safe_mysqld will read all options from the [mysqld], [server] and [safe_mysqld] sections from the option files. See section 4.1.2 my.cnf Option Files.

Note that all options on the command line to safe_mysqld are passed to mysqld. If you wants to use any options in safe_mysqld that mysqld doesn't support, you must specify these in the option file.

Most of the options to safe_mysqld are the same as the options to mysqld. See section 4.1.1 mysqld Command-line Options.

safe_mysqld supports the following options:

--basedir=path
--core-file-size=#
Size of the core file mysqld should be able to create. Passed to ulimit -c.
--datadir=path
--defaults-extra-file=path
--defaults-file=path
--err-log=path
--ledir=path
Path to mysqld
--log=path
--mysqld=mysqld-version
Name of the mysqld version in the ledir directory you want to start.
--mysqld-version=version
Similar to --mysqld= but here you only give the suffix for mysqld. For example if you use --mysqld-version=max, safe_mysqld will start the ledir/mysqld-max version. If the argument to --mysqld-version is empty, ledir/mysqld will be used.
--no-defaults
--open-files-limit=#
Number of files mysqld should be able to open. Passed to ulimit -n. Note that you need to start safe_mysqld as root for this to work properly!
--pid-file=path
--port=#
--socket=path
--timezone=#
Set the timezone (the TZ) variable to the value of this parameter.
--user=#

The safe_mysqld script is written so that it normally is able to start a server that was installed from either a source or a binary version of MySQL, even if these install the server in slightly different locations. safe_mysqld expects one of these conditions to be true:

Because safe_mysqld will try to find the server and databases relative to its own working directory, you can install a binary distribution of MySQL anywhere, as long as you start safe_mysqld from the MySQL installation directory:

shell> cd mysql_installation_directory
shell> bin/safe_mysqld &

If safe_mysqld fails, even when invoked from the MySQL installation directory, you can modify it to use the path to mysqld and the pathname options that are correct for your system. Note that if you upgrade MySQL in the future, your modified version of safe_mysqld will be overwritten, so you should make a copy of your edited version that you can reinstall.

4.7.3 mysqld_multi, program for managing multiple MySQL servers

mysqld_multi is meant for managing several mysqld processes running in different UNIX sockets and TCP/IP ports.

The program will search for group(s) named [mysqld#] from my.cnf (or the given --config-file=...), where # can be any positive number starting from 1. These groups should be the same as the usual [mysqld] group (e.g. options to mysqld, see MySQL manual for detailed information about this group), but with those port, socket etc. options that are wanted for each separate mysqld processes. The number in the group name has another function; it can be used for starting, stopping, or reporting some specific mysqld servers with this program. See the usage and options below for more information.

Usage: mysqld_multi [OPTIONS] {start|stop|report} [GNR,GNR,GNR...]
or     mysqld_multi [OPTIONS] {start|stop|report} [GNR-GNR,GNR,GNR-GNR,...]

The GNR above means the group number. You can start, stop or report any GNR, or several of them at the same time. (See --example) The GNRs list can be comma separated, or a dash combined, of which the latter means that all the GNRs between GNR1-GNR2 will be affected. Without GNR argument all the found groups will be either started, stopped, or reported. Note that you must not have any white spaces in the GNR list. Anything after a white space is ignored.

mysqld_multi supports the following options:

--config-file=...
Alternative config file. NOTE: This will not affect this program's own options (group [mysqld_multi]), but only groups [mysqld#]. Without this option everything will be searched from the ordinary my.cnf file.
--example
Give an example of a config file.
--help
Print this help and exit.
--log=...
Log file. Full path to and the name for the log file. NOTE: If the file exists, everything will be appended.
--mysqladmin=...
mysqladmin binary to be used for a server shutdown.
--mysqld=...
mysqld binary to be used. Note that you can give safe_mysqld to this option also. The options are passed to mysqld. Just make sure you have mysqld in your environment variable PATH or fix safe_mysqld.
--no-log
Print to stdout instead of the log file. By default the log file is turned on.
--password=...
Password for user for mysqladmin.
--tcp-ip
Connect to the MySQL server(s) via the TCP/IP port instead of the UNIX socket. This affects stopping and reporting. If a socket file is missing, the server may still be running, but can be accessed only via the TCP/IP port. By default connecting is done via the UNIX socket.
--user=...
MySQL user for mysqladmin.
--version
Print the version number and exit.

Some notes about mysqld_multi:

See section 4.1.4 Running Multiple MySQL Servers on the Same Machine.

This is an example of the config file on behalf of mysqld_multi.

# This file should probably be in your home dir (~/.my.cnf) or /etc/my.cnf
# Version 2.1 by Jani Tolonen

[mysqld_multi]
mysqld     = /usr/local/bin/safe_mysqld
mysqladmin = /usr/local/bin/mysqladmin
user       = multi_admin
password   = multipass

[mysqld2]
socket     = /tmp/mysql.sock2
port       = 3307
pid-file   = /usr/local/mysql/var2/hostname.pid2
datadir    = /usr/local/mysql/var2
language   = /usr/local/share/mysql/english
user       = john

[mysqld3]
socket     = /tmp/mysql.sock3
port       = 3308
pid-file   = /usr/local/mysql/var3/hostname.pid3
datadir    = /usr/local/mysql/var3
language   = /usr/local/share/mysql/swedish
user       = monty

[mysqld4]
socket     = /tmp/mysql.sock4
port       = 3309
pid-file   = /usr/local/mysql/var4/hostname.pid4
datadir    = /usr/local/mysql/var4
language   = /usr/local/share/mysql/estonia
user       = tonu

[mysqld6]
socket     = /tmp/mysql.sock6
port       = 3311
pid-file   = /usr/local/mysql/var6/hostname.pid6
datadir    = /usr/local/mysql/var6
language   = /usr/local/share/mysql/japanese
user       = jani

See section 4.1.2 my.cnf Option Files.

4.7.4 myisampack, The MySQL Compressed Read-only Table Generator

myisampack is used to compress MyISAM tables, and pack_isam is used to compress ISAM tables. Because ISAM tables are deprecated, we will only discuss myisampack here, but everything said about myisampack should also be true for pack_isam.

myisampack works by compressing each column in the table separately. The information needed to decompress columns is read into memory when the table is opened. This results in much better performance when accessing individual records, because you only have to uncompress exactly one record, not a much larger disk block as when using Stacker on MS-DOS. Usually, myisampack packs the data file 40%-70%.

MySQL uses memory mapping (mmap()) on compressed tables and falls back to normal read/write file usage if mmap() doesn't work.

There are currently two limitations with myisampack:

Fixing these limitations is on our TODO list but with low priority.

myisampack is invoked like this:

shell> myisampack [options] filename ...

Each filename should be the name of an index (`.MYI') file. If you are not in the database directory, you should specify the pathname to the file. It is permissible to omit the `.MYI' extension.

myisampack supports the following options:

-b, --backup
Make a backup of the table as tbl_name.OLD.
-#, --debug=debug_options
Output debug log. The debug_options string often is 'd:t:o,filename'.
-f, --force
Force packing of the table even if it becomes bigger or if the temporary file exists. myisampack creates a temporary file named `tbl_name.TMD' while it compresses the table. If you kill myisampack, the `.TMD' file may not be deleted. Normally, myisampack exits with an error if it finds that `tbl_name.TMD' exists. With --force, myisampack packs the table anyway.
-?, --help
Display a help message and exit.
-j big_tbl_name, --join=big_tbl_name
Join all tables named on the command line into a single table big_tbl_name. All tables that are to be combined MUST be identical (same column names and types, same indexes, etc.).
-p #, --packlength=#
Specify the record length storage size, in bytes. The value should be 1, 2, or 3. (myisampack stores all rows with length pointers of 1, 2, or 3 bytes. In most normal cases, myisampack can determine the right length value before it begins packing the file, but it may notice during the packing process that it could have used a shorter length. In this case, myisampack will print a note that the next time you pack the same file, you could use a shorter record length.)
-s, --silent
Silent mode. Write output only when errors occur.
-t, --test
Don't actually pack table, just test packing it.
-T dir_name, --tmp_dir=dir_name
Use the named directory as the location in which to write the temporary table.
-v, --verbose
Verbose mode. Write information about progress and packing result.
-V, --version
Display version information and exit.
-w, --wait
Wait and retry if table is in use. If the mysqld server was invoked with the --skip-locking option, it is not a good idea to invoke myisampack if the table might be updated during the packing process.

The sequence of commands shown below illustrates a typical table compression session:

shell> ls -l station.*
-rw-rw-r--   1 monty    my         994128 Apr 17 19:00 station.MYD
-rw-rw-r--   1 monty    my          53248 Apr 17 19:00 station.MYI
-rw-rw-r--   1 monty    my           5767 Apr 17 19:00 station.frm

shell> myisamchk -dvv station

MyISAM file:     station
Isam-version:  2
Creation time: 1996-03-13 10:08:58
Recover time:  1997-02-02  3:06:43
Data records:              1192  Deleted blocks:              0
Datafile: Parts:           1192  Deleted data:                0
Datafile pointer (bytes):     2  Keyfile pointer (bytes):     2
Max datafile length:   54657023  Max keyfile length:   33554431
Recordlength:               834
Record format: Fixed length

table description:
Key Start Len Index   Type                       Root  Blocksize    Rec/key
1   2     4   unique  unsigned long              1024       1024          1
2   32    30  multip. text                      10240       1024          1

Field Start Length Type
1     1     1
2     2     4
3     6     4
4     10    1
5     11    20
6     31    1
7     32    30
8     62    35
9     97    35
10    132   35
11    167   4
12    171   16
13    187   35
14    222   4
15    226   16
16    242   20
17    262   20
18    282   20
19    302   30
20    332   4
21    336   4
22    340   1
23    341   8
24    349   8
25    357   8
26    365   2
27    367   2
28    369   4
29    373   4
30    377   1
31    378   2
32    380   8
33    388   4
34    392   4
35    396   4
36    400   4
37    404   1
38    405   4
39    409   4
40    413   4
41    417   4
42    421   4
43    425   4
44    429   20
45    449   30
46    479   1
47    480   1
48    481   79
49    560   79
50    639   79
51    718   79
52    797   8
53    805   1
54    806   1
55    807   20
56    827   4
57    831   4

shell> myisampack station.MYI
Compressing station.MYI: (1192 records)
- Calculating statistics

normal:     20  empty-space:      16  empty-zero:        12  empty-fill:  11
pre-space:   0  end-space:        12  table-lookups:      5  zero:         7
Original trees:  57  After join: 17
- Compressing file
87.14%

shell> ls -l station.*
-rw-rw-r--   1 monty    my         127874 Apr 17 19:00 station.MYD
-rw-rw-r--   1 monty    my          55296 Apr 17 19:04 station.MYI
-rw-rw-r--   1 monty    my           5767 Apr 17 19:00 station.frm

shell> myisamchk -dvv station

MyISAM file:     station
Isam-version:  2
Creation time: 1996-03-13 10:08:58
Recover time:  1997-04-17 19:04:26
Data records:              1192  Deleted blocks:              0
Datafile: Parts:           1192  Deleted data:                0
Datafilepointer (bytes):      3  Keyfile pointer (bytes):     1
Max datafile length:   16777215  Max keyfile length:     131071
Recordlength:               834
Record format: Compressed

table description:
Key Start Len Index   Type                       Root  Blocksize    Rec/key
1   2     4   unique  unsigned long             10240       1024          1
2   32    30  multip. text                      54272       1024          1

Field Start Length Type                         Huff tree  Bits
1     1     1      constant                             1     0
2     2     4      zerofill(1)                          2     9
3     6     4      no zeros, zerofill(1)                2     9
4     10    1                                           3     9
5     11    20     table-lookup                         4     0
6     31    1                                           3     9
7     32    30     no endspace, not_always              5     9
8     62    35     no endspace, not_always, no empty    6     9
9     97    35     no empty                             7     9
10    132   35     no endspace, not_always, no empty    6     9
11    167   4      zerofill(1)                          2     9
12    171   16     no endspace, not_always, no empty    5     9
13    187   35     no endspace, not_always, no empty    6     9
14    222   4      zerofill(1)                          2     9
15    226   16     no endspace, not_always, no empty    5     9
16    242   20     no endspace, not_always              8     9
17    262   20     no endspace, no empty                8     9
18    282   20     no endspace, no empty                5     9
19    302   30     no endspace, no empty                6     9
20    332   4      always zero                          2     9
21    336   4      always zero                          2     9
22    340   1                                           3     9
23    341   8      table-lookup                         9     0
24    349   8      table-lookup                        10     0
25    357   8      always zero                          2     9
26    365   2                                           2     9
27    367   2      no zeros, zerofill(1)                2     9
28    369   4      no zeros, zerofill(1)                2     9
29    373   4      table-lookup                        11     0
30    377   1                                           3     9
31    378   2      no zeros, zerofill(1)                2     9
32    380   8      no zeros                             2     9
33    388   4      always zero                          2     9
34    392   4      table-lookup                        12     0
35    396   4      no zeros, zerofill(1)               13     9
36    400   4      no zeros, zerofill(1)                2     9
37    404   1                                           2     9
38    405   4      no zeros                             2     9
39    409   4      always zero                          2     9
40    413   4      no zeros                             2     9
41    417   4      always zero                          2     9
42    421   4      no zeros                             2     9
43    425   4      always zero                          2     9
44    429   20     no empty                             3     9
45    449   30     no empty                             3     9
46    479   1                                          14     4
47    480   1                                          14     4
48    481   79     no endspace, no empty               15     9
49    560   79     no empty                             2     9
50    639   79     no empty                             2     9
51    718   79     no endspace                         16     9
52    797   8      no empty                             2     9
53    805   1                                          17     1
54    806   1                                           3     9
55    807   20     no empty                             3     9
56    827   4      no zeros, zerofill(2)                2     9
57    831   4      no zeros, zerofill(1)                2     9

The information printed by myisampack is described below:

normal
The number of columns for which no extra packing is used.
empty-space
The number of columns containing values that are only spaces; these will occupy 1 bit.
empty-zero
The number of columns containing values that are only binary 0's; these will occupy 1 bit.
empty-fill
The number of integer columns that don't occupy the full byte range of their type; these are changed to a smaller type (for example, an INTEGER column may be changed to MEDIUMINT).
pre-space
The number of decimal columns that are stored with leading spaces. In this case, each value will contain a count for the number of leading spaces.
end-space
The number of columns that have a lot of trailing spaces. In this case, each value will contain a count for the number of trailing spaces.
table-lookup
The column had only a small number of different values, which were converted to an ENUM before Huffman compression.
zero
The number of columns for which all values are zero.
Original trees
The initial number of Huffman trees.
After join
The number of distinct Huffman trees left after joining trees to save some header space.

After a table has been compressed, myisamchk -dvv prints additional information about each field:

Type
The field type may contain the following descriptors:
constant
All rows have the same value.
no endspace
Don't store endspace.
no endspace, not_always
Don't store endspace and don't do end space compression for all values.
no endspace, no empty
Don't store endspace. Don't store empty values.
table-lookup
The column was converted to an ENUM.
zerofill(n)
The most significant n bytes in the value are always 0 and are not stored.
no zeros
Don't store zeros.
always zero
0 values are stored in 1 bit.
Huff tree
The Huffman tree associated with the field.
Bits
The number of bits used in the Huffman tree.

After you have run pack_isam/myisampack you must run isamchk/myisamchk to re-create the index. At this time you can also sort the index blocks and create statistics needed for the MySQL optimizer to work more efficiently:

myisamchk -rq --analyze --sort-index table_name.MYI
isamchk   -rq --analyze --sort-index table_name.ISM

After you have installed the packed table into the MySQL database directory you should do mysqladmin flush-tables to force mysqld to start using the new table.

If you want to unpack a packed table, you can do this with the --unpack option to isamchk or myisamchk.

4.7.5 mysqld-max, An extended mysqld server

mysqld-max is the MySQL server (mysqld) configured with the following configure options:

Option Comment
--with-server-suffix=-max Add a suffix to the mysqld version string.
--with-bdb Support for Berkeley DB (BDB) tables
--with-innodb Support for InnoDB tables.
CFLAGS=-DUSE_SYMDIR Symbolic links support for Windows.

You can find the MySQL-max binaries at http://www.mysql.com/downloads/mysql-max-3.23.html.

The Windows MySQL 3.23 binary distribution includes both the standard mysqld.exe binary and the mysqld-max.exe binary. http://www.mysql.com/downloads/mysql-3.23.html. See section 2.1.2 Installing MySQL on Windows.

Note that as Berkeley DB and InnoDB are not available for all platforms, some of the Max binaries may not have support for both of these. You can check which table types are supported by doing the following query:

mysql> show variables like "have_%";
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| have_bdb      | YES   |
| have_innodb   | NO    |
| have_isam     | YES   |
| have_raid     | NO    |
| have_ssl      | NO    |
+---------------+-------+

The meaning of the values are:

Value Meaning.
YES The option is activated and usable.
NO MySQL is not compiled with support for this option.
DISABLED The xxxx option is disabled because one started mysqld with --skip-xxxx or because one didn't start mysqld with all needed options to enable the option. In this case the hostname.err file should contain a reason for why the option is disabled.

NOTE: To be able to create InnoDB tables you MUST edit your startup options to include at least the innodb_data_file_path option. See section 7.6.2 InnoDB startup options.

To get better performance for BDB tables, you should add some configuration options for these too. See section 7.5.3 BDB startup options.

safe_mysqld will automatically try to start any mysqld binary with the -max prefix. This makes it very easy to test out a another mysqld binary in an existing installation. Just run configure with the options you want and then install the new mysqld binary as mysqld-max in the same directory where your old mysqld binary is. See section 4.7.2 safe_mysqld, the wrapper around mysqld.

The mysqld-max RPM uses the above mentioned safe_mysqld feature. It just installs the mysqld-max executable and safe_mysqld will automatically use this executable when safe_mysqld is restarted.

The following table shows which table types our standard MySQL-Max binaries includes:

System BDB InnoDB
AIX 4.3 N Y
HP-UX 11.0 N Y
Linux-Alpha N Y
Linux-Intel Y Y
Linux-Ia64 N Y
Solaris-intel N Y
Solaris-sparc Y Y
SCO OSR5 Y Y
UnixWare Y Y
Windows/NT Y Y

4.8 MySQL Client-Side Scripts and Utilities

4.8.1 Overview of the Client-Side Scripts and Utilities

All MySQL clients that communicate with the server using the mysqlclient library use the following environment variables:

Name Description
MYSQL_UNIX_PORT The default socket; used for connections to localhost
MYSQL_TCP_PORT The default TCP/IP port
MYSQL_PWD The default password
MYSQL_DEBUG Debug-trace options when debugging
TMPDIR The directory where temporary tables/files are created

Use of MYSQL_PWD is insecure. See section 4.2.7 Connecting to the MySQL Server.

The `mysql' client uses the file named in the MYSQL_HISTFILE environment variable to save the command-line history. The default value for the history file is `$HOME/.mysql_history', where $HOME is the value of the HOME environment variable. See section H Environment Variables.

All MySQL programs take many different options. However, every MySQL program provides a --help option that you can use to get a full description of the program's different options. For example, try mysql --help.

You can override default options for all standard client programs with an option file. section 4.1.2 my.cnf Option Files.

The list below briefly describes the MySQL programs:

myisamchk
Utility to describe, check, optimize, and repair MySQL tables. Because myisamchk has many functions, it is described in its own chapter. See section 4 MySQL Database Administration.
make_binary_distribution
Makes a binary release of a compiled MySQL. This could be sent by FTP to `/pub/mysql/Incoming' on support.mysql.com for the convenience of other MySQL users.
msql2mysql
A shell script that converts mSQL programs to MySQL. It doesn't handle all cases, but it gives a good start when converting.
mysqlaccess
A script that checks the access privileges for a host, user, and database combination.
mysqladmin
Utility for performing administrative operations, such as creating or dropping databases, reloading the grant tables, flushing tables to disk, and reopening log files. mysqladmin can also be used to retrieve version, process, and status information from the server. See section 4.8.3 mysqladmin, Administrating a MySQL Server.
mysqlbug
The MySQL bug report script. This script should always be used when filing a bug report to the MySQL list.
mysqld
The SQL daemon. This should always be running.
mysqldump
Dumps a MySQL database into a file as SQL statements or as tab-separated text files. Enhanced freeware originally by Igor Romanenko. See section 4.8.5 mysqldump, Dumping Table Structure and Data.
mysqlimport
Imports text files into their respective tables using LOAD DATA INFILE. See section 4.8.7 mysqlimport, Importing Data from Text Files.
mysqlshow
Displays information about databases, tables, columns, and indexes.
mysql_install_db
Creates the MySQL grant tables with default privileges. This is usually executed only once, when first installing MySQL on a system.
replace
A utility program that is used by msql2mysql, but that has more general applicability as well. replace changes strings in place in files or on the standard input. Uses a finite state machine to match longer strings first. Can be used to swap strings. For example, this command swaps a and b in the given files:
shell> replace a b b a -- file1 file2 ...

4.8.2 The Command-line Tool

mysql is a simple SQL shell (with GNU readline capabilities). It supports interactive and non-interactive use. When used interactively, query results are presented in an ASCII-table format. When used non-interactively (for example, as a filter), the result is presented in tab-separated format. (The output format can be changed using command-line options.) You can run scripts simply like this:

shell> mysql database < script.sql > output.tab

If you have problems due to insufficient memory in the client, use the --quick option! This forces mysql to use mysql_use_result() rather than mysql_store_result() to retrieve the result set.

Using mysql is very easy. Just start it as follows: mysql database or mysql --user=user_name --password=your_password database. Type a SQL statement, end it with `;', `\g', or `\G' and press RETURN/ENTER.

mysql supports the following options:

-?, --help
Display this help and exit.
-A, --no-auto-rehash
No automatic rehashing. One has to use 'rehash' to get table and field completion. This gives a quicker start of mysql.
-B, --batch
Print results with a tab as separator, each row on a new line. Doesn't use history file.
--character-sets-dir=...
Directory where character sets are located.
-C, --compress
Use compression in server/client protocol.
-#, --debug[=...]
Debug log. Default is 'd:t:o,/tmp/mysql.trace'.
-D, --database=...
Database to use. This is mainly useful in the my.cnf file.
--default-character-set=...
Set the default character set.
-e, --execute=...
Execute command and quit. (Output like with --batch)
-E, --vertical
Print the output of a query (rows) vertically. Without this option you can also force this output by ending your statements with \G.
-f, --force
Continue even if we get a SQL error.
-g, --no-named-commands
Named commands are disabled. Use \* form only, or use named commands only in the beginning of a line ending with a semicolon (;). Since Version 10.9, the client now starts with this option ENABLED by default! With the -g option, long format commands will still work from the first line, however.
-G, --enable-named-commands
Named commands are enabled. Long format commands are allowed as well as shortened \* commands.
-i, --ignore-space
Ignore space after function names.
-h, --host=...
Connect to the given host.
-H, --html
Produce HTML output.
-L, --skip-line-numbers
Don't write line number for errors. Useful when one wants to compare result files that includes error messages
--no-pager
Disable pager and print to stdout. See interactive help (\h) also.
--no-tee
Disable outfile. See interactive help (\h) also.
-n, --unbuffered
Flush buffer after each query.
-N, --skip-column-names
Don't write column names in results.
-O, --set-variable var=option
Give a variable a value. --help lists variables.
-o, --one-database
Only update the default database. This is useful for skipping updates to other database in the update log.
--pager[=...]
Output type. Default is your ENV variable PAGER. Valid pagers are less, more, cat [> filename], etc. See interactive help (\h) also. This option does not work in batch mode. Pager works only in UNIX.
-p[password], --password[=...]
Password to use when connecting to server. If a password is not given on the command line, you will be prompted for it. Note that if you use the short form -p you can't have a space between the option and the password.
-P --port=...
TCP/IP port number to use for connection.
-q, --quick
Don't cache result, print it row-by-row. This may slow down the server if the output is suspended. Doesn't use history file.
-r, --raw
Write column values without escape conversion. Used with --batch
-s, --silent
Be more silent.
-S --socket=...
Socket file to use for connection.
-t --table
Output in table format. This is default in non-batch mode.
-T, --debug-info
Print some debug information at exit.
--tee=...
Append everything into outfile. See interactive help (\h) also. Does not work in batch mode.
-u, --user=#
User for login if not current user.
-U, --safe-updates[=#], --i-am-a-dummy[=#]
Only allow UPDATE and DELETE that uses keys. See below for more information about this option. You can reset this option if you have it in your my.cnf file by using --safe-updates=0.
-v, --verbose
More verbose output (-v -v -v gives the table output format).
-V, --version
Output version information and exit.
-w, --wait
Wait and retry if connection is down instead of aborting.

You can also set the following variables with -O or --set-variable:

Variable Name Default Description
connect_timeout 0 Number of seconds before timeout connection.
max_allowed_packet 16777216 Max packetlength to send/receive from to server
net_buffer_length 16384 Buffer for TCP/IP and socket communication
select_limit 1000 Automatic limit for SELECT when using --i-am-a-dummy
max_join_size 1000000 Automatic limit for rows in a join when using --i-am-a-dummy.

If you type 'help' on the command line, mysql will print out the commands that it supports:

mysql> help

MySQL commands:
help    (\h)    Display this text.
?       (\h)    Synonym for `help'.
clear   (\c)    Clear command.
connect (\r)    Reconnect to the server. Optional arguments are db and host.
edit    (\e)    Edit command with $EDITOR.
ego     (\G)    Send command to mysql server, display result vertically.
exit    (\q)    Exit mysql. Same as quit.
go      (\g)    Send command to mysql server.
nopager (\n)    Disable pager, print to stdout.
notee   (\t)    Don't write into outfile.
pager   (\P)    Set PAGER [to_pager]. Print the query results via PAGER.
print   (\p)    Print current command.
quit    (\q)    Quit mysql.
rehash  (\#)    Rebuild completion hash.
source  (\.)    Execute a SQL script file. Takes a file name as an argument.
status  (\s)    Get status information from the server.
tee     (\T)    Set outfile [to_outfile]. Append everything into given outfile.
use     (\u)    Use another database. Takes database name as argument.

From the above, pager only works in UNIX.

The status command gives you some information about the connection and the server you are using. If you are running in the --safe-updates mode, status will also print the values for the mysql variables that affect your queries.

A useful startup option for beginners (introduced in MySQL Version 3.23.11) is --safe-updates (or --i-am-a-dummy for users that has at some time done a DELETE FROM table_name but forgot the WHERE clause). When using this option, mysql sends the following command to the MySQL server when opening the connection:

SET SQL_SAFE_UPDATES=1,SQL_SELECT_LIMIT=#select_limit#,
    SQL_MAX_JOIN_SIZE=#max_join_size#"

where #select_limit# and #max_join_size# are variables that can be set from the mysql command line. See section 5.5.6 SET Syntax.

The effect of the above is:

Some useful hints about the mysql client:

Some data is much more readable when displayed vertically, instead of the usual horizontal box type output. For example longer text, which includes new lines, is often much easier to be read with vertical output.

mysql> select * from mails where length(txt) < 300 limit 300,1\G
*************************** 1. row ***************************
  msg_nro: 3068
     date: 2000-03-01 23:29:50
time_zone: +0200
mail_from: Monty
    reply: monty@no.spam.com
  mail_to: "Thimble Smith" <tim@no.spam.com>
      sbj: UTF-8
      txt: >>>>> "Thimble" == Thimble Smith writes:

Thimble> Hi.  I think this is a good idea.  Is anyone familiar with UTF-8
Thimble> or Unicode?  Otherwise I'll put this on my TODO list and see what
Thimble> happens.

Yes, please do that.

Regards,
Monty
     file: inbox-jani-1
     hash: 190402944
1 row in set (0.09 sec)

4.8.3 mysqladmin, Administrating a MySQL Server

A utility for performing administrative operations. The syntax is:

shell> mysqladmin [OPTIONS] command [command-option] command ...

You can get a list of the options your version of mysqladmin supports by executing mysqladmin --help.

The current mysqladmin supports the following commands:

create databasename
Create a new database.
drop databasename
Delete a database and all its tables.
extended-status
Gives an extended status message from the server.
flush-hosts
Flush all cached hosts.
flush-logs
Flush all logs.
flush-tables
Flush all tables.
flush-privileges
Reload grant tables (same as reload).
kill id,id,...
Kill mysql threads.
password
Set a new password. Change old password to new-password.
ping
Check if mysqld is alive.
processlist
Show list of active threads in server.
reload
Reload grant tables.
refresh
Flush all tables and close and open logfiles.
shutdown
Take server down.
slave-start
Start slave replication thread.
slave-stop
Stop slave replication thread.
status
Gives a short status message from the server.
variables
Prints variables available.
version
Get version info from server.

All commands can be shortened to their unique prefix. For example:

shell> mysqladmin proc stat
+----+-------+-----------+----+-------------+------+-------+------+
| Id | User  | Host      | db | Command     | Time | State | Info |
+----+-------+-----------+----+-------------+------+-------+------+
| 6  | monty | localhost |    | Processlist | 0    |       |      |
+----+-------+-----------+----+-------------+------+-------+------+
Uptime: 10077  Threads: 1  Questions: 9  Slow queries: 0  Opens: 6  Flush tables: 1  Open tables: 2  Memory in use: 1092K  Max memory used: 1116K

The mysqladmin status command result has the following columns:

Uptime Number of seconds the MySQL server has been up.
Threads Number of active threads (clients).
Questions Number of questions from clients since mysqld was started.
Slow queries Queries that have taken more than long_query_time seconds. See section 4.9.5 The Slow Query Log.
Opens How many tables mysqld has opened.
Flush tables Number of flush ..., refresh, and reload commands.
Open tables Number of tables that are open now.
Memory in use Memory allocated directly by the mysqld code (only available when MySQL is compiled with --with-debug=full).
Max memory used Maximum memory allocated directly by the mysqld code (only available when MySQL is compiled with --with-debug=full).

If you do myslqadmin shutdown on a socket (in other words, on a the computer where mysqld is running), mysqladmin will wait until the MySQL pid-file is removed to ensure that the mysqld server has stopped properly.

4.8.4 Using mysqlcheck for Table Maintenance and Crash Recovery

Since MySQL version 3.23.38 you will be able to use a new checking and repairing tool for MyISAM tables. The difference to myisamchk is that mysqlcheck should be used when the mysqld server is running, where as myisamchk should be used when it is not. The benefit is that you no longer have to take the server down for checking or repairing your tables.

mysqlcheck uses MySQL server commands CHECK, REPAIR, ANALYZE and OPTIMIZE in a convenient way for the user.

There are three alternative ways to invoke mysqlcheck:

shell> mysqlcheck [OPTIONS] database [tables]
shell> mysqlcheck [OPTIONS] --databases DB1 [DB2 DB3...]
shell> mysqlcheck [OPTIONS] --all-databases

So it can be used in a similar way as mysqldump when it comes to what databases and tables you want to choose.

mysqlcheck does have a special feature compared to the other clients; the default behavior, checking tables (-c), can be changed by renaming the binary. So if you want to have a tool that repairs tables by default, you should just copy mysqlcheck to your harddrive with a new name, mysqlrepair, or alternatively make a symbolic link to mysqlrepair and name the symbolic link as mysqlrepair. If you invoke mysqlrepair now, it will repair tables by default.

The names that you can use to change mysqlcheck default behavior are here:

mysqlrepair:   The default option will be -r
mysqlanalyze:  The default option will be -a
mysqloptimize: The default option will be -o

The options available for mysqlcheck are listed here, please check what your version supports with mysqlcheck --help.

-A, --all-databases
Check all the databases. This will be same as --databases with all databases selected
-1, --all-in-1
Instead of making one query for each table, execute all queries in 1 query separately for each database. Table names will be in a comma separated list.
-a, --analyze
Analyze given tables.
--auto-repair
If a checked table is corrupted, automatically fix it. Repairing will be done after all tables have been checked, if corrupted ones were found.
-#, --debug=...
Output debug log. Often this is 'd:t:o,filename'
--character-sets-dir=...
Directory where character sets are
-c, --check
Check table for errors
-C, --check-only-changed
Check only tables that have changed since last check or haven't been closed properly.
--compress
Use compression in server/client protocol.
-?, --help
Display this help message and exit.
-B, --databases
To check several databases. Note the difference in usage; In this case no tables are given. All name arguments are regarded as database names.
--default-character-set=...
Set the default character set
-F, --fast
Check only tables that hasn't been closed properly
-f, --force
Continue even if we get an sql-error.
-e, --extended
If you are using this option with CHECK TABLE, it will ensure that the table is 100 percent consistent, but will take a long time. If you are using this option with REPAIR TABLE, it will run an extended repair on the table, which may not only take a long time to execute, but may produce a lot of garbage rows also!
-h, --host=...
Connect to host.
-m, --medium-check
Faster than extended-check, but only finds 99.99 percent of all errors. Should be good enough for most cases.
-o, --optimize
Optimize table
-p, --password[=...]
Password to use when connecting to server. If password is not given it's solicited on the tty.
-P, --port=...
Port number to use for connection.
-q, --quick
If you are using this option with CHECK TABLE, it prevents the check from scanning the rows to check for wrong links. This is the fastest check. If you are using this option with REPAIR TABLE, it will try to repair only the index tree. This is the fastest repair method for a table.
-r, --repair
Can fix almost anything except unique keys that aren't unique.
-s, --silent
Print only error messages.
-S, --socket=...
Socket file to use for connection.
--tables
Overrides option --databases (-B).
-u, --user=#
User for login if not current user.
-v, --verbose
Print info about the various stages.
-V, --version
Output version information and exit.

4.8.5 mysqldump, Dumping Table Structure and Data

Utility to dump a database or a collection of database for backup or for transferring the data to another SQL server (not necessarily a MySQL server). The dump will contain SQL statements to create the table and/or populate the table.

If you are doing a backup on the server, you should consider using the mysqlhotcopy instead. See section 4.8.6 mysqlhotcopy, Copying MySQL Databases and Tables.

shell> mysqldump [OPTIONS] database [tables]
OR     mysqldump [OPTIONS] --databases [OPTIONS] DB1 [DB2 DB3...]
OR     mysqldump [OPTIONS] --all-databases [OPTIONS]

If you don't give any tables or use the --databases or --all-databases, the whole database(s) will be dumped.

You can get a list of the options your version of mysqldump supports by executing mysqldump --help.

Note that if you run mysqldump without --quick or --opt, mysqldump will load the whole result set into memory before dumping the result. This will probably be a problem if you are dumping a big database.

Note that if you are using a new copy of the mysqldump program and you are going to do a dump that will be read into a very old MySQL server, you should not use the --opt or -e options.

mysqldump supports the following options:

--add-locks
Add LOCK TABLES before and UNLOCK TABLE after each table dump. (To get faster inserts into MySQL.)
--add-drop-table
Add a drop table before each create statement.
-A, --all-databases
Dump all the databases. This will be same as --databases with all databases selected.
-a, --all
Include all MySQL-specific create options.
--allow-keywords
Allow creation of column names that are keywords. This works by prefixing each column name with the table name.
-c, --complete-insert
Use complete insert statements (with column names).
-C, --compress
Compress all information between the client and the server if both support compression.
-B, --databases
To dump several databases. Note the difference in usage. In this case no tables are given. All name arguments are regarded as database names. USE db_name; will be included in the output before each new database.
--delayed
Insert rows with the INSERT DELAYED command.
-e, --extended-insert
Use the new multiline INSERT syntax. (Gives more compact and faster inserts statements.)
-#, --debug[=option_string]
Trace usage of the program (for debugging).
--help
Display a help message and exit.
--fields-terminated-by=...
--fields-enclosed-by=...
--fields-optionally-enclosed-by=...
--fields-escaped-by=...
--lines-terminated-by=...
These options are used with the -T option and have the same meaning as the corresponding clauses for LOAD DATA INFILE. See section 6.4.8 LOAD DATA INFILE Syntax.
-F, --flush-logs
Flush log file in the MySQL server before starting the dump.
-f, --force,
Continue even if we get a SQL error during a table dump.
-h, --host=..
Dump data from the MySQL server on the named host. The default host is localhost.
-l, --lock-tables.
Lock all tables before starting the dump. The tables are locked with READ LOCAL to allow concurrent inserts in the case of MyISAM tables.
-n, --no-create-db
'CREATE DATABASE /*!32312 IF NOT EXISTS*/ db_name;' will not be put in the output. The above line will be added otherwise, if --databases or --all-databases option was given.
-t, --no-create-info
Don't write table creation information (The CREATE TABLE statement.)
-d, --no-data
Don't write any row information for the table. This is very useful if you just want to get a dump of the structure for a table!
--opt
Same as --quick --add-drop-table --add-locks --extended-insert --lock-tables. Should give you the fastest possible dump for reading into a MySQL server.
-pyour_pass, --password[=your_pass]
The password to use when connecting to the server. If you specify no `=your_pass' part, mysqldump you will be prompted for a password.
-P port_num, --port=port_num
The TCP/IP port number to use for connecting to a host. (This is used for connections to hosts other than localhost, for which Unix sockets are used.)
-q, --quick
Don't buffer query, dump directly to stdout. Uses mysql_use_result() to do this.
-r, --result-file=...
Direct output to a given file. This option should be used in MSDOS, because it prevents new line '\n' from being converted to '\n\r' (new line + carriage return).
-S /path/to/socket, --socket=/path/to/socket
The socket file to use when connecting to localhost (which is the default host).
--tables
Overrides option --databases (-B).
-T, --tab=path-to-some-directory
Creates a table_name.sql file, that contains the SQL CREATE commands, and a table_name.txt file, that contains the data, for each give table. NOTE: This only works if mysqldump is run on the same machine as the mysqld daemon. The format of the .txt file is made according to the --fields-xxx and --lines--xxx options.
-u user_name, --user=user_name
The MySQL user name to use when connecting to the server. The default value is your Unix login name.
-O var=option, --set-variable var=option
Set the value of a variable. The possible variables are listed below.
-v, --verbose
Verbose mode. Print out more information on what the program does.
-V, --version
Print version information and exit.
-w, --where='where-condition'
Dump only selected records. Note that QUOTES are mandatory:
"--where=user='jimf'" "-wuserid>1" "-wuserid<1"
-O net_buffer_length=#, where # < 16M
When creating multi-row-insert statements (as with option --extended-insert or --opt), mysqldump will create rows up to net_buffer_length length. If you increase this variable, you should also ensure that the max_allowed_packet variable in the MySQL server is bigger than the net_buffer_length.

The most normal use of mysqldump is probably for making a backup of whole databases. See section 4.4.1 Database Backups.

mysqldump --opt database > backup-file.sql

You can read this back into MySQL with:

mysql database < backup-file.sql

or

mysql -e "source /patch-to-backup/backup-file.sql" database

However, it's also very useful to populate another MySQL server with information from a database:

mysqldump --opt database | mysql --host=remote-host -C database

It is possible to dump several databases with one command:

mysqldump --databases database1 [database2 database3...] > my_databases.sql

If all the databases are wanted, one can use:

mysqldump --all-databases > all_databases.sql

4.8.6 mysqlhotcopy, Copying MySQL Databases and Tables

mysqlhotcopy is a perl script that uses LOCK TABLES, FLUSH TABLES and cp or scp to quickly make a backup of a database. It's the fastest way to make a backup of the database, of single tables but it can only be run on the same machine where the database directories are.

mysqlhotcopy db_name [/path/to/new_directory]

mysqlhotcopy db_name_1 ... db_name_n /path/to/new_directory

mysqlhotcopy db_name./regex/

mysqlhotcopy supports the following options:

-?, --help
Display a help screen and exit
-u, --user=#
User for database login
-p, --password=#
Password to use when connecting to server
-P, --port=#
Port to use when connecting to local server
-S, --socket=#
Socket to use when connecting to local server
--allowold
Don't abort if target already exists (rename it _old)
--keepold
Don't delete previous (now renamed) target when done
--noindices
Don't include full index files in copy to make the backup smaller and faster The indexes can later be reconstructed with myisamchk -rq..
--method=#
Method for copy (cp or scp).
-q, --quiet
Be silent except for errors
--debug
Enable debug
-n, --dryrun
Report actions without doing them
--regexp=#
Copy all databases with names matching regexp
--suffix=#
Suffix for names of copied databases
--checkpoint=#
Insert checkpoint entry into specified db.table
--flushlog
Flush logs once all tables are locked.
--tmpdir=#
Temporary directory (instead of /tmp).

You can use perldoc mysqlhotcopy to get a more complete documentation for mysqlhotcopy.

mysqlhotcopy reads the groups [client] and [mysqlhotcopy] from the option files.

To be able to execute mysqlhotcopy you need write access to the backup directory, SELECT privilege to the tables you are about to copy and the MySQL Reload privilege (to be able to execute FLUSH TABLES).

4.8.7 mysqlimport, Importing Data from Text Files

mysqlimport provides a command-line interface to the LOAD DATA INFILE SQL statement. Most options to mysqlimport correspond directly to the same options to LOAD DATA INFILE. See section 6.4.8 LOAD DATA INFILE Syntax.

mysqlimport is invoked like this:

shell> mysqlimport [options] database textfile1 [textfile2....]

For each text file named on the command line, mysqlimport strips any extension from the filename and uses the result to determine which table to import the file's contents into. For example, files named `patient.txt', `patient.text', and `patient' would all be imported into a table named patient.

mysqlimport supports the following options:

-c, --columns=...
This option takes a comma-separated list of field names as an argument. The field list is used to create a proper LOAD DATA INFILE command, which is then passed to MySQL. See section 6.4.8 LOAD DATA INFILE Syntax.
-C, --compress
Compress all information between the client and the server if both support compression.
-#, --debug[=option_string]
Trace usage of the program (for debugging).
-d, --delete
Empty the table before importing the text file.
--fields-terminated-by=...
--fields-enclosed-by=...
--fields-optionally-enclosed-by=...
--fields-escaped-by=...
--lines-terminated-by=...
These options have the same meaning as the corresponding clauses for LOAD DATA INFILE. See section 6.4.8 LOAD DATA INFILE Syntax.
-f, --force
Ignore errors. For example, if a table for a text file doesn't exist, continue processing any remaining files. Without --force, mysqlimport exits if a table doesn't exist.
--help
Display a help message and exit.
-h host_name, --host=host_name
Import data to the MySQL server on the named host. The default host is localhost.
-i, --ignore
See the description for the --replace option.
-l, --lock-tables
Lock ALL tables for writing before processing any text files. This ensures that all tables are synchronized on the server.
-L, --local
Read input files from the client. By default, text files are assumed to be on the server if you connect to localhost (which is the default host).
-pyour_pass, --password[=your_pass]
The password to use when connecting to the server. If you specify no `=your_pass' part, mysqlimport you will be prompted for a password.
-P port_num, --port=port_num
The TCP/IP port number to use for connecting to a host. (This is used for connections to hosts other than localhost, for which Unix sockets are used.)
-r, --replace
The --replace and --ignore options control handling of input records that duplicate existing records on unique key values. If you specify --replace, new rows replace existing rows that have the same unique key value. If you specify --ignore, input rows that duplicate an existing row on a unique key value are skipped. If you don't specify either option, an error occurs when a duplicate key value is found, and the rest of the text file is ignored.
-s, --silent
Silent mode. Write output only when errors occur.
-S /path/to/socket, --socket=/path/to/socket
The socket file to use when connecting to localhost (which is the default host).
-u user_name, --user=user_name
The MySQL user name to use when connecting to the server. The default value is your Unix login name.
-v, --verbose
Verbose mode. Print out more information what the program does.
-V, --version
Print version information and exit.

Here is a sample run using mysqlimport:

$ mysql --version
mysql  Ver 9.33 Distrib 3.22.25, for pc-linux-gnu (i686)
$ uname -a
Linux xxx.com 2.2.5-15 #1 Mon Apr 19 22:21:09 EDT 1999 i586 unknown
$ mysql -e 'CREATE TABLE imptest(id INT, n VARCHAR(30))' test
$ ed
a
100     Max Sydow
101     Count Dracula
.
w imptest.txt
32
q
$ od -c imptest.txt
0000000   1   0   0  \t   M   a   x       S   y   d   o   w  \n   1   0
0000020   1  \t   C   o   u   n   t       D   r   a   c   u   l   a  \n
0000040
$ mysqlimport --local test imptest.txt
test.imptest: Records: 2  Deleted: 0  Skipped: 0  Warnings: 0
$ mysql -e 'SELECT * FROM imptest' test
+------+---------------+
| id   | n             |
+------+---------------+
|  100 | Max Sydow     |
|  101 | Count Dracula |
+------+---------------+

4.8.8 Showing Databases, Tables, and Columns

mysqlshow can be used to quickly look at which databases exist, their tables, and the table's columns.

With the mysql program you can get the same information with the SHOW commands. See section 4.5.5 SHOW Syntax.

mysqlshow is invoked like this:

shell> mysqlshow [OPTIONS] [database [table [column]]]

Note that in newer MySQL versions, you only see those database/tables/columns for which you have some privileges.

If the last argument contains a shell or SQL wild-card (*, ?, % or _) then only what's matched by the wild card is shown. This may cause some confusion when you try to display the columns for a table with a _ as in this case mysqlshow only shows you the table names that match the pattern. This is easily fixed by adding an extra % last on the command line (as a separate argument).

4.8.9 perror, Explaining Error Codes

perror can be used to print error message(s). perror can be invoked like this:

shell> perror [OPTIONS] [ERRORCODE [ERRORCODE...]]

For example:

shell> perror 64 79
Error code  64:  Machine is not on the network
Error code  79:  Can not access a needed shared library

perror can be used to display a description for a system error code, or an MyISAM/ISAM table handler error code. The error messages are mostly system dependent.

4.8.10 How to Run SQL Commands from a Text File

The mysql client typically is used interactively, like this:

shell> mysql database

However, it's also possible to put your SQL commands in a file and tell mysql to read its input from that file. To do so, create a text file `text_file' that contains the commands you wish to execute. Then invoke mysql as shown below:

shell> mysql database < text_file

You can also start your text file with a USE db_name statement. In this case, it is unnecessary to specify the database name on the command line:

shell> mysql < text_file

See section 4.8 MySQL Client-Side Scripts and Utilities.

4.9 The MySQL Log Files

MySQL has several different log files that can help you find out what's going on inside mysqld:

The error log Problems encountering starting, running or stopping mysqld.
The isam log Logs all changes to the ISAM tables. Used only for debugging the isam code.
The query log Established connections and executed queries.
The update log Deprecated: Stores all statements that changes data
The binary log Stores all statements that changes something. Used also for replication
The slow log Stores all queries that took more than long_query_time to execute or didn't use indexes.

All logs can be found in the mysqld data directory. You can force mysqld to reopen the log files (or in some cases switch to a new log) by executing FLUSH LOGS. See section 4.5.3 FLUSH Syntax.

4.9.1 The Error Log

mysqld writes all errors to the stderr, which the safe_mysqld script redirects to a file called 'hostname'.err. (On Windows, mysqld writes this directly to `\mysql\data\mysql.err').

This contains information indicating when mysqld was started and stopped and also any critical errors found when running. If mysqld dies unexpectedly and safe_mysqld needs to restart mysqld, safe_mysqld will write a restarted mysqld row in this file. This log also holds a warning if mysqld notices a table that needs to be automatically checked or repaired.

On some operating systems, the error log will contain a stack trace for where mysqld died. This can be used to find out where mysqld died. See section G.1.4 Using a stack trace.

4.9.2 The General Query Log

If you want to know what happens within mysqld, you should start it with --log[=file]. This will log all connections and queries to the log file (by default named `'hostname'.log'). This log can be very useful when you suspect an error in a client and want to know exactly what mysqld thought the client sent to it.

By default, the mysql.server script starts the MySQL server with the -l option. If you need better performance when you start using MySQL in a production environment, you can remove the -l option from mysql.server or change it to --log-binary.

The entries in this log are written as mysqld receives the questions. This may be different than the order in which the statements are executed. This is in contrast to the update log and the binary log which are written after the query is executed, but before any locks are released.

4.9.3 The Update Log

NOTE: The update log is replaced by the binary log. See section 4.9.4 The Binary Update Log. With this you can do anything that you can do with the update log.

When started with the --log-update[=file_name] option, mysqld writes a log file containing all SQL commands that update data. If no filename is given, it defaults to the name of the host machine. If a filename is given, but it doesn't contain a path, the file is written in the data directory. If `file_name' doesn't have an extension, mysqld will create log file names like so: `file_name.###', where ### is a number that is incremented each time you execute mysqladmin refresh, execute mysqladmin flush-logs, execute the FLUSH LOGS statement, or restart the server.

NOTE: For the above scheme to work, you should NOT create your own files with the same filename as the update log + some extensions that may be regarded as a number, in the directory used by the update log!

If you use the --log or -l options, mysqld writes a general log with a filename of `hostname.log', and restarts and refreshes do not cause a new log file to be generated (although it is closed and reopened). In this case you can copy it (on Unix) by doing:

mv hostname.log hostname-old.log
mysqladmin flush-logs
cp hostname-old.log to-backup-directory
rm hostname-old.log

Update logging is smart because it logs only statements that really update data. So an UPDATE or a DELETE with a WHERE that finds no rows is not written to the log. It even skips UPDATE statements that set a column to the value it already has.

The update logging is done immediately after a query completes but before any locks are released or any commit is done. This ensures that the log will be logged in the execution order.

If you want to update a database from update log files, you could do the following (assuming your update logs have names of the form `file_name.###'):

shell> ls -1 -t -r file_name.[0-9]* | xargs cat | mysql

ls is used to get all the log files in the right order.

This can be useful if you have to revert to backup files after a crash and you want to redo the updates that occurred between the time of the backup and the crash.

4.9.4 The Binary Update Log

In the future the binary log will replace the update log, so we recommend you to switch to this log format as soon as possible!

The binary log contains all information that is available in the update log in a more efficient format. It also contains information about how long every query that updated the database took.

The binary log is also used when you are replicating a slave from a master. See section 4.10 Replication in MySQL.

When started with the --log-bin[=file_name] option, mysqld writes a log file containing all SQL commands that update data. If no file name is given, it defaults to the name of the host machine followed by -bin. If file name is given, but it doesn't contain a path, the file is written in the data directory.

You can use the following options to mysqld to affect what is logged to the binary log:

binlog-do-db=database_name Tells the master it should log updates for the specified database, and exclude all others not explicitly mentioned. (Example: binlog-do-db=some_database)
binlog-ignore-db=database_name Tells the master that updates to the given database should not be logged to the binary log (Example: binlog-ignore-db=some_database)

To the binary log filename mysqld will append an extension that is a number that is incremented each time you execute mysqladmin refresh, execute mysqladmin flush-logs, execute the FLUSH LOGS statement or restart the server.

To be able to know which different binary log files have been used, mysqld will also create a binary log index file that contains the name of all used binary log files. By default this has the same name as the binary log file, with the extension '.index'. You can change the name of the binary log index file with the --log-bin-index=[filename] option.

If you are using replication, you should not delete old binary log files until you are sure that no slave will ever need to use them. One way to do this is to do mysqladmin flush-logs once a day and then remove any logs that are more than 3 days old.

You can examine the binary log file with the mysqlbinlog command. For example, you can update a MySQL server from the binary log as follows:

mysqlbinlog log-file | mysql -h server_name

You can also use the mysqlbinlog program to read the binary log directly from a remote MySQL server!

mysqlbinlog --help will give you more information of how to use this program!

If you are using BEGIN [WORK] or SET AUTOCOMMIT=0, you must use the MySQL binary log for backups instead of the old update log.

The binary logging is done immediately after a query completes but before any locks are released or any commit is done. This ensures that the log will be logged in the execution order.

All updates (UPDATE, DELETE or INSERT) that change a transactional table (like BDB tables) are cached until a COMMIT. Any updates to a non-transactional table are stored in the binary log at once. Every thread will, on start, allocate a buffer of binlog_cache_size to buffer queries. If a query is bigger than this, the thread will open a temporary file to handle the bigger cache. The temporary file will be deleted when the thread ends.

The max_binlog_cache_size can be used to restrict the total size used to cache a multi-transaction query.

If you are using the update or binary log, concurrent inserts will not work together with CREATE ... INSERT and INSERT ... SELECT. This is to ensure that you can recreate an exact copy of your tables by applying the log on a backup.

4.9.5 The Slow Query Log

When started with the --log-slow-queries[=file_name] option, mysqld writes a log file containing all SQL commands that took more than long_query_time to execute. The time to get the initial table locks are not counted as execution time.

The slow query log is logged after the query is executed and after all locks has been released. This may be different than the order in which the statements are executed.

If no file name is given, it defaults to the name of the host machine suffixed with -slow.log. If a filename is given, but doesn't contain a path, the file is written in the data directory.

The slow query log can be used to find queries that take a long time to execute and are thus candidates for optimization. With a large log, that can become a difficult task. You can pipe the slow query log through the mysqldumpslow command to get a summary of the queries which appear in the log.

You are using --log-long-format then also queries that are not using indexes are printed. See section 4.1.1 mysqld Command-line Options.

4.9.6 Log File Maintenance

MySQL has a lot of log files which make it easy to see what is going. See section 4.9 The MySQL Log Files. One must however from time to time clean up after MysQL to ensure that the logs don't take up too much disk space.

When using MySQL with log files, you will, from time to time, want to remove/backup old log files and tell MySQL to start logging on new files. See section 4.4.1 Database Backups.

On a Linux (Redhat) installation, you can use the mysql-log-rotate script for this. If you installed MySQL from an RPM distribution, the script should have been installed automatically. Note that you should be careful with this if you are using the log for replication!

On other systems you must install a short script yourself that you start from cron to handle log files.

You can force MySQL to start using new log files by using mysqladmin flush-logs or by using the SQL command FLUSH LOGS. If you are using MySQL Version 3.21 you must use mysqladmin refresh.

The above command does the following:

If you are using only an update log, you only have to flush the logs and then move away the old update log files to a backup. If you are using the normal logging, you can do something like:

shell> cd mysql-data-directory
shell> mv mysql.log mysql.old
shell> mysqladmin flush-logs

and then take a backup and remove `mysql.old'.

4.10 Replication in MySQL

This chapter describes the various replication features in MySQL. It serves as a reference to the options available with replication. You will be introduced to replication and learn how to implement it. Towards the end, there are some frequently asked questions and descriptions of problems and how to solve them.

4.10.1 Introduction

One way replication can be used is to increase both robustness and speed. For robustness you can have two systems and can switch to the backup if you have problems with the master. The extra speed is achieved by sending a part of the non-updating queries to the replica server. Of course this only works if non-updating queries dominate, but that is the normal case.

Starting in Version 3.23.15, MySQL supports one-way replication internally. One server acts as the master, while the other acts as the slave. Note that one server could play the roles of master in one pair and slave in the other. The master server keeps a binary log of updates (See section 4.9.4 The Binary Update Log.) and an index file to binary logs to keep track of log rotation. The slave, upon connecting, informs the master where it left off since the last successfully propagated update, catches up on the updates, and then blocks and waits for the master to notify it of the new updates.

Note that if you are replicating a database, all updates to this database should be done through the master!

Another benefit of using replication is that one can get live backups of the system by doing a backup on a slave instead of doing it on the master. See section 4.4.1 Database Backups.

4.10.2 Replication Implementation Overview

MySQL replication is based on the server keeping track of all changes to your database (updates, deletes, etc) in the binary log. (See section 4.9.4 The Binary Update Log.) and the slave server(s) reading the saved queries from the master server's binary log so that the slave can execute the same queries on its copy of the data.

It is very important to realize that the binary log is simply a record starting from a fixed point in time (the moment you enable binary logging). Any slaves which you set up will need copies of all the data from your master as it existed the moment that you enabled binary logging on the master. If you start your slaves with data that doesn't agree with what was on the master when the binary log was started, your slaves may fail.

A future version (4.0) of MySQL will remove the need to keep a (possibly large) snapshot of data for new slaves that you might wish to set up through the live backup functionality with no locking required. However, at this time, it is necessary to block all writes either with a global read lock or by shutting down the master while taking a snapshot.

Once a slave is properly configured and running, it will simply connect to the master and wait for updates to process. If the master goes away or the slave loses connectivity with your master, it will keep trying to connect every master-connect-retry seconds until it is able to reconnect and resume listening for updates.

Each slave keeps track of where it left off. The master server has no knowledge of how many slaves there are or which ones are up-to-date at any given time.

The next section explains the master/slave setup process in more detail.

4.10.3 How To Set Up Replication

Below is a quick description of how to set up complete replication on your current MySQL server. It assumes you want to replicate all your databases and have not configured replication before. You will need to shutdown your master server briefly to complete the steps outlined below.

  1. Make sure you have a recent version of MySQL installed on the master and slave(s). Use Version 3.23.29 or higher. Previous releases used a different binary log format and had bugs which have been fixed in newer releases. Please, do not report bugs until you have verified that the problem is present in the latest release.
  2. Set up special a replication user on the master with the FILE privilege and permission to connect from all the slaves. If the user is only doing replication (which is recommended), you don't need to grant any additional privileges. For example, to create a user named repl which can access your master from any host, you might use this command:
    GRANT FILE ON *.* TO repl@"%" IDENTIFIED BY '<password>';
    
  3. Shut down MySQL on the master.
    mysqladmin -u root -p<password> shutdown
    
  4. Snapshot all the data on your master server. The easiest way to do this (on Unix) is to simply use tar to produce an archive of your entire data directory. The exact data directory location depends on your installation.
    tar -cvf /tmp/mysql-snapshot.tar /path/to/data-dir
    
    Windows users can use WinZip or similar software to create an archive of the data directory.
  5. In my.cnf on the master add log-bin and server-id=unique number to the [mysqld] section and restart it. It is very important that the id of the slave is different from the id of the master. Think of server-id as something similar to the IP address - it uniquely identifies the server instance in the community of replication partners.
    [mysqld]
    log-bin
    server-id=1
    
  6. Restart MySQL on the master.
  7. Add the following to my.cnf on the slave(s):
    master-host=<hostname of the master>
    master-user=<replication user name>
    master-password=<replication user password>
    master-port=<TCP/IP port for master>
    server-id=<some unique number between 2 and 2^32-1>
    
    replacing the values in <> with what is relevant to your system. server-id must be different for each server participating in replication. If you don't specify a server-id, it will be set to 1 if you have not defined master-host, else it will be set to 2. Note that in the case of server-id omission the master will refuse connections from all slaves, and the slave will refuse to connect to a master. Thus, omitting server-id is only good for backup with a binary log.
  8. Copy the snapshot data into your data directory on your slave(s). Make sure that the privileges on the files and directories are correct. The user which MySQL runs as needs to be able to read and write to them, just as on the master.
  9. Restart the slave(s).

After you have done the above, the slave(s) should connect to the master and catch up on any updates which happened since the snapshot was taken.

If you have forgotten to set server-id for the slave you will get the following error in the error log file:

Warning: one should set server_id to a non-0 value if master_host is set.
The server will not act as a slave.

If you have forgot to do this for the master, the slaves will not be able to connect to the master.

If a slave is not able to replicate for any reason, you will find error messages in the error log on the slave.

Once a slave is replicating, you will find a file called master.info in the same directory as your error log. The master.info file is used by the slave to keep track of how much of the master's binary log is has processed. Do not remove or edit the file, unless you really know what you are doing. Even in that case, it is preferred that you use CHANGE MASTER TO command.

4.10.4 Replication Features and Known Problems

Below is an explanation of what is supported and what is not:

4.10.5 Replication Options in my.cnf

If you are using replication, we recommend you to use MySQL Version 3.23.30 or later. Older versions work, but they do have some bugs and are missing some features.

On both master and slave you need to use the server-id option. This sets an unique replication id. You should pick a unique value in the range between 1 to 2^32-1 for each master and slave. Example: server-id=3

The following table has the options you can use for the MASTER:

Option Description
log-bin=filename Write to a binary update log to the specified location. Note that if you give it a parameter with an extension (for example, log-bin=/mysql/logs/replication.log ) versions up to 3.23.24 will not work right during replication if you do FLUSH LOGS . The problem is fixed in Version 3.23.25. If you are using this kind of log name, FLUSH LOGS will be ignored on binlog. To clear the log, run FLUSH MASTER, and do not forget to run FLUSH SLAVE on all slaves. In Version 3.23.26 and in later versions you should use RESET MASTER and RESET SLAVE
log-bin-index=filename Because the user could issue the FLUSH LOGS command, we need to know which log is currently active and which ones have been rotated out and in what sequence. This information is stored in the binary log index file. The default is `hostname`.index. You can use this option if you want to be a rebel. Example: log-bin-index=db.index.
sql-bin-update-same If set, setting SQL_LOG_BIN to a value will automatically set SQL_LOG_UPDATE to the same value and vice versa.
binlog-do-db=database_name Tells the master that it should log updates to the binary log if the current database is 'database_name'. All others database are ignored. Note that if you use this you should ensure that you only do updates in the current database. Example: binlog-do-db=some_database.
binlog-ignore-db=database_name Tells the master that updates where the current database is 'database_name' should not be stored in the binary log. Note that if you use this you should ensure that you only do updates in the current database. Example: binlog-ignore-db=some_database

The following table has the options you can use for the SLAVE:

Option Description
master-host=host Master hostname or IP address for replication. If not set, the slave thread will not be started. Example: master-host=db-master.mycompany.com.
master-user=username The user the slave thread will us for authentication when connecting to the master. The user must have FILE privilege. If the master user is not set, user test is assumed. Example: master-user=scott.
master-password=password The password the slave thread will authenticate with when connecting to the master. If not set, an empty password is assumed. Example: master-password=tiger.
master-port=portnumber The port the master is listening on. If not set, the compiled setting of MYSQL_PORT is assumed. If you have not tinkered with configure options, this should be 3306. Example: master-port=3306.
master-connect-retry=seconds The number of seconds the slave thread will sleep before retrying to connect to the master in case the master goes down or the connection is lost. Default is 60. Example: master-connect-retry=60.
master-info-file=filename The location of the file that remembers where we left off on the master during the replication process. The default is master.info in the data directory. Sasha: The only reason I see for ever changing the default is the desire to be rebelious. Example: master-info-file=master.info.
replicate-do-table=db_name.table_name Tells the slave thread to restrict replication to the specified table. To specify more than one table, use the directive multiple times, once for each table. This will work for cross-database updates, in contrast to replicate-do-db. Example: replicate-do-table=some_db.some_table.
replicate-ignore-table=db_name.table_name Tells the slave thread to not replicate to the specified table. To specify more than one table to ignore, use the directive multiple times, once for each table. This will work for cross-datbase updates, in contrast to replicate-ignore-db. Example: replicate-ignore-table=db_name.some_table.
replicate-wild-do-table=db_name.table_name Tells the slave thread to restrict replication to the tables that match the specified wildcard pattern. To specify more than one table, use the directive multiple times, once for each table. This will work for cross-database updates. Example: replicate-wild-do-table=foo%.bar% will replicate only updates to tables in all databases that start with foo and whose table names start with bar.
replicate-wild-ignore-table=db_name.table_name Tells the slave thread to not replicate to the tables that match the given wild card pattern. To specify more than one table to ignore, use the directive multiple times, once for each table. This will work for cross-database updates. Example: replicate-wild-ignore-table=foo%.bar% will not do updates to tables in databases that start with foo and whose table names start with bar.
replicate-ignore-db=database_name Tells the slave thread to not replicate to the specified database. To specify more than one database to ignore, use the directive multiple times, once for each database. This option will not work if you use cross database updates. If you need cross database updates to work, make sure you have 3.23.28 or later, and use replicate-wild-ignore-table=db_name.% Example: replicate-ignore-db=some_db.
replicate-do-db=database_name Tells the slave thread to restrict replication to the specified database. To specify more than one database, use the directive multiple times, once for each database. Note that this will only work if you do not use cross-database queries such as UPDATE some_db.some_table SET foo='bar' while having selected a different or no database. If you need cross database updates to work, make sure you have 3.23.28 or later, and use replicate-wild-do-table=db_name.% Example: replicate-do-db=some_db.
log-slave-updates Tells the slave to log the updates from the slave thread to the binary log. Off by default. You will need to turn it on if you plan to daisy-chain the slaves.
replicate-rewrite-db=from_name->to_name Updates to a database with a different name than the original Example: replicate-rewrite-db=master_db_name->slave_db_name.
skip-slave-start Tells the slave server not to start the slave on the startup. The user can start it later with SLAVE START.
slave_read_timeout=# Number of seconds to wait for more data from the master before aborting the read.

4.10.6 SQL Commands Related to Replication

Replication can be controlled through the SQL interface. Below is the summary of commands:

Command Description
SLAVE START Starts the slave thread. (Slave)
SLAVE STOP Stops the slave thread. (Slave)
SET SQL_LOG_BIN=0 Disables update logging if the user has process privilege. Ignored otherwise. (Master)
SET SQL_LOG_BIN=1 Re-enables update logging if the user has process privilege. Ignored otherwise. (Master)
SET SQL_SLAVE_SKIP_COUNTER=n Skip the next n events from the master. Only valid when the slave thread is not running, otherwise, gives an error. Useful for recovering from replication glitches.
RESET MASTER Deletes all binary logs listed in the index file, resetting the binlog index file to be empty. In pre-3.23.26 versions, FLUSH MASTER (Master)
RESET SLAVE Makes the slave forget its replication position in the master logs. In pre 3.23.26 versions the command was called FLUSH SLAVE(Slave)
LOAD TABLE tblname FROM MASTER Downloads a copy of the table from master to the slave. (Slave)
CHANGE MASTER TO master_def_list Changes the master parameters to the values specified in master_def_list and restarts the slave thread. master_def_list is a comma-separated list of master_def where master_def is one of the following: MASTER_HOST, MASTER_USER, MASTER_PASSWORD, MASTER_PORT, MASTER_CONNECT_RETRY, MASTER_LOG_FILE, MASTER_LOG_POS. Example:

CHANGE MASTER TO
  MASTER_HOST='master2.mycompany.com',
  MASTER_USER='replication',
  MASTER_PASSWORD='bigs3cret',
  MASTER_PORT=3306,
  MASTER_LOG_FILE='master2-bin.001',
  MASTER_LOG_POS=4;

You only need to specify the values that need to be changed. The values that you omit will stay the same with the exception of when you change the host or the port. In that case, the slave will assume that since you are connecting to a different host or a different port, the master is different. Therefore, the old values of log and position are not applicable anymore, and will automatically be reset to an empty string and 0, respectively (the start values). Note that if you restart the slave, it will remember its last master. If this is not desirable, you should delete the `master.info' file before restarting, and the slave will read its master from my.cnf or the command line. (Slave)
SHOW MASTER STATUS Provides status information on the binlog of the master. (Master)
SHOW SLAVE STATUS Provides status information on essential parameters of the slave thread. (Slave)
SHOW MASTER LOGS Only available starting in Version 3.23.28. Lists the binary logs on the master. You should use this command prior to PURGE MASTER LOGS TO to find out how far you should go.
PURGE MASTER LOGS TO 'logname' Available starting in Version 3.23.28. Deletes all the replication logs that are listed in the log index as being prior to the specified log, and removed them from the log index, so that the given log now becomes first. Example:
PURGE MASTER LOGS TO 'mysql-bin.010'
This command will do nothing and fail with an error if you have an active slave that is currently reading one of the logs you are trying to delete. However, if you have a dormant slave, and happen to purge one of the logs it wants to read, the slave will be unable to replicate once it comes up. The command is safe to run while slaves are replicating - you do not need to stop them. You must first check all the slaves with SHOW SLAVE STATUS to see which log they are on, then do a listing of the logs on the master with SHOW MASTER LOGS, find the earliest log among all the slaves (if all the slaves are up to date, this will be the last log on the list), backup all the logs you are about to delete (optional) and purge up to the target log.

4.10.7 Replication FAQ

Q: Why do I sometimes see more than one Binlog_Dump thread on the master after I have restarted the slave?

A: Binlog_Dump is a continuous process that is handled by the server in the following way:

So if the slave thread stops on the slave, the corresponding Binlog_Dump thread on the master will not notice it until after at least one update to the master (or a kill), which is needed to wake it up from pthread_cond_wait(). In the meantime, the slave could have opened another connection, which resulted in another Binlog_Dump thread.

The above problem should not be present in Version 3.23.26 and later versions. In Version 3.23.26 we added server-id to each replication server, and now all the old zombie threads are killed on the master when a new replication thread connects from the same slave

Q: How do I rotate replication logs?

A: In Version 3.23.28 you should use PURGE MASTER LOGS TO command after determining which logs can be deleted, and optionally backing them up first. In earlier versions the process is much more painful, and cannot be safely done without stopping all the slaves in the case that you plan to re-use log names. You will need to stop the slave threads, edit the binary log index file, delete all the old logs, restart the master, start slave threads, and then remove the old log files.

Q: How do I upgrade on a hot replication setup?

A: If you are upgrading pre-3.23.26 versions, you should just lock the master tables, let the slave catch up, then run FLUSH MASTER on the master, and FLUSH SLAVE on the slave to reset the logs, then restart new versions of the master and the slave. Note that the slave can stay down for some time - since the master is logging all the updates, the slave will be able to catch up once it is up and can connect.

After 3.23.26, we have locked the replication protocol for modifications, so you can upgrade masters and slave on the fly to a newer 3.23 version and you can have different versions of MySQL running on the slave and the master, as long as they are both newer than 3.23.26.

Q: What issues should I be aware of when setting up two-way replication?

A: MySQL replication currently does not support any locking protocol between master and slave to guarantee the atomicity of a distributed (cross-server) update. In in other words, it is possible for client A to make an update to co-master 1, and in the meantime, before it propagates to co-master 2, client B could make an update to co-master 2 that will make the update of client A work differently than it did on co-master 1. Thus when the update of client A will make it to co-master 2, it will produce tables that will be different than what you have on co-master 1, even after all the updates from co-master 2 have also propagated. So you should not co-chain two servers in a two-way replication relationship, unless you are sure that you updates can safely happen in any order, or unless you take care of mis-ordered updates somehow in the client code.

You must also realize that two-way replication actually does not improve performance very much, if at all, as far as updates are concerned. Both servers need to do the same amount of updates each, as you would have one server do. The only difference is that there will be a little less lock contention, because the updates originating on another server will be serialized in one slave thread. This benefit, though, might be offset by network delays.

Q: How can I use replication to improve performance of my system?

A: You should set up one server as the master, and direct all writes to it, and configure as many slaves as you have the money and rackspace for, distributing the reads among the master and the slaves. You can also start the slaves with --skip-bdb, --low-priority-updates and --delay-key-write-for-all-tables to get speed improvements for the slave. In this case the slave will use non-transactional MyISAM tables instead of BDB tables to get more speed.

Q: What should I do to prepare my client code to use performance-enhancing replication?

A: If the part of your code that is responsible for database access has been properly abstracted/modularized, converting it to run with the replicated setup should be very smooth and easy - just change the implementation of your database access to read from some slave or the master, and to always write to the master. If your code does not have this level of abstraction, setting up a replicated system will give you an opportunity/motivation to it clean up. You should start by creating a wrapper library /module with the following functions:

safe_ means that the function will take care of handling all the error conditions.

You should then convert your client code to use the wrapper library. It may be a painful and scary process at first, but it will pay off in the long run. All applications that follow the above pattern will be able to take advantage of one-master/many slaves solution. The code will be a lot easier to maintain, and adding troubleshooting options will be trivial. You will just need to modify one or two functions, for example, to log how long each query took, or which query, among your many thousands, gave you an error. If you have written a lot of code already, you may want to automate the conversion task by using Monty's replace utility, which comes with the standard distribution of MySQL, or just write your own Perl script. Hopefully, your code follows some recognizable pattern. If not, then you are probably better off re-writing it anyway, or at least going through and manually beating it into a pattern.

Note that, of course, you can use different names for the functions. What is important is having unified interface for connecting for reads, connecting for writes, doing a read, and doing a write.

Q: When and how much can MySQL replication improve the performance of my system?

A: MySQL replication is most beneficial for a system with frequent reads and not so frequent writes. In theory, by using a one master/many slaves setup you can scale by adding more slaves until you either run out of network bandwidth, or your update load grows to the point that the master cannot handle it.

In order to determine how many slaves you can get before the added benefits begin to level out, and how much you can improve performance of your site, you need to know your query patterns, and empirically (by benchmarking) determine the relationship between the throughput on reads (reads per second, or max_reads) and on writes max_writes) on a typical master and a typical slave. The example below will show you a rather simplified calculation of what you can get with replication for our imagined system.

Let's say our system load consists of 10% writes and 90% reads, and we have determined that max_reads = 1200 - 2 * max_writes, or in other words, our system can do 1200 reads per second with no writes, our average write is twice as slow as average read, and the relationship is linear. Let us suppose that our master and slave are of the same capacity, and we have N slaves and 1 master. Then we have for each server (master or slave):

reads = 1200 - 2 * writes (from bencmarks)

reads = 9* writes / (N + 1) (reads split, but writes go to all servers)

9*writes/(N+1) + 2 * writes = 1200

writes = 1200/(2 + 9/(N+1)

So if N = 0, which means we have no replication, our system can handle 1200/11, about 109 writes per second (which means we will have 9 times as many reads due to the nature of our application).

If N = 1, we can get up to 184 writes per second.

If N = 8, we get up to 400.

If N = 17, 480 writes.

Eventually as N approaches infinity (and our budget negative infinity), we can get very close to 600 writes per second, increasing system throughput about 5.5 times. However, with only 8 servers, we increased it almost 4 times already.

Note that our computations assumed infinite network bandwidth, and neglected several other factors that could turn out to be significant on your system. In many cases, you may not be able to make a computation similar to the one above that will accurately predict what will happen on your system if you add N replication slaves. However, answering the following questions should help you decided whether and how much, if at all, the replication will improve the performance of your system:

Q: How can I use replication to provide redundancy/high availability?

A: With the currently available features, you would have to set up a master and a slave (or several slaves), and write a script that will monitor the master to see if it is up, and instruct your applications and the slaves of the master change in case of failure. Some suggestions:

We are currently working on integrating an automatic master election system into MySQL, but until it is ready, you will have to create your own monitoring tools.

4.10.8 Troubleshooting Replication

If you have followed the instructions, and your replication setup is not working, first eliminate the user error factor by checking the following:

When you have determined that there is no user error involved, and replication still either does not work at all or is unstable, it is time to start working on a bug report. We need to get as much info as possible from you to be able to track down the bug. Please do spend some time and effort preparing a good bug report. Ideally, we would like to have a test case in the format found in mysql-test/t/rpl* directory of the source tree. If you submit a test case like that, you can expect a patch within a day or two in most cases, although, of course, you mileage may vary depending on a number of factors.

Second best option is a just program with easily configurable connection arguments for the master and the slave that will demonstrate the problem on our systems. You can write one in Perl or in C, depending on which language you know better.

If you have one of the above ways to demonstrate the bug, use mysqlbug to prepare a bug report and send it to bugs@lists.mysql.com. If you have a phantom - a problem that does occur but you cannot duplicate "at will":

Once you have collected the evidence on the phantom problem, try hard to isolate it into a separate test case first. Then report the problem to bugs@lists.mysql.com with as much info as possible.

5 MySQL Optimization

Optimization is a complicated task because it ultimately requires understanding of the whole system. While it may be possible to do some local optimizations with small knowledge of your system or application, the more optimal you want your system to become the more you will have to know about it.

This chapter will try to explain and give some examples of different ways to optimize MySQL. Remember, however, that there are always some (increasingly harder) additional ways to make the system even faster.

5.1 Optimization Overview

The most important part for getting a system fast is of course the basic design. You also need to know what kinds of things your system will be doing, and what your bottlenecks are.

The most common bottlenecks are:

5.1.1 MySQL Design Limitations/Tradeoffs

Because MySQL uses extremely fast table locking (multiple readers / single writers) the biggest remaining problem is a mix of a steady stream of inserts and slow selects on the same table.

We believe that for a huge number of systems the extremely fast performance in other cases make this choice a win. This case is usually also possible to solve by having multiple copies of the table, but it takes more effort and hardware.

We are also working on some extensions to solve this problem for some common application niches.

5.1.2 Portability

Because all SQL servers implement different parts of SQL, it takes work to write portable SQL applications. For very simple selects/inserts it is very easy, but the more you need the harder it gets. If you want an application that is fast with many databases it becomes even harder!

To make a complex application portable you need to choose a number of SQL servers that it should work with.

You can use the MySQL crash-me program/web-page http://www.mysql.com/information/crash-me.php to find functions, types, and limits you can use with a selection of database servers. Crash-me now tests far from everything possible, but it is still comprehensive with about 450 things tested.

For example, you shouldn't have column names longer than 18 characters if you want to be able to use Informix or DB2.

Both the MySQL benchmarks and crash-me programs are very database-independent. By taking a look at how we have handled this, you can get a feeling for what you have to do to write your application database-independent. The benchmarks themselves can be found in the `sql-bench' directory in the MySQL source distribution. They are written in Perl with DBI database interface (which solves the access part of the problem).

See http://www.mysql.com/information/benchmarks.html for the results from this benchmark.

As you can see in these results, all databases have some weak points. That is, they have different design compromises that lead to different behavior.

If you strive for database independence, you need to get a good feeling for each SQL server's bottlenecks. MySQL is VERY fast in retrieving and updating things, but will have a problem in mixing slow readers/writers on the same table. Oracle, on the other hand, has a big problem when you try to access rows that you have recently updated (until they are flushed to disk). Transaction databases in general are not very good at generating summary tables from log tables, as in this case row locking is almost useless.

To get your application really database-independent, you need to define an easy extendable interface through which you manipulate your data. As C++ is available on most systems, it makes sense to use a C++ classes interface to the databases.

If you use some specific feature for some database (like the REPLACE command in MySQL), you should code a method for the other SQL servers to implement the same feature (but slower). With MySQL you can use the /*! */ syntax to add MySQL-specific keywords to a query. The code inside /**/ will be treated as a comment (ignored) by most other SQL servers.

If REAL high performance is more important than exactness, as in some Web applications, a possibility is to create an application layer that caches all results to give you even higher performance. By letting old results 'expire' after a while, you can keep the cache reasonably fresh. This is quite nice in case of extremely high load, in which case you can dynamically increase the cache and set the expire timeout higher until things get back to normal.

In this case the table creation information should contain information of the initial size of the cache and how often the table should normally be refreshed.

5.1.3 What Have We Used MySQL For?

During MySQL initial development, the features of MySQL were made to fit our largest customer. They handle data warehousing for a couple of the biggest retailers in Sweden.

From all stores, we get weekly summaries of all bonus card transactions, and we are expected to provide useful information for the store owners to help them find how their advertisement campaigns are affecting their customers.

The data is quite huge (about 7 million summary transactions per month), and we have data for 4-10 years that we need to present to the users. We got weekly requests from the customers that they want to get 'instant' access to new reports from this data.

We solved this by storing all information per month in compressed 'transaction' tables. We have a set of simple macros (script) that generates summary tables grouped by different criteria (product group, customer id, store ...) from the transaction tables. The reports are Web pages that are dynamically generated by a small Perl script that parses a Web page, executes the SQL statements in it, and inserts the results. We would have used PHP or mod_perl instead but they were not available at that time.

For graphical data we wrote a simple tool in C that can produce GIFs based on the result of a SQL query (with some processing of the result). This is also dynamically executed from the Perl script that parses the HTML files.

In most cases a new report can simply be done by copying an existing script and modifying the SQL query in it. In some cases, we will need to add more fields to an existing summary table or generate a new one, but this is also quite simple, as we keep all transactions tables on disk. (Currently we have at least 50G of transactions tables and 200G of other customer data.)

We also let our customers access the summary tables directly with ODBC so that the advanced users can themselves experiment with the data.

We haven't had any problems handling this with quite modest Sun Ultra SPARCstation (2x200 Mhz). We recently upgraded one of our servers to a 2 CPU 400 Mhz UltraSPARC, and we are now planning to start handling transactions on the product level, which would mean a ten-fold increase of data. We think we can keep up with this by just adding more disk to our systems.

We are also experimenting with Intel-Linux to be able to get more CPU power cheaper. Now that we have the binary portable database format (new in Version 3.23), we will start to use this for some parts of the application.

Our initial feelings are that Linux will perform much better on low-to-medium load and Solaris will perform better when you start to get a high load because of extreme disk IO, but we don't yet have anything conclusive about this. After some discussion with a Linux Kernel developer, this might be a side effect of Linux giving so much resources to the batch job that the interactive performance gets very low. This makes the machine feel very slow and unresponsive while big batches are going. Hopefully this will be better handled in future Linux Kernels.

5.1.4 The MySQL Benchmark Suite

This should contain a technical description of the MySQL benchmark suite (and crash-me), but that description is not written yet. Currently, you can get a good idea of the benchmark by looking at the code and results in the `sql-bench' directory in any MySQL source distributions.

This benchmark suite is meant to be a benchmark that will tell any user what things a given SQL implementation performs well or poorly at.

Note that this benchmark is single threaded, so it measures the minimum time for the operations. We plan to in the future add a lot of multi-threaded tests to the benchmark suite.

For example, (run on the same NT 4.0 machine):

Reading 2000000 rows by index
Seconds Seconds
mysql 367 249
mysql_odbc 464
db2_odbc 1206
informix_odbc 121126
ms-sql_odbc 1634
oracle_odbc 20800
solid_odbc 877
sybase_odbc 17614
Inserting (350768) rows
Seconds Seconds
mysql 381 206
mysql_odbc 619
db2_odbc 3460
informix_odbc 2692
ms-sql_odbc 4012
oracle_odbc 11291
solid_odbc 1801
sybase_odbc 4802

In the above test MySQL was run with a 8M index cache.

We have gather some more benchmark results at http://www.mysql.com/information/benchmarks.html.

Note that Oracle is not included because they asked to be removed. All Oracle benchmarks have to be passed by Oracle! We believe that makes Oracle benchmarks VERY biased because the above benchmarks are supposed to show what a standard installation can do for a single client.

To run the benchmark suite, you have to download a MySQL source distribution, install the perl DBI driver, the perl DBD driver for the database you want to test and then do:

cd sql-bench
perl run-all-tests --server=#

where # is one of supported servers. You can get a list of all options and supported servers by doing run-all-tests --help.

crash-me tries to determine what features a database supports and what its capabilities and limitations are by actually running queries. For example, it determines:

We can find the result from crash-me on a lot of different databases at http://www.mysql.com/information/crash-me.php.

5.1.5 Using Your Own Benchmarks

You should definitely benchmark your application and database to find out where the bottlenecks are. By fixing it (or by replacing the bottleneck with a 'dummy module') you can then easily identify the next bottleneck (and so on). Even if the overall performance for your application is sufficient, you should at least make a plan for each bottleneck, and decide how to solve it if someday you really need the extra performance.

For an example of portable benchmark programs, look at the MySQL benchmark suite. See section 5.1.4 The MySQL Benchmark Suite. You can take any program from this suite and modify it for your needs. By doing this, you can try different solutions to your problem and test which is really the fastest solution for you.

It is very common that some problems only occur when the system is very heavily loaded. We have had many customers who contact us when they have a (tested) system in production and have encountered load problems. In every one of these cases so far, it has been problems with basic design (table scans are NOT good at high load) or OS/Library issues. Most of this would be a LOT easier to fix if the systems were not already in production.

To avoid problems like this, you should put some effort into benchmarking your whole application under the worst possible load! You can use Super Smack for this, and it is available at: http://www.mysql.com/Downloads/super-smack/super-smack-1.0.tar.gz. As the name suggests, it can bring your system down to its knees if you ask it, so make sure to use it only on your development systems.

5.2 Optimizing SELECTs and Other Queries

First, one thing that affects all queries: The more complex permission system setup you have, the more overhead you get.

If you do not have any GRANT statements done, MySQL will optimize the permission checking somewhat. So if you have a very high volume it may be worth the time to avoid grants. Otherwise more permission check results in a larger overhead.

If your problem is with some explicit MySQL function, you can always time this in the MySQL client:

mysql> select benchmark(1000000,1+1);
+------------------------+
| benchmark(1000000,1+1) |
+------------------------+
|                      0 |
+------------------------+
1 row in set (0.32 sec)

The above shows that MySQL can execute 1,000,000 + expressions in 0.32 seconds on a PentiumII 400MHz.

All MySQL functions should be very optimized, but there may be some exceptions, and the benchmark(loop_count,expression) is a great tool to find out if this is a problem with your query.

5.2.1 EXPLAIN Syntax (Get Information About a SELECT)

    EXPLAIN tbl_name
or  EXPLAIN SELECT select_options

EXPLAIN tbl_name is a synonym for DESCRIBE tbl_name or SHOW COLUMNS FROM tbl_name.

When you precede a SELECT statement with the keyword EXPLAIN, MySQL explains how it would process the SELECT, providing information about how tables are joined and in which order.

With the help of EXPLAIN, you can see when you must add indexes to tables to get a faster SELECT that uses indexes to find the records. You can also see if the optimizer joins the tables in an optimal order. To force the optimizer to use a specific join order for a SELECT statement, add a STRAIGHT_JOIN clause.

For non-simple joins, EXPLAIN returns a row of information for each table used in the SELECT statement. The tables are listed in the order they would be read. MySQL resolves all joins using a single-sweep multi-join method. This means that MySQL reads a row from the first table, then finds a matching row in the second table, then in the third table and so on. When all tables are processed, it outputs the selected columns and backtracks through the table list until a table is found for which there are more matching rows. The next row is read from this table and the process continues with the next table.

Output from EXPLAIN includes the following columns:

table
The table to which the row of output refers.
type
The join type. Information about the various types is given below.
possible_keys
The possible_keys column indicates which indexes MySQL could use to find the rows in this table. Note that this column is totally independent of the order of the tables. That means that some of the keys in possible_keys may not be usable in practice with the generated table order. If this column is empty, there are no relevant indexes. In this case, you may be able to improve the performance of your query by examining the WHERE clause to see if it refers to some column or columns that would be suitable for indexing. If so, create an appropriate index and check the query with EXPLAIN again. See section 6.5.4 ALTER TABLE Syntax. To see what indexes a table has, use SHOW INDEX FROM tbl_name.
key
The key column indicates the key that MySQL actually decided to use. The key is NULL if no index was chosen. If MySQL chooses the wrong index, you can probably force MySQL to use another index by using myisamchk --analyze, See section 4.4.6.1 myisamchk Invocation Syntax, or by using USE INDEX/IGNORE INDEX. See section 6.4.1.1 JOIN Syntax.
key_len
The key_len column indicates the length of the key that MySQL decided to use. The length is NULL if the key is NULL. Note that this tells us how many parts of a multi-part key MySQL will actually use.
ref
The ref column shows which columns or constants are used with the key to select rows from the table.
rows
The rows column indicates the number of rows MySQL believes it must examine to execute the query.
Extra
This column contains additional information of how MySQL will resolve the query. Here is an explanation of the different text strings that can be found in this column:
Distinct
MySQL will not continue searching for more rows for the current row combination after it has found the first matching row.
Not exists
MySQL was able to do a LEFT JOIN optimization on the query and will not examine more rows in this table for the previous row combination after it finds one row that matches the LEFT JOIN criteria. Here is an example for this:
SELECT * FROM t1 LEFT JOIN t2 ON t1.id=t2.id WHERE t2.id IS NULL;
Assume that t2.id is defined with NOT NULL. In this case MySQL will scan t1 and look up the rows in t2 through t1.id. If MySQL finds a matching row in t2, it knows that t2.id can never be NULL, and will not scan through the rest of the rows in t2 that has the same id. In other words, for each row in t1, MySQL only needs to do a single lookup in t2, independent of how many matching rows there are in t2.
range checked for each record (index map: #)
MySQL didn't find a real good index to use. It will, instead, for each row combination in the preceding tables, do a check on which index to use (if any), and use this index to retrieve the rows from the table. This isn't very fast but is faster than having to do a join without an index.
Using filesort
MySQL will need to do an extra pass to find out how to retrieve the rows in sorted order. The sort is done by going through all rows according to the join type and storing the sort key + pointer to the row for all rows that match the WHERE. Then the keys are sorted. Finally the rows are retrieved in sorted order.
Using index
The column information is retrieved from the table using only information in the index tree without having to do an additional seek to read the actual row. This can be done when all the used columns for the table are part of the same index.
Using temporary
To resolve the query MySQL will need to create a temporary table to hold the result. This typically happens if you do an ORDER BY on a different column set than you did a GROUP BY on.
Where used
A WHERE clause will be used to restrict which rows will be matched against the next table or sent to the client. If you don't have this information and the table is of type ALL or index, you may have something wrong in your query (if you don't intend to fetch/examine all rows from the table).
If you want to get your queries as fast as possible, you should look out for Using filesort and Using temporary.

The different join types are listed below, ordered from best to worst type:

system
The table has only one row (= system table). This is a special case of the const join type.
const
The table has at most one matching row, which will be read at the start of the query. Because there is only one row, values from the column in this row can be regarded as constants by the rest of the optimizer. const tables are very fast as they are read only once!
eq_ref
One row will be read from this table for each combination of rows from the previous tables. This is the best possible join type, other than the const types. It is used when all parts of an index are used by the join and the index is UNIQUE or a PRIMARY KEY.
ref
All rows with matching index values will be read from this table for each combination of rows from the previous tables. ref is used if the join uses only a leftmost prefix of the key, or if the key is not UNIQUE or a PRIMARY KEY (in other words, if the join cannot select a single row based on the key value). If the key that is used matches only a few rows, this join type is good.
range
Only rows that are in a given range will be retrieved, using an index to select the rows. The key column indicates which index is used. The key_len contains the longest key part that was used. The ref column will be NULL for this type.
index
This is the same as ALL, except that only the index tree is scanned. This is usually faster than ALL, as the index file is usually smaller than the data file.
ALL
A full table scan will be done for each combination of rows from the previous tables. This is normally not good if the table is the first table not marked const, and usually very bad in all other cases. You normally can avoid ALL by adding more indexes, so that the row can be retrieved based on constant values or column values from earlier tables.

You can get a good indication of how good a join is by multiplying all values in the rows column of the EXPLAIN output. This should tell you roughly how many rows MySQL must examine to execute the query. This number is also used when you restrict queries with the max_join_size variable. See section 5.5.2 Tuning Server Parameters.

The following example shows how a JOIN can be optimized progressively using the information provided by EXPLAIN.

Suppose you have the SELECT statement shown below, that you examine using EXPLAIN:

EXPLAIN SELECT tt.TicketNumber, tt.TimeIn,
            tt.ProjectReference, tt.EstimatedShipDate,
            tt.ActualShipDate, tt.ClientID,
            tt.ServiceCodes, tt.RepetitiveID,
            tt.CurrentProcess, tt.CurrentDPPerson,
            tt.RecordVolume, tt.DPPrinted, et.COUNTRY,
            et_1.COUNTRY, do.CUSTNAME
        FROM tt, et, et AS et_1, do
        WHERE tt.SubmitTime IS NULL
            AND tt.ActualPC = et.EMPLOYID
            AND tt.AssignedPC = et_1.EMPLOYID
            AND tt.ClientID = do.CUSTNMBR;

For this example, assume that:

Initially, before any optimizations have been performed, the EXPLAIN statement produces the following information:

table type possible_keys                key  key_len ref  rows  Extra
et    ALL  PRIMARY                      NULL NULL    NULL 74
do    ALL  PRIMARY                      NULL NULL    NULL 2135
et_1  ALL  PRIMARY                      NULL NULL    NULL 74
tt    ALL  AssignedPC,ClientID,ActualPC NULL NULL    NULL 3872
      range checked for each record (key map: 35)

Because type is ALL for each table, this output indicates that MySQL is doing a full join for all tables! This will take quite a long time, as the product of the number of rows in each table must be examined! For the case at hand, this is 74 * 2135 * 74 * 3872 = 45,268,558,720 rows. If the tables were bigger, you can only imagine how long it would take.

One problem here is that MySQL can't (yet) use indexes on columns efficiently if they are declared differently. In this context, VARCHAR and CHAR are the same unless they are declared as different lengths. Because tt.ActualPC is declared as CHAR(10) and et.EMPLOYID is declared as CHAR(15), there is a length mismatch.

To fix this disparity between column lengths, use ALTER TABLE to lengthen ActualPC from 10 characters to 15 characters:

mysql> ALTER TABLE tt MODIFY ActualPC VARCHAR(15);

Now tt.ActualPC and et.EMPLOYID are both VARCHAR(15). Executing the EXPLAIN statement again produces this result:

table type   possible_keys   key     key_len ref         rows    Extra
tt    ALL    AssignedPC,ClientID,ActualPC NULL NULL NULL 3872    where used
do    ALL    PRIMARY         NULL    NULL    NULL        2135
      range checked for each record (key map: 1)
et_1  ALL    PRIMARY         NULL    NULL    NULL        74
      range checked for each record (key map: 1)
et    eq_ref PRIMARY         PRIMARY 15      tt.ActualPC 1

This is not perfect, but is much better (the product of the rows values is now less by a factor of 74). This version is executed in a couple of seconds.

A second alteration can be made to eliminate the column length mismatches for the tt.AssignedPC = et_1.EMPLOYID and tt.ClientID = do.CUSTNMBR comparisons:

mysql> ALTER TABLE tt MODIFY AssignedPC VARCHAR(15),
                      MODIFY ClientID   VARCHAR(15);

Now EXPLAIN produces the output shown below:

table type   possible_keys   key     key_len ref            rows     Extra
et    ALL    PRIMARY         NULL    NULL    NULL           74
tt    ref    AssignedPC,ClientID,ActualPC ActualPC 15 et.EMPLOYID 52 where used
et_1  eq_ref PRIMARY         PRIMARY 15      tt.AssignedPC  1
do    eq_ref PRIMARY         PRIMARY 15      tt.ClientID    1

This is almost as good as it can get.

The remaining problem is that, by default, MySQL assumes that values in the tt.ActualPC column are evenly distributed, and that isn't the case for the tt table. Fortunately, it is easy to tell MySQL about this:

shell> myisamchk --analyze PATH_TO_MYSQL_DATABASE/tt
shell> mysqladmin refresh

Now the join is perfect, and EXPLAIN produces this result:

table type   possible_keys   key     key_len ref            rows    Extra
tt    ALL    AssignedPC,ClientID,ActualPC NULL NULL NULL    3872    where used
et    eq_ref PRIMARY         PRIMARY 15      tt.ActualPC    1
et_1  eq_ref PRIMARY         PRIMARY 15      tt.AssignedPC  1
do    eq_ref PRIMARY         PRIMARY 15      tt.ClientID    1

Note that the rows column in the output from EXPLAIN is an educated guess from the MySQL join optimizer. To optimize a query, you should check if the numbers are even close to the truth. If not, you may get better performance by using STRAIGHT_JOIN in your SELECT statement and trying to list the tables in a different order in the FROM clause.

5.2.2 Estimating Query Performance

In most cases you can estimate the performance by counting disk seeks. For small tables, you can usually find the row in 1 disk seek (as the index is probably cached). For bigger tables, you can estimate that (using B++ tree indexes) you will need: log(row_count) / log(index_block_length / 3 * 2 / (index_length + data_pointer_length)) + 1 seeks to find a row.

In MySQL an index block is usually 1024 bytes and the data pointer is usually 4 bytes. A 500,000 row table with an index length of 3 (medium integer) gives you: log(500,000)/log(1024/3*2/(3+4)) + 1 = 4 seeks.

As the above index would require about 500,000 * 7 * 3/2 = 5.2M, (assuming that the index buffers are filled to 2/3, which is typical) you will probably have much of the index in memory and you will probably only need 1-2 calls to read data from the OS to find the row.

For writes, however, you will need 4 seek requests (as above) to find where to place the new index and normally 2 seeks to update the index and write the row.

Note that the above doesn't mean that your application will slowly degenerate by N log N! As long as everything is cached by the OS or SQL server things will only go marginally slower while the table gets bigger. After the data gets too big to be cached, things will start to go much slower until your applications is only bound by disk-seeks (which increase by N log N). To avoid this, increase the index cache as the data grows. See section 5.5.2 Tuning Server Parameters.

5.2.3 Speed of SELECT Queries

In general, when you want to make a slow SELECT ... WHERE faster, the first thing to check is whether or not you can add an index. See section 5.4.3 How MySQL Uses Indexes. All references between different tables should usually be done with indexes. You can use the EXPLAIN command to determine which indexes are used for a SELECT. See section 5.2.1 EXPLAIN Syntax (Get Information About a SELECT).

Some general tips:

5.2.4 How MySQL Optimizes WHERE Clauses

The WHERE optimizations are put in the SELECT part here because they are mostly used with SELECT, but the same optimizations apply for WHERE in DELETE and UPDATE statements.

Also note that this section is incomplete. MySQL does many optimizations, and we have not had time to document them all.

Some of the optimizations performed by MySQL are listed below:

Some examples of queries that are very fast:

mysql> SELECT COUNT(*) FROM tbl_name;
mysql> SELECT MIN(key_part1),MAX(key_part1) FROM tbl_name;
mysql> SELECT MAX(key_part2) FROM tbl_name
           WHERE key_part_1=constant;
mysql> SELECT ... FROM tbl_name
           ORDER BY key_part1,key_part2,... LIMIT 10;
mysql> SELECT ... FROM tbl_name
           ORDER BY key_part1 DESC,key_part2 DESC,... LIMIT 10;

The following queries are resolved using only the index tree (assuming the indexed columns are numeric):

mysql> SELECT key_part1,key_part2 FROM tbl_name WHERE key_part1=val;
mysql> SELECT COUNT(*) FROM tbl_name
           WHERE key_part1=val1 AND key_part2=val2;
mysql> SELECT key_part2 FROM tbl_name GROUP BY key_part1;

The following queries use indexing to retrieve the rows in sorted order without a separate sorting pass:

mysql> SELECT ... FROM tbl_name ORDER BY key_part1,key_part2,... ;
mysql> SELECT ... FROM tbl_name ORDER BY key_part1 DESC,key_part2 DESC,... ;

5.2.5 How MySQL Optimizes DISTINCT

DISTINCT is converted to a GROUP BY on all columns, DISTINCT combined with ORDER BY will in many cases also need a temporary table.

When combining LIMIT # with DISTINCT, MySQL will stop as soon as it finds # unique rows.

If you don't use columns from all used tables, MySQL will stop the scanning of the not used tables as soon as it has found the first match.

SELECT DISTINCT t1.a FROM t1,t2 where t1.a=t2.a;

In the case, assuming t1 is used before t2 (check with EXPLAIN), then MySQL will stop reading from t2 (for that particular row in t1) when the first row in t2 is found.

5.2.6 How MySQL Optimizes LEFT JOIN and RIGHT JOIN

A LEFT JOIN B in MySQL is implemented as follows:

RIGHT JOIN is implemented analogously as LEFT JOIN.

The table read order forced by LEFT JOIN and STRAIGHT JOIN will help the join optimizer (which calculates in which order tables should be joined) to do its work much more quickly, as there are fewer table permutations to check.

Note that the above means that if you do a query of type:

SELECT * FROM a,b LEFT JOIN c ON (c.key=a.key) LEFT JOIN d (d.key=a.key) WHERE b.key=d.key

MySQL will do a full scan on b as the LEFT JOIN will force it to be read before d.

The fix in this case is to change the query to:

SELECT * FROM b,a LEFT JOIN c ON (c.key=a.key) LEFT JOIN d (d.key=a.key) WHERE b.key=d.key

5.2.7 How MySQL Optimizes LIMIT

In some cases MySQL will handle the query differently when you are using LIMIT # and not using HAVING:

5.2.8 Speed of INSERT Queries

The time to insert a record consists approximately of:

where the numbers are somewhat proportional to the overall time. This does not take into consideration the initial overhead to open tables (which is done once for each concurrently running query).

The size of the table slows down the insertion of indexes by N log N (B-trees).

Some ways to speed up inserts:

To get some more speed for both LOAD DATA INFILE and INSERT, enlarge the key buffer. See section 5.5.2 Tuning Server Parameters.

5.2.9 Speed of UPDATE Queries

Update queries are optimized as a SELECT query with the additional overhead of a write. The speed of the write is dependent on the size of the data that is being updated and the number of indexes that are updated. Indexes that are not changed will not be updated.

Also, another way to get fast updates is to delay updates and then do many updates in a row later. Doing many updates in a row is much quicker than doing one at a time if you lock the table.

Note that, with dynamic record format, updating a record to a longer total length may split the record. So if you do this often, it is very important to OPTIMIZE TABLE sometimes. See section 4.5.1 OPTIMIZE TABLE Syntax.

5.2.10 Speed of DELETE Queries

If you want to delete all rows in the table, you should use TRUNCATE TABLE table_name. See section 6.4.6 TRUNCATE Syntax.

The time to delete a record is exactly proportional to the number of indexes. To delete records more quickly, you can increase the size of the index cache. See section 5.5.2 Tuning Server Parameters.

5.2.11 Other Optimization Tips

Unsorted tips for faster systems:

5.3 Locking Issues

5.3.1 How MySQL Locks Tables

You can find a discussion about different locking methods in the appendix. See section G.4 Locking methods.

All locking in MySQL is deadlock-free. This is managed by always requesting all needed locks at once at the beginning of a query and always locking the tables in the same order.

The locking method MySQL uses for WRITE locks works as follows:

The locking method MySQL uses for READ locks works as follows:

When a lock is released, the lock is made available to the threads in the write lock queue, then to the threads in the read lock queue.

This means that if you have many updates on a table, SELECT statements will wait until there are no more updates.

To work around this for the case where you want to do many INSERT and SELECT operations on a table, you can insert rows in a temporary table and update the real table with the records from the temporary table once in a while.

This can be done with the following code:

mysql> LOCK TABLES real_table WRITE, insert_table WRITE;
mysql> insert into real_table select * from insert_table;
mysql> TRUNCATE TABLE insert_table;
mysql> UNLOCK TABLES;

You can use the LOW_PRIORITY options with INSERT, UPDATE or DELETE or HIGH_PRIORITY with SELECT if you want to prioritize retrieval in some specific cases. You can also start mysqld with --low-priority-updates to get the same behaveour.

Using SQL_BUFFER_RESULT can also help making table locks shorter. See section 6.4.1 SELECT Syntax.

You could also change the locking code in `mysys/thr_lock.c' to use a single queue. In this case, write locks and read locks would have the same priority, which might help some applications.

5.3.2 Table Locking Issues

The table locking code in MySQL is deadlock free.

MySQL uses table locking (instead of row locking or column locking) on all table types, except BDB tables, to achieve a very high lock speed. For large tables, table locking is MUCH better than row locking for most applications, but there are, of course, some pitfalls.

For BDB and InnoDB tables, MySQL only uses table locking if you explicitely lock the table with LOCK TABLES or execute a command that will modify every row in the table, like ALTER TABLE. For these table types we recommend you to not use LOCK TABLES at all.

In MySQL Version 3.23.7 and above, you can insert rows into MyISAM tables at the same time other threads are reading from the table. Note that currently this only works if there are no holes after deleted rows in the table at the time the insert is made. When all holes has been filled with new data, concurrent inserts will automatically be enabled again.

Table locking enables many threads to read from a table at the same time, but if a thread wants to write to a table, it must first get exclusive access. During the update, all other threads that want to access this particular table will wait until the update is ready.

As updates on tables normally are considered to be more important than SELECT, all statements that update a table have higher priority than statements that retrieve information from a table. This should ensure that updates are not 'starved' because one issues a lot of heavy queries against a specific table. (You can change this by using LOW_PRIORITY with the statement that does the update or HIGH_PRIORITY with the SELECT statement.)

Starting from MySQL Version 3.23.7 one can use the max_write_lock_count variable to force MySQL to temporary give all SELECT statements, that wait for a table, a higher priority after a specific number of inserts on a table.

Table locking is, however, not very good under the following senario:

Some possible solutions to this problem are:

5.4 Optimizing Database Structure

5.4.1 Design Choices

MySQL keeps row data and index data in separate files. Many (almost all) other databases mix row and index data in the same file. We believe that the MySQL choice is better for a very wide range of modern systems.

Another way to store the row data is to keep the information for each column in a separate area (examples are SDBM and Focus). This will cause a performance hit for every query that accesses more than one column. Because this degenerates so quickly when more than one column is accessed, we believe that this model is not good for general purpose databases.

The more common case is that the index and data are stored together (like in Oracle/Sybase et al). In this case you