mysqld Linux Service

mysqld service:

MySQL server.
Now lets see the manual of mysqld service.
Manual mysqld:
NAME
mysqld - the MySQL server
SYNOPSIS
mysqld [options]
DESCRIPTION
mysqld is the MySQL server. The following discussion covers these MySQL server configuration topics:
· Startup options that the server supports
· Server system variables
· Server status variables
· How to set the server SQL mode
· The server shutdown process
MYSQLD COMMAND OPTIONS
When you start the mysqld server, you can specify program options using any of the methods described in Section 3, 'Specifying Program Options'. The most common methods are to provide options in an option file or on the command line. However, in most cases it is desirable to make sure that the server uses the same options each time it runs. The best way to ensure this is to list them in an option file. See Section 3.2, 'Using Option Files'.
mysqld reads options from the [mysqld] and [server] groups. mysqld_safe reads options from the [mysqld], [server], [mysqld_safe], and [safe_mysqld] groups. mysql.server reads options from the [mysqld] and [mysql.server] groups.
An embedded MySQL server usually reads options from the [server], [embedded], and [xxxxx_SERVER] groups, where xxxxx is the name of the application into which the server is embedded.
mysqld accepts many command options. For a list, execute mysqld --help. Before MySQL 4.1.1, --help prints the full help message. As of 4.1.1, it prints a brief message; to see the full list, use mysqld --verbose --help.
The following list shows some of the most common server options. Additional options are described in other sections:
· Options that affect security: See Section 5.3, 'Security-Related mysqld Options'.
· SSL-related options: See Section 7.7.3, 'SSL Command Options'.
· Binary log control options: See Section 10.4, 'The Binary Log'.
· Replication-related options: See Section 8, 'Replication Startup Options'.
· Options specific to particular storage engines: See Section 1.1, 'MyISAM Startup Options', Section 5.3, 'BDB Startup Options', Section 2.5, 'InnoDB Startup Options and System Variables', and Section 6.5.1, 'MySQL Cluster-Related Command Options for mysqld'.
You can also set the values of server system variables by using variable names as options, as described later in this section.
· --help, -?
Display a short help message and exit. Before MySQL 4.1.1, --help displays the full help message. As of 4.1.1, it displays an abbreviated message only. Use both the --verbose and --help options to see the full message.
· --allow-suspicious-udfs
This option controls whether user-defined functions that have only an xxx symbol for the main function can be loaded. By default, the option is off and only UDFs that have at least one auxiliary symbol can be loaded; this prevents attempts at loading functions from shared object files other than those containing legitimate UDFs. This option was added in MySQL 4.0.24, and 4.1.10a. See Section 2.4.6, 'User-Defined Function Security Precautions'.
· --ansi
Use standard (ANSI) SQL syntax instead of MySQL syntax. For more precise control over the server SQL mode, use the --sql-mode option instead. See Section 9.3, 'Running MySQL in ANSI Mode', and the section called 'THE SERVER SQL MODE'.
· --basedir=path, -b path
The path to the MySQL installation directory. All paths are usually resolved relative to this directory.
· --big-tables
Allow large result sets by saving all temporary sets in files. This option prevents most 'table full' errors, but also slows down queries for which in-memory tables would suffice. Since MySQL 3.23.2, the server is able to handle large result sets automatically by using memory for small temporary tables and switching to disk tables where necessary.
· --bind-address=IP
The IP address to bind to.
· --bootstrap
This option is used by the mysql_install_db script to create the MySQL privilege tables without having to start a full MySQL server.
· --character-sets-dir=path
The directory where character sets are installed. See Section 9.1, 'The Character Set Used for Data and Sorting'.
· --character-set-client-handshake
Don't ignore character set information sent by the client. To ignore client information and use the default server character set, use --skip-character-set-client-handshake; this makes MySQL 4.1 and higher behave like MySQL 4.0. This option was added in MySQL 4.1.15.
· --character-set-server=charset_name, -C charset_name
Use charset_name as the default server character set. This option is available as of MySQL 4.1.3. See Section 9.1, 'The Character Set Used for Data and Sorting'.
· --chroot=path
Put the mysqld server in a closed environment during startup by using the chroot() system call. This is a recommended security measure as of MySQL 4.0. (MySQL 3.23 is not able to provide a chroot() jail that is 100% closed.) Note that use of this option somewhat limits LOAD DATA INFILE and SELECT ... INTO OUTFILE.
· --collation-server=collation_name
Use collation_name as the default server collation. This option is available as of MySQL 4.1.3. See Section 9.1, 'The Character Set Used for Data and Sorting'.
· --console
(Windows only.) Write error log messages to stderr and stdout even if --log-error is specified. mysqld does not close the console window if this option is used.
· --core-file
Write a core file if mysqld dies. For some systems, you must also specify the --core-file-size option to mysqld_safe. See mysqld_safe(1). Note that on some systems, such as Solaris, you do not get a core file if you are also using the --user option.
· --datadir=path, -h path
The path to the data directory.
· --debug[=debug_options], -# [debug_options]
If MySQL is configured with --with-debug, you can use this option to get a trace file of what mysqld is doing. The debug_options string often is ´d:t:o,file_name'. The default is ´d:t:i:o,mysqld.trace'. See Section 1.2, 'Creating Trace Files'.
· --default-character-set=charset_name, -C charset_name
Use charset_name as the default character set. This option is deprecated in favor of --character-set-server as of MySQL 4.1.3. See Section 9.1, 'The Character Set Used for Data and Sorting'.
· --default-collation=collation_name
Use collation_name as the default collation. This option is deprecated in favor of --collation-server as of MySQL 4.1.3. See Section 9.1, 'The Character Set Used for Data and Sorting'.
· --default-storage-engine=type
This option is a synonym for --default-table-type. It is available as of MySQL 4.1.2.
· --default-table-type=type
Set the default table type (storage engine) for tables. See Chapter 14, Storage Engines and Table Types.
· --default-time-zone=timezone
Set the default server time zone. This option sets the global time_zone system variable. If this option is not given, the default time zone is the same as the system time zone (given by the value of the system_time_zone system variable. This option is available as of MySQL 4.1.3.
· --delay-key-write[={OFF|ON|ALL}]
Specify how to use delayed key writes. Delayed key writing causes key buffers not to be flushed between writes for MyISAM tables. OFF disables delayed key writes. ON enables delayed key writes for those tables that were created with the DELAY_KEY_WRITE option. ALL delays key writes for all MyISAM tables. Available as of MySQL 4.0.3. See Section 5.2, 'Tuning Server Parameters', and Section 1.1, 'MyISAM Startup Options'.
Note: If you set this variable to ALL, you should not use MyISAM tables from within another program (such as another MySQL server or myisamchk) when the tables are in use. Doing so leads to index corruption.
· --delay-key-write-for-all-tables
Old form of --delay-key-write=ALL for use prior to MySQL 4.0.3. As of 4.0.3, use --delay-key-write instead.
· --des-key-file=file_name
Read the default DES keys from this file. These keys are used by the DES_ENCRYPT() and DES_DECRYPT() functions.
· --enable-named-pipe
Enable support for named pipes. This option applies only on Windows NT, 2000, XP, and 2003 systems, and can be used only with the mysqld-nt and mysqld-max-nt servers that support named-pipe connections.
· --exit-info[=flags], -T [flags]
This is a bit mask of different flags that you can use for debugging the mysqld server. Do not use this option unless you know exactly what it does!
· --external-locking
Enable external locking (system locking), which is disabled by default as of MySQL 4.0. Note that if you use this option on a system on which lockd does not fully work (such as Linux), it is easy for mysqld to deadlock. This option was named --enable-locking before MySQL 4.0.3.
Note: If you use this option to enable updates to MyISAM tables from many MySQL processes, you must ensure that the following conditions are satisfied:
· You should not use the query cache for queries that use tables that are updated by another process.
· You should not use --delay-key-write=ALL or DELAY_KEY_WRITE=1 on any shared tables.
The easiest way to ensure this is to always use --external-locking together with --delay-key-write=OFF and --query-cache-size=0. (This is not done by default because in many setups it is useful to have a mixture of the preceding options.)
· --flush
Flush (synchronize) all changes to disk after each SQL statement. Normally, MySQL does a write of all changes to disk only after each SQL statement and lets the operating system handle the synchronizing to disk. See Section 4.2, 'What to Do If MySQL Keeps Crashing'.
· --init-file=file_name
Read SQL statements from this file at startup. Each statement must be on a single line and should not include comments.
· --innodb-safe-binlog
Adds consistency guarantees between the content of InnoDB tables and the binary log. See Section 10.4, 'The Binary Log'.
· --innodb-xxx
The InnoDB options are listed in Section 2.5, 'InnoDB Startup Options and System Variables'.
· --language=lang_name, -L lang_name
Return client error messages in the given language. lang_name can be given as the language name or as the full pathname to the directory where the language files are installed. See Section 9.2, 'Setting the Error Message Language'.
· --log[=file_name], -l [file_name]
Log connections and SQL statements received from clients to this file. See Section 10.2, 'The General Query Log'. If you omit the filename, MySQL uses host_name.log as the filename.
· --log-bin[=base_name]
Enable binary logging. The server logs all statements that change data to the binary log, which is used for backup and replication. See Section 10.4, 'The Binary Log'.
The option value, if given, is the basename for the log sequence. The server creates binary log files in sequence by adding a numeric suffix to the basename. It is recommended that you specify a basename (see Section 8.4, 'Open Issues in MySQL', for the reason). Otherwise, MySQL uses host_name-bin as the basename.
· --log-bin-index[=file_name]
The index file for binary log filenames. See Section 10.4, 'The Binary Log'. If you omit the filename, and if you didn't specify one with --log-bin, MySQL uses host_name-bin.index as the filename.
· --log-error[=file_name]
Log errors and startup messages to this file. See Section 10.1, 'The Error Log'. If you omit the filename, MySQL uses host_name.err. If the filename has no extension, the server adds an extension of .err.
· --log-isam[=file_name]
Log all ISAM/MyISAM changes to this file (used only when debugging ISAM/MyISAM).
· --log-long-format
Log extra information to the update log, binary update log, and slow query log, if they have been activated. For example, the username and timestamp are logged for queries. Before MySQL 4.1, if you are using --log-slow-queries and --log-long-format, queries that are not using indexes also are logged to the slow query log. --log-long-format is deprecated as of MySQL version 4.1, when --log-short-format was introduced. (Long log format is the default setting since version 4.1.) Also note that starting with MySQL 4.1, the --log-queries-not-using-indexes option is available for the purpose of logging queries that do not use indexes to the slow query log.
· --log-queries-not-using-indexes
If you are using this option with --log-slow-queries, queries that do not use indexes also are logged to the slow query log. This option is available as of MySQL 4.1. See Section 10.5, 'The Slow Query Log'.
· --log-short-format
Log less information to the update log, binary update log, and slow query log, if they have been activated. For example, the username and timestamp are not logged for queries. This option was introduced in MySQL 4.1.
· --log-slow-admin-statements
Log slow administrative statements such as OPTIMIZE TABLE, ANALYZE TABLE, and ALTER TABLE to the slow query log.
This option was added in MySQL 4.1.13. (It is unnecessary in MySQL 4.0 because slow administrative statements are logged by default.)
· --log-slow-queries[=file_name]
Log all queries that have taken more than long_query_time seconds to execute to this file. See Section 10.5, 'The Slow Query Log'. Note that the default for the amount of information logged has changed in MySQL 4.1. See the --log-long-format and --log-short-format options for details.
· --log-update[=file_name]
Log updates to fileN where N is a unique number if not given. See Section 10.3, 'The Update Log'. The update log is now deprecated; you should use the binary log instead (--log-bin). See Section 10.4, 'The Binary Log'.
· --log-warnings[=level], -W [level]
Print out warnings such as Aborted connection... to the error log. Enabling this option is recommended, for example, if you use replication (you get more information about what is happening, such as messages about network failures and reconnections). This option is enabled by default as of MySQL 4.0.19 and 4.1.2; to disable it, use --log-warnings=0. As of MySQL 4.0.21 and 4.1.3, a level argument can be given. If omitted, the default level is 1. Aborted connections are not logged to the error log unless the value is greater than 1. See Section 2.10, 'Communication Errors and Aborted Connections'.
Before MySQL 4.0.21 and 4.1.3, this is a boolean option, not an integer-valued option. Before 4.0, this option was named --warnings.
· --low-priority-updates
Give table-modifying operations (INSERT, REPLACE, DELETE, UPDATE) lower priority than selects. This can also be done via {INSERT | REPLACE | DELETE | UPDATE} LOW_PRIORITY ... to lower the priority of only one query, or by SET LOW_PRIORITY_UPDATES=1 to change the priority in one thread. See Section 3.2, 'Table Locking Issues'.
· --memlock
Lock the mysqld process in memory. This works on systems such as Solaris that support the mlockall() system call. This might help if you have a problem where the operating system is causing mysqld to swap on disk. Note that use of this option requires that you run the server as root, which is normally not a good idea for security reasons. See Section 5.5, 'How to Run MySQL as a Normal User'.
· --myisam-recover[=option[,option]...]]
Set the MyISAM storage engine recovery mode. The option value is any combination of the values of DEFAULT, BACKUP, FORCE, or QUICK. If you specify multiple values, separate them by commas. You can also use a value of "" to disable this option. If this option is used, each time mysqld opens a MyISAM table, it checks whether the table is marked as crashed or wasn't closed properly. (The last option works only if you are running with external locking disabled.) If this is the case, mysqld runs a check on the table. If the table was corrupted, mysqld attempts to repair it.
The following options affect how the repair works: OptionDescriptionDEFAULTThe same as not giving any option to --myisam-recover.BACKUPIf the data file was changed during recovery, save a backup of the
tbl_name.MYD
file as
tbl_name-datetime.BAK.FORCERun recovery even if we would lose more than one row from the MYD file.QUICKdo not check the rows in the table if there are not any delete blocks.Before the server automatically repairs a table, it writes a note about the repair to the error log. If you want to be able to recover from most problems without user intervention, you should use the options BACKUP,FORCE. This forces a repair of a table even if some rows would be deleted, but it keeps the old data file as a backup so that you can later examine what happened.
This option is available as of MySQL 3.23.25.
· --ndb-connectstring=connect_string
When using the NDB storage engine, it is possible to point out the management server that distributes the cluster configuration by setting the connect string option. See Section 4.4.2, 'The MySQL Cluster connectstring', for syntax.
· --ndbcluster
If the binary includes support for the NDB Cluster storage engine (from version 4.1.3, the MySQL-Max binaries are built with NDB Cluster enabled), this option enables the engine, which is disabled by default. Using the NDB Cluster storage engine is necessary for using MySQL Cluster. See Chapter 15, MySQL Cluster.
· --new
The --new option can be used to make the server behave as 4.1 in certain respects, easing a 4.0 to 4.1 upgrade:
· Hexadecimal strings such as 0xFF are treated as strings by default rather than as numbers. (Works in 4.0.12 and up.)
· TIMESTAMP is returned as a string with the format 'YYYY-MM-DD HH:MM:SS'. (Works in 4.0.13 and up.) See Chapter 11, Data Types.
This option can be used to help you see how your applications behave in MySQL 4.1, without actually upgrading to 4.1.
· --old-passwords
Force the server to generate short (pre-4.1) password hashes for new passwords. This is useful for compatibility when the server must support older client programs. See Section 6.9, 'Password Hashing as of MySQL 4.1'.
· --old-protocol, -o
Use the 3.20 protocol for compatibility with some very old clients.
· --one-thread
Only use one thread (for debugging under Linux). This option is available only if the server is built with debugging enabled. See Section 1, 'Debugging a MySQL Server'.
· --open-files-limit=count
Change the number of file descriptors available to mysqld. If this option is not set or is set to 0, mysqld uses the value to reserve file descriptors with setrlimit(). If the value is 0, mysqld reserves max_connections×5 or max_connections + table_open_cache×2 files (whichever is larger). You should try increasing this value if mysqld gives you the error Too many open files.
· --pid-file=path
The pathname of the process ID file. This file is used by other programs such as mysqld_safe to determine the server's process ID.
· --port=port_num, -P port_num
The port number to use when listening for TCP/IP connections. The port number must be 1024 or higher unless the server is started by the root system user.
· --safe-mode
Skip some optimization stages.
· --safe-show-database
With this option, the SHOW DATABASES statement displays only the names of those databases for which the user has some kind of privilege. As of MySQL 4.0.2, this option is deprecated and does not do anything (it is enabled by default), because there is a SHOW DATABASES privilege that can be used to control access to database names on a per-account basis. See Section 6.3, 'Privileges Provided by MySQL'.
· --safe-user-create
If this option is enabled, a user cannot create new MySQL users by using the GRANT statement, if the user doesn't have the INSERT privilege for the mysql.user table or any column in the table.
· --secure-auth
Disallow authentication by clients that attempt to use accounts that have old (pre-4.1) passwords. This option is available as of MySQL 4.1.1.
· --shared-memory
Enable shared-memory connections by local clients. This option is available only on Windows. It was added in MySQL 4.1.0.
· --shared-memory-base-name=name
The name of shared memory to use for shared-memory connections. This option is available only on Windows. The default name is MYSQL. The name is case sensitive. This option was added in MySQL 4.1.0.
· --skip-bdb
Disable the BDB storage engine. This saves memory and might speed up some operations. Do not use this option if you require BDB tables.
· --skip-concurrent-insert
Turn off the ability to select and insert at the same time on MyISAM tables. (This is to be used only if you think you have found a bug in this feature.) See Section 3.3, 'Concurrent Inserts'.
· --skip-delay-key-write
Ignore the DELAY_KEY_WRITE option for all tables. As of MySQL 4.0.3, you should use --delay-key-write=OFF instead. See Section 5.2, 'Tuning Server Parameters'.
· --skip-external-locking
Do not use external locking (system locking). With external locking disabled, you must shut down the server to use myisamchk or isamchk. See Section 4.3, 'MySQL Stability'. As of MySQL 3.23, you can use the CHECK TABLE and REPAIR TABLE statements to check and repair MyISAM tables. This option previously was named --skip-locking.
External locking has been disabled by default since MySQL 4.0.
· --skip-grant-tables
This option causes the server not to use the privilege system at all, which gives anyone with access to the server unrestricted access to all databases. You can cause a running server to start using the grant tables again by executing mysqladmin flush-privileges or mysqladmin reload command from a system shell, or by issuing a MySQL FLUSH PRIVILEGES statement after connecting to the server. This option also suppresses loading of user-defined functions (UDFs).
· --skip-host-cache
Do not use the internal hostname cache for faster name-to-IP resolution. Instead, query the DNS server every time a client connects. See Section 5.5, 'How MySQL Uses DNS'.
· --skip-innodb
Disable the InnoDB storage engine. This saves memory and disk space and might speed up some operations. Do not use this option if you require InnoDB tables.
· --skip-isam
Disable the ISAM storage engine. As of MySQL 4.1, ISAM is disabled by default, so this option applies only if the server was configured with support for ISAM. This option was added in MySQL 4.1.1.
· --skip-name-resolve
Do not resolve hostnames when checking client connections. Use only IP numbers. If you use this option, all Host column values in the grant tables must be IP numbers or localhost. See Section 5.5, 'How MySQL Uses DNS'.
· --skip-ndbcluster
Disable the NDB Cluster storage engine. This is the default for binaries that were built with NDB Cluster storage engine support; the server allocates memory and other resources for this storage engine only if the --ndbcluster option is given explicitly. See Section 4.3, 'Quick Test Setup of MySQL Cluster', for an example of usage.
· --skip-networking
Do not listen for TCP/IP connections at all. All interaction with mysqld must be made via named pipes or shared memory (on Windows) or Unix socket files (on Unix). This option is highly recommended for systems where only local clients are allowed. See Section 5.5, 'How MySQL Uses DNS'.
· --skip-new
do not use new, possibly wrong routines.
· --skip-symlink
This is the old form of --skip-symbolic-links, for use before MySQL 4.0.13.
· --ssl*
Options that begin with --ssl specify whether to allow clients to connect via SSL and indicate where to find SSL keys and certificates. See Section 7.7.3, 'SSL Command Options'.
· --standalone
Available on Windows NT-based systems only; instructs the MySQL server not to run as a service.
· --symbolic-links, --skip-symbolic-links
Enable or disable symbolic link support. This option has different effects on Windows and Unix:
· On Windows, enabling symbolic links allows you to establish a symbolic link to a database directory by creating a db_name.sym file that contains the path to the real directory. See Section 6.1.3, 'Using Symbolic Links for Databases on Windows'.
· On Unix, enabling symbolic links means that you can link a MyISAM index file or data file to another directory with the INDEX DIRECTORY or DATA DIRECTORY options of the CREATE TABLE statement. If you delete or rename the table, the files that its symbolic links point to also are deleted or renamed. See Section 6.1.2, 'Using Symbolic Links for Tables on Unix'.
This option was added in MySQL 4.0.13.
· --skip-safemalloc
If MySQL is configured with --with-debug=full, all MySQL programs check for memory overruns during each memory allocation and memory freeing operation. This checking is very slow, so for the server you can avoid it when you do not need it by using the --skip-safemalloc option.
· --skip-show-database
With this option, the SHOW DATABASES statement is allowed only to users who have the SHOW DATABASES privilege, and the statement displays all database names. Without this option, SHOW DATABASES is allowed to all users, but displays each database name only if the user has the SHOW DATABASES privilege or some privilege for the database. Note that any global privilege is considered a privilege for the database.
· --skip-stack-trace
do not write stack traces. This option is useful when you are running mysqld under a debugger. On some systems, you also must use this option to get a core file. See Section 1, 'Debugging a MySQL Server'.
· --skip-thread-priority
Disable using thread priorities for faster response time.
· --socket=path
On Unix, this option specifies the Unix socket file to use when listening for local connections. The default value is /tmp/mysql.sock. On Windows, the option specifies the pipe name to use when listening for local connections that use a named pipe. The default value is MySQL (not case sensitive).
· --sql-mode=value[,value[,value...]]
Set the SQL mode. See the section called 'THE SERVER SQL MODE'. This option was added in 3.23.41.
· --temp-pool
This option causes most temporary files created by the server to use a small set of names, rather than a unique name for each new file. This works around a problem in the Linux kernel dealing with creating many new files with different names. With the old behavior, Linux seems to 'leak' memory, because it is being allocated to the directory entry cache rather than to the disk cache.
· --transaction-isolation=level
Sets the default transaction isolation level. The level value can be READ-UNCOMMITTED, READ-COMMITTED, REPEATABLE-READ, or SERIALIZABLE. See Section 4.6, 'SET TRANSACTION Syntax'.
· --tmpdir=path, -t path
The path of the directory to use for creating temporary files. It might be useful if your default /tmp directory resides on a partition that is too small to hold temporary tables. Starting from MySQL 4.1.0, this option accepts several paths that are used in round-robin fashion. Paths should be separated by colon characters (':') on Unix and semicolon characters (';') on Windows, NetWare, and OS/2. If the MySQL server is acting as a replication slave, you should not set --tmpdir to point to a directory on a memory-based filesystem or to a directory that is cleared when the server host restarts. For more information about the storage location of temporary files, see Section 4.4, 'Where MySQL Stores Temporary Files'. A replication slave needs some of its temporary files to survive a machine restart so that it can replicate temporary tables or LOAD DATA INFILE operations. If files in the temporary file directory are lost when the server restarts, replication fails.
· --user={user_name|user_id}, -u {user_name|user_id}
Run the mysqld server as the user having the name user_name or the numeric user ID user_id. ('User' in this context refers to a system login account, not a MySQL user listed in the grant tables.)
This option is mandatory when starting mysqld as root. The server changes its user ID during its startup sequence, causing it to run as that particular user rather than as root. See Section 5.1, 'General Security Guidelines'.
Starting from MySQL 3.23.56 and 4.0.12: To avoid a possible security hole where a user adds a --user=root option to a my.cnf file (thus causing the server to run as root), mysqld uses only the first --user option specified and produces a warning if there are multiple --user options. Options in /etc/my.cnf and $MYSQL_HOME/my.cnf are processed before command-line options, so it is recommended that you put a --user option in /etc/my.cnf and specify a value other than root. The option in /etc/my.cnf is found before any other --user options, which ensures that the server runs as a user other than root, and that a warning results if any other --user option is found.
· --version, -V
Display version information and exit.
As of MySQL 4.0, you can assign a value to a server system variable by using an option of the form --var_name=value. For example, --key_buffer_size=32M sets the key_buffer_size variable to a value of 32MB.
Note that when you assign a value to a variable, MySQL might automatically correct the value to stay within a given range, or adjust the value to the closest allowable value if only certain values are allowed.
If you want to restrict the maximum value to which a variable can be set at runtime with SET, you can define this by using the --maximum-var_name=value command-line option.
It is also possible to set variables by using --set-variable=var_name=value or --var_name=value syntax. This syntax is deprecated as of MySQL 4.0.
You can change the values of most system variables for a running server with the SET statement. See Section 5.3, 'SET Syntax'.
the section called 'SERVER SYSTEM VARIABLES', provides a full description for all variables, and additional information for setting them at server startup and runtime. Section 5.2, 'Tuning Server Parameters', includes information on optimizing the server by tuning system variables.
SERVER SYSTEM VARIABLES
The mysql server maintains many system variables that indicate how it is configured. Each system variable has a default value. System variables can be set at server startup using options on the command line or in an option file. As of MySQL 4.0.3, most of them can be changed dynamically while the server is running by means of the SET statement, which enables you to modify operation of the server without having to stop and restart it. You can refer to system variable values in expressions.
There are several ways to see the names and values of system variables:
· To see the values that a server will use based on its compiled-in defaults and any option files that it reads, use this command (omit --verbose before MySQL 4.1.1):
mysqld --verbose --help
· To see the values that a server will use based on its compiled-in defaults, ignoring the settings in any option files, use this command (omit --verbose before MySQL 4.1.1):
mysqld --no-defaults --verbose --help
· To see the current values used by a running server, use the SHOW VARIABLES statement.
This section provides a description of each system variable. Variables with no version indicated have been present since at least MySQL 3.22.
For additional system variable information, see these sections:
· the section called 'USING SYSTEM VARIABLES', discusses the syntax for setting and displaying system variable values.
· the section called 'Dynamic System Variables', lists the variables that can be set at runtime.
· Information on tuning sytem variables can be found in Section 5.2, 'Tuning Server Parameters'.
· Section 2.5, 'InnoDB Startup Options and System Variables', lists InnoDB system variables.
Note: Some of the following variable descriptions refer to 'enabling' or 'disabling' a variable. These variables can be enabled with the SET statement by setting them to ON or 1, or disabled by setting them to OFF or 0. However, to set such a variable on the command line or in an option file, you must set it to 1 or 0; setting it to ON or OFF will not work. For example, on the command line, --delay_key_write=1 works but --delay_key_write=ON does not.
Values for buffer sizes, lengths, and stack sizes are given in bytes unless otherwise specified.
· ansi_mode
This is ON if mysqld was started with --ansi. See Section 9.3, 'Running MySQL in ANSI Mode'. This variable was added in MySQL 3.23.6 and removed in 3.23.41. See the description for sql_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() system call should have more details. Check your OS documentation for the maximum value for this variable. back_log cannot be set higher than your operating system limit.
· basedir
The MySQL installation base directory. This variable can be set with the --basedir option.
· bdb_cache_size
The size of the buffer that is allocated for caching indexes and rows for BDB tables. If you do not use BDB tables, you should start mysqld with --skip-bdb to not allocate memory for this cache. This variable was added in MySQL 3.23.14.
· bdb_home
The base directory for BDB tables. This should be assigned the same value as the datadir variable. This variable was added in MySQL 3.23.14.
· bdb_log_buffer_size
The size of the buffer that is allocated for caching indexes and rows for BDB tables. If you do not use BDB tables, you should set this to 0 or start mysqld with --skip-bdb in order not to allocate memory for this cache. This variable was added in MySQL 3.23.31.
· bdb_logdir
The directory where the BDB storage engine writes its log files. This variable can be set with the --bdb-logdir option. This variable was added in MySQL 3.23.14.
· bdb_max_lock
The maximum number of locks that can be active for a BDB table (10,000 by default). You should increase this value if errors such as the following occur when you perform long transactions or when mysqld has to examine many rows to calculate a query:
bdb: Lock table is out of available locks
Got error 12 from ...
This variable was added in MySQL 3.23.29.
· bdb_shared_data
This is ON if you are using --bdb-shared-data to start Berkeley DB in multi-process mode. (Do not use DB_PRIVATE when initializing Berkeley DB.) This variable was added in MySQL 3.23.29.
· bdb_tmpdir
The BDB temporary file directory. This variable was added in MySQL 3.23.14.
· bdb_version
See the description for version_bdb.
· binlog_cache_size
The size of the cache to hold the SQL statements for the binary log during a transaction. A binary log cache is allocated for each client if the server supports any transactional storage engines and, starting from MySQL 4.1.2, if the server has the binary log enabled (--log-bin option). If you often use large, multiple-statement transactions, you can increase this cache size to get more performance. The Binlog_cache_use and Binlog_cache_disk_use status variables can be useful for tuning the size of this variable. This variable was added in MySQL 3.23.29. See Section 10.4, 'The Binary Log'.
· bulk_insert_buffer_size
MyISAM uses a special tree-like cache to make bulk inserts faster for INSERT ... SELECT, INSERT ... VALUES (...), (...), ..., and LOAD DATA INFILE when adding data to non-empty tables. This variable limits the size of the cache tree in bytes per thread. Setting it to 0 disables this optimization. The default value is 8MB. This variable was added in MySQL 4.0.3. This variable previously was named myisam_bulk_insert_tree_size.
· character_set
The default character set. This variable was added in MySQL 3.23.3, then removed in MySQL 4.1.1 and replaced by the various character_set_xxx variables.
· character_set_client
The character set for statements that arrive from the client. This variable was added in MySQL 4.1.1.
· character_set_connection
The character set used for literals that do not have a character set introducer and for number-to-string conversion. This variable was added in MySQL 4.1.1.
· character_set_database
The character set used by the default database. The server sets this variable whenever the default database changes. If there is no default database, the variable has the same value as character_set_server. This variable was added in MySQL 4.1.1.
· character_set_results
The character set used for returning query results to the client. This variable was added in MySQL 4.1.1.
· character_set_server
The server default character set. This variable was added in MySQL 4.1.1.
· character_set_system
The character set used by the server for storing identifiers. The value is always utf8. This variable was added in MySQL 4.1.1.
· character_sets
The supported character sets. This variable was added in MySQL 3.23.15 and removed in MySQL 4.1.1. (Use SHOW CHARACTER SET for a list of character sets.)
· character_sets_dir
The directory where character sets are installed. This variable was added in MySQL 4.1.2.
· collation_connection
The collation of the connection character set. This variable was added in MySQL 4.1.1.
· collation_database
The collation used by the default database. The server sets this variable whenever the default database changes. If there is no default database, the variable has the same value as collation_server. This variable was added in MySQL 4.1.1.
· collation_server
The server default collation. This variable was added in MySQL 4.1.1.
· concurrent_insert
If ON (the default), MySQL allows INSERT and SELECT statements to run concurrently for MyISAM tables that have no free blocks in the middle. You can turn this option off by starting mysqld with --safe or --skip-new. This variable was added in MySQL 3.23.7.
See also Section 3.3, 'Concurrent Inserts'.
· connect_timeout
The number of seconds that the mysqld server waits for a connect packet before responding with Bad handshake.
· convert_character_set
The current character set mapping that was set by SET CHARACTER SET. This variable was removed in MySQL 4.1.
· datadir
The MySQL data directory. This variable can be set with the --datadir option.
· date_format
This variable is not implemented.
· datetime_format
This variable is not implemented.
· default_week_format
The default mode value to use for the WEEK() function. See Section 5, 'Date and Time Functions'. This variable is available as of MySQL 4.0.14.
· delay_key_write
This option applies only to MyISAM tables. It can have one of the following values to affect handling of the DELAY_KEY_WRITE table option that can be used in CREATE TABLE statements. OptionDescriptionOFFDELAY_KEY_WRITE is ignored.ONMySQL honors any DELAY_KEY_WRITE option specified in CREATE TABLE statements. This is the default value. ALLAll new opened tables are treated as if they were created with the DELAY_KEY_WRITE option enabled.If DELAY_KEY_WRITE is enabled for a table, the key buffer is not flushed for the table on every index update, but only when the table is closed. This speeds up writes on keys a lot, but if you use this feature, you should add automatic checking of all MyISAM tables by starting the server with the --myisam-recover option (for example, --myisam-recover=BACKUP,FORCE). See the section called 'FBMYSQLDFR COMMAND OPTIONS', and Section 1.1, 'MyISAM Startup Options'.
Note that enabling external locking with --external-locking offers no protection against index corruption for tables that use delayed key writes.
This variable was added in MySQL 3.23.8.
· delayed_insert_limit
After inserting delayed_insert_limit delayed rows, the INSERT DELAYED handler thread checks whether there are any SELECT statements pending. If so, it allows them to execute before continuing to insert delayed rows.
· delayed_insert_timeout
How many seconds an INSERT DELAYED handler thread should wait for INSERT statements before terminating.
· delayed_queue_size
This is a per-table limit on the number of rows to queue when handling INSERT DELAYED statements. If the queue becomes full, any client that issues an INSERT DELAYED statement waits until there is room in the queue again.
· expire_logs_days
The number of days for automatic binary log removal. The default is 0, which means 'no automatic removal.' Possible removals happen at startup and at binary log rotation. This variable was added in MySQL 4.1.0.
· flush
If ON, the server flushes (synchronizes) all changes to disk after each SQL statement. Normally, MySQL does a write of all changes to disk only after each SQL statement and lets the operating system handle the synchronizing to disk. See Section 4.2, 'What to Do If MySQL Keeps Crashing'. This variable is set to ON if you start mysqld with the --flush option. This variable was added in MySQL 3.22.9.
· flush_time
If this is set to a non-zero value, all tables are closed every flush_time seconds to free up resources and synchronize unflushed data to disk. We recommend that this option be used only on Windows 9x or Me, or on systems with minimal resources. This variable was added in MySQL 3.22.18.
· ft_boolean_syntax
The list of operators supported by boolean full-text searches performed using IN BOOLEAN MODE. See Section 7.1, 'Boolean Full-Text Searches'. This variable was added as a read-only variable in MySQL 4.0.1. It can be modified as of MySQL 4.1.2.
The default variable value is '+ -><()~*:""&|'. The rules for changing the value are as follows:
· Operator function is determined by position within the string.
· The replacement value must be 14 characters.
· Each character must be an ASCII non-alphanumeric character.
· Either the first or second character must be a space.
· No duplicates are allowed except the phrase quoting operators in positions 11 and 12. These two characters are not required to be the same, but they are the only two that may be.
· Positions 10, 13, and 14 (which by default are set to ':', '&', and '|') are reserved for future extensions.
· ft_max_word_len
The maximum length of the word to be included in a FULLTEXT index. This variable was added in MySQL 4.0.0.
Note: FULLTEXT indexes must be rebuilt after changing this variable. Use REPAIR TABLE tbl_name QUICK.
· ft_min_word_len
The minimum length of the word to be included in a FULLTEXT index. This variable was added in MySQL 4.0.0.
Note: FULLTEXT indexes must be rebuilt after changing this variable. Use REPAIR TABLE tbl_name QUICK.
· ft_query_expansion_limit
The number of top matches to use for full-text searches performed using WITH QUERY EXPANSION. This variable was added in MySQL 4.1.1.
· ft_stopword_file
The file from which to read the list of stopwords for full-text searches. All the words from the file are used; comments are not honored. By default, a built-in list of stopwords is used (as defined in the myisam/ft_static.c file). Setting this variable to the empty string ('') disables stopword filtering. This variable was added in MySQL 4.0.10.
Note: FULLTEXT indexes must be rebuilt after changing this variable or the contents of the stopword file. Use REPAIR TABLE tbl_name QUICK.
· group_concat_max_len
The maximum allowed result length for the GROUP_CONCAT() function. The default is 1024. This variable was added in MySQL 4.1.0.
· have_archive
YES if mysqld supports ARCHIVE tables, NO if not. This variable was added in MySQL 4.1.3.
· have_bdb
YES if mysqld supports BDB tables. DISABLED if --skip-bdb is used. This variable was added in MySQL 3.23.30.
· have_blackhole_engine
YES if mysqld supports BLACKHOLE tables, NO if not. This variable was added in MySQL 4.1.11.
· have_compress
YES if the zlib compression library is available to the server, NO if not. If not, the COMPRESS() and UNCOMPRESS() functions cannot be used. This variable was added in MySQL 4.1.1.
· have_crypt
YES if the crypt() system call is available to the server, NO if not. If not, the ENCRYPT() function cannot be used. This variable was added in MySQL 4.0.10.
· have_csv
YES if mysqld supports ARCHIVE tables, NO if not. This variable was added in MySQL 4.1.4.
· have_example_engine
YES if mysqld supports EXAMPLE tables, NO if not. This variable was added in MySQL 4.1.4.
· have_geometry
YES if the server supports spatial data types, NO if not. This variable was added in MySQL 4.1.3.
· have_innodb
YES if mysqld supports InnoDB tables. DISABLED if --skip-innodb is used. This variable was added in MySQL 3.23.37.
· have_isam
YES if mysqld supports ISAM tables. DISABLED if --skip-isam is used. This variable was added in MySQL 3.23.30.
· have_ndbcluster
YES if mysqld supports NDB Cluster tables. DISABLED if --skip-ndbcluster is used. This variable was added in MySQL 4.1.2.
· have_openssl
YES if mysqld supports SSL (encryption) connections, NO if not. This variable was added in MySQL 3.23.43.
· have_query_cache
YES if mysqld supports the query cache, NO if not. This variable was added in MySQL 4.0.2.
· have_raid
YES if mysqld supports the RAID option, NO if not. This variable was added in MySQL 3.23.30.
· have_rtree_keys
YES if RTREE indexes are available, NO if not. (These are used for spatial indexes in MyISAM tables.) This variable was added in MySQL 4.1.3.
· have_symlink
YES if symbolic link support is enabled, NO if not. This is required on Unix for support of the DATA DIRECTORY and INDEX DIRECTORY table options, and on Windows for support of data directory symlinks.
This variable was added in MySQL 4.0.0.
· init_connect
A string to be executed by the server for each client that connects. The string consists of one or more SQL statements. To specify multiple statements, separate them by semicolon characters. For example, each client begins by default with autocommit mode enabled. There is no global system variable to specify that autocommit should be disabled by default, but init_connect can be used to achieve the same effect:
SET GLOBAL init_connect='SET AUTOCOMMIT=0';
This variable can also be set on the command line or in an option file. To set the variable as just shown using an option file, include these lines:
[mysqld]
init_connect='SET AUTOCOMMIT=0'
Note that the content of init_connect is not executed for users that have the SUPER privilege. This is done so that an erroneous value for init_connect does not prevent all clients from connecting. For example, the value might contain a statement that has a syntax error, thus causing client connections to fail. Not executing init_connect for users that have the SUPER privilege enables them to open a connection and fix the init_connect value.
This variable was added in MySQL 4.1.2.
· init_file
The name of the file specified with the --init-file option when you start the server. This should be a file containing SQL statements that you want the server to execute when it starts. Each statement must be on a single line and should not include comments. This variable was added in MySQL 3.23.2.
· init_slave
This variable is similar to init_connect, but is a string to be executed by a slave server each time the SQL thread starts. The format of the string is the same as for the init_connect variable. This variable was added in MySQL 4.1.2.
· innodb_xxx
InnoDB system variables are listed in Section 2.5, 'InnoDB Startup Options and System Variables'.
· 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 joins that do not use indexes and thus perform full table scans. Normally, the best way to get fast joins is to add indexes. Increase the value of join_buffer_size to get a faster full join when adding indexes is not possible. One join buffer is allocated for each full join between two tables. For a complex join between several tables for which indexes are not used, multiple join buffers might be necessary.
· key_buffer_size
Index blocks for MyISAM and ISAM tables are buffered and are shared by all threads. key_buffer_size is the size of the buffer used for index blocks. The key buffer is also known as the key cache.
The maximum allowable setting for key_buffer_size is 4GB. The effective maximum size might be less, depending on your available physical RAM and per-process RAM limits imposed by your operating system or hardware platform.
Increase the value to get better index handling (for all reads and multiple writes) to as much as you can afford. Using a value that is 25% of total memory on a machine that mainly runs MySQL is quite common. However, if you make the value too large (for example, more than 50% of your total memory) your system might start to page and become extremely slow. MySQL relies on the operating system to perform filesystem caching for data reads, so you must leave some room for the filesystem cache. Consider also the memory requirements of other storage engines.
For even more speed when writing many rows at the same time, use LOCK TABLES. See Section 2.13, 'Speed of INSERT Statements'.
You can check the performance of the key buffer by issuing a SHOW STATUS statement and examining the Key_read_requests, Key_reads, Key_write_requests, and Key_writes status variables. (See Section 5.4, 'SHOW Syntax'.) The Key_reads/Key_read_requests ratio should normally be less than 0.01. The Key_writes/Key_write_requests ratio is usually near 1 if you are using mostly updates and deletes, but might be much smaller if you tend to do updates that affect many rows at the same time or if you are using the DELAY_KEY_WRITE table option.
The fraction of the key buffer in use can be determined using key_buffer_size in conjunction with the Key_blocks_unused status variable and the buffer block size. From MySQL 4.1.1 on, the buffer block size is available from the key_cache_block_size server variable. The fraction of the buffer in use is:
1 - ((Key_blocks_unused × key_cache_block_size) / key_buffer_size)
This value is an approximation because some space in the key buffer may be allocated internally for administrative structures.
Before MySQL 4.1.1, key cache blocks are 1024 bytes, and before MySQL 4.1.2, Key_blocks_unused is unavailable. The Key_blocks_used variable can be used as follows to determine the fraction of the key buffer in use:
(Key_blocks_used × 1024) / key_buffer_size
However, Key_blocks_used indicates the maximum number of blocks that have ever been in use at once, so this formula does not necessary represent the current fraction of the buffer that is in use.
As of MySQL 4.1, it is possible to create multiple MyISAM key caches. The size limit of 4GB applies to each cache individually, not as a group. See Section 4.6, 'The MyISAM Key Cache'.
· key_cache_age_threshold
This value controls the demotion of buffers from the hot sub-chain of a key cache to the warm sub-chain. Lower values cause demotion to happen more quickly. The minimum value is 100. The default value is 300. This variable was added in MySQL 4.1.1. See Section 4.6, 'The MyISAM Key Cache'.
· key_cache_block_size
The size in bytes of blocks in the key cache. The default value is 1024. This variable was added in MySQL 4.1.1. See Section 4.6, 'The MyISAM Key Cache'.
· key_cache_division_limit
The division point between the hot and warm sub-chains of the key cache buffer chain. The value is the percentage of the buffer chain to use for the warm sub-chain. Allowable values range from 1 to 100. The default value is 100. This variable was added in MySQL 4.1.1. See Section 4.6, 'The MyISAM Key Cache'.
· language
The language used for error messages.
· large_file_support
Whether mysqld was compiled with options for large file support. This variable was added in MySQL 3.23.28.
· large_pages
Whether large page support is enabled. This variable was added in MySQL 5.0.3.
· license
The type of license the server has. This variable was added in MySQL 4.0.19.
· local_infile
Whether LOCAL is supported for LOAD DATA INFILE statements. See Section 5.4, 'Security Issues with LOAD DATA LOCAL'. This variable was added in MySQL 4.0.3.
· locked_in_memory
Whether mysqld was locked in memory with --memlock. This variable was added in MySQL 3.23.25.
· log
Whether logging of all statements to the general query log is enabled. See Section 10.2, 'The General Query Log'.
· log_bin
Whether the binary log is enabled. This variable was added in MySQL 3.23.14. See Section 10.4, 'The Binary Log'.
· log_error
The location of the error log. This variable was added in MySQL 4.0.10.
· log_slave_updates
Whether updates received by a slave server from a master server should be logged to the slave's own binary log. Binary logging must be enabled on the slave for this variable to have any effect. This variable was added in MySQL 3.23.17. See Section 8, 'Replication Startup Options'.
· log_slow_queries
Whether slow queries should be logged. 'Slow' is determined by the value of the long_query_time variable. This variable was added in MySQL 4.0.2. See Section 10.5, 'The Slow Query Log'.
· log_update
Whether the update log is enabled. This variable was added in MySQL 3.22.18. Note that the binary log is preferable to the update log, which is unavailable as of MySQL 5.0. See Section 10.3, 'The Update Log'.
· log_warnings
Whether to produce additional warning messages. This variable was added in MySQL 4.0.3. It is enabled by default as of MySQL 4.0.19 and 4.1.2. As of MySQL 4.0.21 and 4.1.3, the variable can take values greater than 1 and aborted connections are not logged to the error log unless the value is greater than 1.
· long_query_time
If a query takes longer than this many seconds, the server increments the Slow_queries status variable. If you are using the --log-slow-queries option, the query is logged to the slow query log file. This value is measured in real time, not CPU time, so a query that is under the threshold on a lightly loaded system might be above the threshold on a heavily loaded one. The minimum value is 1. The default is 10. See Section 10.5, 'The Slow Query Log'.
· low_priority_updates
If set to 1, all INSERT, UPDATE, DELETE, and LOCK TABLE WRITE statements wait until there is no pending SELECT or LOCK TABLE READ on the affected table. This variable previously was named sql_low_priority_updates. It was added in MySQL 3.22.5.
· lower_case_file_system
This variable describes the case sensitivity of filenames on the filesystem where the data directory is located. OFF means filenames are case sensitive, ON means they are not case sensitive. This variable was added in MySQL 4.0.19.
· lower_case_table_names
If set to 1 table names are stored in lowercase on disk and table name comparisons are not case sensitive. This variable was added in MySQL 3.23.6. If set to 2 (new in 4.0.18), table names are stored as given but compared in lowercase. From MySQL 4.0.2, this option also applies to database names. From 4.1.1, it also applies to table aliases. See Section 2.2, 'Identifier Case Sensitivity'.
Note: If you are using InnoDB tables, you should set this variable to 1 on all platforms to force names to be converted to lowercase.
You should not set this variable to 0 if you are running MySQL on a system that does not have case-sensitive filenames (such as Windows or Mac OS X). New in 4.0.18: If this variable is not set at startup and the filesystem on which the data directory is located does not have case-sensitive filenames, MySQL automatically sets lower_case_table_names to 2.
· max_allowed_packet
The maximum size of one packet or any generated/intermediate string.
The packet 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 large (possibly incorrect) packets.
You must increase this value if you are using large BLOB columns or long strings. It should be as big as the largest BLOB you want to use. The protocol limit for max_allowed_packet is 16MB before MySQL 4.0 and 1GB thereafter.
· max_binlog_cache_size
If a multiple-statement transaction requires more than this amount of memory, the server generates a Multi-statement transaction required more than 'max_binlog_cache_size' bytes of storage error. This variable was added in MySQL 3.23.29.
· max_binlog_size
If a write to the binary log causes the current log file size to exceed the value of this variable, the server rotates the binary logs (closes the current file and opens the next one). You cannot set this variable to more than 1GB or to less than 4096 bytes. (The minimum before MYSQL 4.0.14 is 1024 bytes.) The default value is 1GB. This variable was added in MySQL 3.23.33.
A transaction is written in one chunk to the binary log, so it is never split between several binary logs. Therefore, if you have big transactions, you might see binary logs larger than max_binlog_size.
If max_relay_log_size is 0, the value of max_binlog_size applies to relay logs as well. max_relay_log_size was added in MySQL 4.0.14.
· max_connect_errors
If there are more than this number of interrupted connections from a host, that host is blocked from further connections. You can unblock blocked hosts with the FLUSH HOSTS statement.
· max_connections
The number of simultaneous client connections allowed. Increasing this value increases the number of file descriptors that mysqld requires. See Section 4.8, 'How MySQL Opens and Closes Tables', for comments on file descriptor limits. See also Section 2.6, 'Too many connections'.
· max_delayed_threads
Do not 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 is inserted as if the DELAYED attribute wasn't specified. If you set this to 0, MySQL never creates a thread to handle DELAYED rows; in effect, doing so disables DELAYED entirely. This variable was added in MySQL 3.23.0.
· max_error_count
The maximum number of error, warning, and note messages to be stored for display by the SHOW ERRORS or SHOW WARNINGS statements. This variable was added in MySQL 4.1.0.
· max_heap_table_size
This variable sets the maximum size to which MEMORY (HEAP) tables are allowed to grow. The value of the variable is used to calculate MEMORY table MAX_ROWS values. Setting this variable has no effect on any existing MEMORY table, unless the table is re-created with a statement such as CREATE TABLE, or altered with ALTER TABLE or TRUNCATE TABLE. This variable was added in MySQL 3.23.0.
· max_insert_delayed_threads
This variable is a synonym for max_delayed_threads. It was added in MySQL 4.0.19.
· max_join_size
Do not allow SELECT statements that probably need to examine more than max_join_size rows (for single-table statements) or row combinations (for multiple-table statements) or that are likely to do more than max_join_size disk seeks. By setting this value, you can catch SELECT statements where keys are not used properly and that would probably take a long time. Set it if your users tend to perform joins that lack a WHERE clause, that take a long time, or that return millions of rows.
Setting this variable to a value other than DEFAULT resets the value of SQL_BIG_SELECTS to 0. If you set the SQL_BIG_SELECTS value again, the max_join_size variable is ignored.
If a query result is in the query cache, no result size check is performed, because the result has previously been computed and it does not burden the server to send it to the client.
This variable previously was named sql_max_join_size.
· max_length_for_sort_data
The cutoff on the size of index values that determines which filesort algorithm to use. See Section 2.9, 'ORDER BY Optimization'. This variable was added in MySQL 4.1.1
· max_prepared_stmt_count
This variable limits the total number of prepared statements in the server. It can be used in environments where there is the potential for denial-of-service attacks based on running the server out of memory by preparing huge numbers of statements. The default value is 16,382. The allowable range of values is from 0 to 1 milliion. If the value is set lower than the current number of prepared statements, existing statements are not affected and can be used, but no new statements can be prepared until the current number drops below the limit. This variable was added in MySQL 4.1.19.
· max_relay_log_size
If a write by a replication slave to its relay log causes the current log file size to exceed the value of this variable, the slave rotates the relay logs (closes the current file and opens the next one). If max_relay_log_size is 0, the server uses max_binlog_size for both the binary log and the relay log. If max_relay_log_size is greater than 0, it constrains the size of the relay log, which enables you to have different sizes for the two logs. You must set max_relay_log_size to between 4096 bytes and 1GB (inclusive), or to 0. The default value is 0. This variable was added in MySQL 4.0.14. See Section 3, 'Replication Implementation Details'.
· max_seeks_for_key
Limit the assumed maximum number of seeks when looking up rows based on a key. The MySQL optimizer assumes that no more than this number of key seeks are required when searching for matching rows in a table by scanning an index, regardless of the actual cardinality of the index (see Section 5.4.11, 'SHOW INDEX Syntax'). By setting this to a low value (say, 100), you can force MySQL to prefer indexes instead of table scans.
This variable was added in MySQL 4.0.14.
· 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_tmp_tables
The maximum number of temporary tables a client can keep open at the same time. (This option does not yet do anything.)
· max_user_connections
The maximum number of simultaneous connections allowed to any given MySQL account. A value of 0 means 'no limit.' This variable was added in MySQL 3.23.34.
This variable has only a global form.
· max_write_lock_count
After this many write locks, allow some pending read lock requests to be processed in between. This variable was added in MySQL 3.23.7.
· myisam_data_pointer_size
The default pointer size in bytes, to be used by CREATE TABLE for MyISAM tables when no MAX_ROWS option is specified. This variable cannot be less than 2 or larger than 7. The default value is 4. This variable was added in MySQL 4.1.2. See Section 2.11, 'The table is full'.
· myisam_max_extra_sort_file_size
If the temporary file used for fast MyISAM index creation would be larger than using the key cache by the amount specified here, 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. This variable was added in MySQL 3.23.37. Note: The value is given in megabytes before 4.0.3 and in bytes thereafter.
· myisam_max_sort_file_size
The maximum size of the temporary file that MySQL is allowed to use while re-creating a MyISAM index (during REPAIR TABLE, ALTER TABLE, or LOAD DATA INFILE). If the file size would be larger than this value, the index is created using the key cache instead, which is slower. This variable was added in MySQL 3.23.37. Note: The value is given in megabytes before 4.0.3 and in bytes thereafter.
· myisam_recover_options
The value of the --myisam-recover option. See the section called '\FBMYSQLD\FR COMMAND OPTIONS'. This variable was added in MySQL 3.23.36.
· myisam_repair_threads
If this value is greater than 1, MyISAM table indexes are created in parallel (each index in its own thread) during the Repair by sorting process. The default value is 1.
Note: Multi-threaded repair is still beta-quality code. This variable was added in MySQL 4.0.13.
· myisam_sort_buffer_size
The size of the buffer that is allocated when sorting MyISAM indexes during a REPAIR TABLE or when creating indexes with CREATE INDEX or ALTER TABLE. This variable was added in MySQL 3.23.16.
· myisam_stats_method
How the server treats NULL values when collecting statistics about the distribution of index values for MyISAM tables. This variable has two possible values, nulls_equal and nulls_unequal. For nulls_equal, all NULL index values are considered equal and form a single value group that has a size equal to the number of NULL values. For nulls_unequal, NULL values are considered unequal, and each NULL forms a distinct value group of size 1.
The method that is used for generating table statistics influences how the optimizer chooses indexes for query execution, as described in Section 4.7, 'MyISAM Index Statistics Collection'.
This variable was added in MySQL 4.1.15/5.0.14. For older versions, the statistics collection method is equivalent to nulls_equal.
· named_pipe
On Windows, indicates whether the server supports connections over named pipes. This variable was added in MySQL 3.23.50.
· ndb_autoincrement_prefetch_sz
Determines the probability of gaps in an autoincremented column. Set to 1 to minimize this. Set to a high value for optimization - makes inserts faster, but decreases the likelihood that consecutive autoincrement numbers will be used in a batch of inserts. Default value: 32. Mimimum value: 1.
· ndb_cache_check_time
The number of seconds to wait before checking the NDB query cache. Setting this to 0 (the default and minimum value) means that the NDB query cache will be checked for validation upon each query. A large value means the NDB query cache will seldom be checked and invalidated because of updates on another mysqld. It is generally not desirable to set this to a value greater than 2.
· ndb_force_send
Forces sending of buffers to NDB immediately, without waiting for other threads. Defaults to ON.
· ndb_index_stat_cache_entries
Sets the granularity of the statistics by determining the number of starting and ending keys to store in the statistics memory cache. Zero means no caching takes place; in this case, the data nodes are always queries directly. Default value: 32.
· ndb_index_stat_enable
Use NDB index statistics in query optimization. Defaults to ON.
· ndb_index_stat_update_freq
How often to query data nodes instead of the statistics cache. For example, a value of 20 (the default) means to direct every 20th query to the data nodes.
· ndb_report_thresh_binlog_epoch_slip
This is a threshold on the number of epochs to be behind before reporting binlog status. For example, a value of 3 (the default) means that if the difference between which epoch has been received from the storage nodes and which epoch has been applied to the binlog is 3 or more, a status message will be sent to the cluster log.
· ndb_report_thresh_binlog_mem_usage
This is a threshold on the percentage of free memory remaining before reporting binlog status. For example, a value of 10 (the default) means that if the amount of available memory for receiving binlog data from the data nodes falls below 10%, a status message will be sent to the cluster log.
· ndb_use_exact_count
Forces NDB to use an count of records during SELECT COUNT(*) query planning to speed up this type of query. The default value is ON. For faster queries overall, disable this feature by setting the value of ndb_use_exact_count to OFF.
· ndb_use_transactions
You can disable NDB transaction support by setting this variable's values to OFF (not recommended). The default is ON.
· net_buffer_length
The communication buffer is reset to this size between SQL statements. This variable should not normally be changed, but if you have very little memory, you can set it to the expected length of statements sent by clients. If statements exceed this length, the buffer is automatically enlarged, up to max_allowed_packet bytes.
· net_read_timeout
The number of seconds to wait for more data from a connection before aborting the read. This timeout applies only to TCP/IP connections, not to connections made via Unix socket files, named pipes, or shared memory. When the server is reading from the client, net_read_timeout is the timeout value controlling when to abort. When the server is writing to the client, net_write_timeout is the timeout value controlling when to abort. See also slave_net_timeout. This variable was added in MySQL 3.23.20.
· net_retry_count
If a read on a communication port is interrupted, retry this many times before giving up. This value should be set quite high on FreeBSD because internal interrupts are sent to all threads. This variable was added in MySQL 3.23.7.
· net_write_timeout
The number of seconds to wait for a block to be written to a connection before aborting the write. This timeout applies only to TCP/IP connections, not to connections made via Unix socket files, named pipes, or shared memory. See also net_read_timeout. This variable was added in MySQL 3.23.20.
· new
This variable is used in MySQL 4.0 to turn on some 4.1 behaviors. This variable was added in MySQL 4.0.12.
· old_passwords
Whether the server should use pre-4.1-style passwords for MySQL user accounts. This variable was added in MySQL 4.1.1.
· one_shot
This is not a variable, but it can be used when setting some variables. It is described in Section 5.3, 'SET Syntax'.
· open_files_limit
The number of files that the operating system allows mysqld to open. This is the real value allowed by the system and might be different from the value you gave using the --open-files-limit option to mysqld or mysqld_safe. The value is 0 on systems where MySQL can't change the number of open files. This variable was added in MySQL 3.23.20.
· pid_file
The pathname of the process ID (PID) file. This variable can be set with the --pid-file option. This variable was added in MySQL 3.23.23.
· port
The number of the port on which the server listens for TCP/IP connections. This variable can be set with the --port option.
· preload_buffer_size
The size of the buffer that is allocated when preloading indexes. This variable was added in MySQL 4.1.1.
· prepared_stmt_count
The current number of prepared statements. (The maximum number of statements is given by the max_prepared_stmt_count system variable.) This variable was added in MySQL 4.1.19.
· protocol_version
The version of the client/server protocol used by the MySQL server. This variable was added in MySQL 3.23.18.
· query_alloc_block_size
The allocation size of memory blocks that are allocated for objects created during statement parsing and execution. If you have problems with memory fragmentation, it might help to increase this a bit. This variable was added in MySQL 4.0.16.
· query_cache_limit
Don't cache results that are larger than this number of bytes. The default value is 1MB. This variable was added in MySQL 4.0.1.
· query_cache_min_res_unit
The minimum size for blocks allocated by the query cache. The default value is 4KB. Tuning information for this variable is given in Section 12.3, 'Query Cache Configuration'. This variable is present from MySQL 4.1.
· query_cache_size
The amount of memory allocated for caching query results. The default value is 0, which disables the query cache. The allowable values are multiples of 1024; other values are rounded down to the nearest multiple. Note that query_cache_size bytes of memory are allocated even if if query_cache_type is set to 0. This variable was added in MySQL 4.0.1.
· query_cache_type
Set the query cache type. Setting the GLOBAL value sets the type for all clients that connect thereafter. Individual clients can set the SESSION value to affect their own use of the query cache. OptionDescription0 or OFFDon't cache results in or retrieve results from the query cache. Note that this does not deallocate the query cache buffer. To do that, you should set query_cache_size to 0.1 or ONCache all query results except for those that begin with SELECT SQL_NO_CACHE.2 or DEMANDCache results only for queries that begin with SELECT SQL_CACHE. This variable was added in MySQL 4.0.3.
· query_cache_wlock_invalidate
Normally, when one client acquires a WRITE lock on a MyISAM table, other clients are not blocked from issuing statements that read from the table if the query results are present in the query cache. Setting this variable to 1 causes acquisition of a WRITE lock for a table to invalidate any queries in the query cache that refer to the table. This forces other clients that attempt to access the table to wait while the lock is in effect. This variable was added in MySQL 4.0.19.
· query_prealloc_size
The size of the persistent buffer used for statement parsing and execution. This buffer is not freed between statements. If you are running complex queries, a larger query_prealloc_size value might be helpful in improving performance, because it can reduce the need for the server to perform memory allocation during query execution operations.
This variable was added in MySQL 4.0.16.
· range_alloc_block_size
The size of blocks that are allocated when doing range optimization. This variable was added in MySQL 4.0.16.
· read_buffer_size
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 might want to increase this value. This variable was added in MySQL 4.0.3. Previously, it was named record_buffer.
· read_only
When the variable is set to ON for a replication slave server, it causes the slave to allow no updates except from slave threads or from users that have the SUPER privilege. This can be useful to ensure that a slave server accepts updates only from its master server and not from clients. This variable was added in MySQL 4.0.14.
· relay_log_purge
Disables or enables automatic purging of relay logs as soon as they are not needed any more. The default value is 1 (ON). This variable was added in MySQL 4.1.1.
· read_rnd_buffer_size
When reading rows in sorted order following a key-sorting operation, the rows are read through this buffer to avoid disk seeks. Setting the variable to a large value can improve ORDER BY performance by a lot. However, this is a buffer allocated for each client, so you should not set the global variable to a large value. Instead, change the session variable only from within those clients that need to run large queries. This variable was added in MySQL 4.0.3. Previously, it was named record_rnd_buffer.
· safe_show_database
Do not show databases for which the user has no database or table privileges. This can improve security if you are concerned about people being able to see what databases other users have. See also skip_show_database.
This variable was removed in MySQL 4.0.5. Beginning with this version, you should instead use the SHOW DATABASES privilege to control access by MySQL accounts to databases.
· secure_auth
If the MySQL server has been started with the --secure-auth option, it blocks connections from all accounts that have passwords stored in the old (pre-4.1) format. In that case, the value of this variable is ON, otherwise it is OFF.
You should enable this option if you want to prevent all use of passwords in the old format (and hence insecure communication over the network). This variable was added in MySQL 4.1.1.
Server startup fails with an error if this option is enabled and the privilege tables are in pre-4.1 format.
· server_id
The server ID. This value is set by the --server-id option. It is used for replication to enable master and slave servers to identify themselves uniquely. This variable was added in MySQL 3.23.26.
· shared_memory
(Windows only.) Whether the server allows shared-memory connections. This variable was added in MySQL 4.1.1.
· shared_memory_base_name
(Windows only.) The name of shared memory to use for shared-memory connections. This is useful when running multiple MYSQL instances on a single physical machine. This variable was added in MySQL 4.1.0.
· skip_external_locking
This is OFF if mysqld uses external locking, ON if external locking is disabled. This variable was added in MySQL 4.0.3. Previously, it was named skip_locking.
· skip_networking
This is ON if the server allows only local (non-TCP/IP) connections. On Unix, local connections use a Unix socket file. On Windows, local connections use a named pipe or shared memory. On NetWare, only TCP/IP connections are supported, so do not set this variable to ON. This variable can be set to ON with the --skip-networking option. This variable was added in MySQL 3.22.23.
· skip_show_database
This prevents people from using the SHOW DATABASES statement if they do not have the SHOW DATABASES privilege. This can improve security if you are concerned about people being able to see what databases other users have. See also safe_show_database. This variable was added in MySQL 3.23.4. As of MySQL 4.0.2, its effect also depends on the SHOW DATABASES privilege: If the variable value is ON, the SHOW DATABASES statement is allowed only to users who have the SHOW DATABASES privilege, and the statement displays all database names. If the value is OFF, SHOW DATABASES is allowed to all users, but displays each database name only if the user has the SHOW DATABASES privilege or some privilege for the database. Note that any global privilege is a privilege for the database.
· slave_compressed_protocol
Whether to use compression of the master/slave protocol if both the slave and the master support it. This variable was added in MySQL 4.0.3.
· slave_load_tmpdir
The name of the directory where the slave creates temporary files for replicating LOAD DATA INFILE statements. This variable was added in MySQL 4.0.0.
· slave_net_timeout
The number of seconds to wait for more data from a master/slave connection before aborting the read. This timeout applies only to TCP/IP connections, not to connections made via Unix socket files, named pipes, or shared memory. This variable was added in MySQL 3.23.40.
· slave_skip_errors
The replication errors that the slave should skip (ignore). This variable was added in MySQL 3.23.47.
· slave_transaction_retries
If a replication slave SQL thread fails to execute a transaction because of an InnoDB deadlock or InnoDB's innodb_lock_wait_timeout or NDB Cluster's TransactionDeadlockDetectionTimeout or TransactionInactiveTimeout was exceeded, it automatically retries slave_transaction_retries times before stopping with an error. The default in MySQL 4.1 is 0. You must explicitly set the value to greater than 0 to enable the 'retry' behavior, which is probably a good idea.
· slow_launch_time
If creating a thread takes longer than this many seconds, the server increments the Slow_launch_threads status variable. This variable was added in MySQL 3.23.15.
· socket
On Unix platforms, this variable is the name of the socket file that is used for local client connections. The default is /tmp/mysql.sock. (For some distribution formats, the directory might be different, such as /var/lib/mysql for RPMs.)
On Windows, this variable is the name of the named pipe that is used for local client connections. The default value is MySQL (not case sensitive).
· sort_buffer_size
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 4.4, 'Where MySQL Stores Temporary Files'.
· sql_mode
The current server SQL mode. This variable was added in MySQL 3.23.41. It can be set dynamically as of MySQL 4.1.1. See the section called 'THE SERVER SQL MODE'.
· sql_slave_skip_counter
The number of events from the master that a slave server should skip. See Section 6.2.6, 'SET GLOBAL SQL_SLAVE_SKIP_COUNTER Syntax'. This variable was added in MySQL 3.23.33.
· storage_engine
This variable is a synonym for table_type. It was added in MySQL 4.1.2.
· sync_binlog
If the value of this variable is positive, the MySQL server synchronizes its binary log to disk (using fdatasync()) after every sync_binlog writes to the binary log. Note that there is one write to the binary log per statement if autocommit is enabled, and one write per transaction otherwise. The default value is 0, which does no synchronizing to disk. A value of 1 is the safest choice, because in the event of a crash you lose at most one statement or transaction from the binary log. However, it is also the slowest choice (unless the disk has a battery-backed cache, which makes synchronization very fast). This variable was added in MySQL 4.1.3.
If the value of sync_binlog is 0 (the default), no extra flushing is done. The server relies on the operating system to flush the file contents occasionaly as for any other file.
· sync_frm
If this variable is set to 1, when any non-temporary table is created its .frm file is synchronized to disk (using fdatasync()). This is slower but safer in case of a crash. The default is 1. This was added as a command-line option in MySQL 4.0.18. It is also a settable global variable as of MySQL 4.1.3.
· system_time_zone
The server system time zone. When the server begins executing, it inherits a time zone setting from the machine defaults, possibly modified by the environment of the account used for running the server or the startup script. The value is used to set system_time_zone. Typically the time zone is specified by the TZ environment variable. It also can be specified using the --timezone option of the mysqld_safe script. This variable was added in MySQL 4.1.3.
· table_cache
The number of open tables for all threads. Increasing this value increases the number of file descriptors that mysqld requires. You can check whether you need to increase the table cache by checking the Opened_tables status variable. See the section called 'SERVER STATUS VARIABLES'. If the value of Opened_tables is large and you do not do FLUSH TABLES often (which just forces all tables to be closed and reopened), then you should increase the value of the table_cache variable. For more information about the table cache, see Section 4.8, 'How MySQL Opens and Closes Tables'.
· table_type
The default table type (storage engine). To set the table type at server startup, use the --default-table-type option. This variable was added in MySQL 3.23.0. See the section called '\FBMYSQLD\FR COMMAND OPTIONS'.
· thread_cache_size
How many threads the server should cache for reuse. When a client disconnects, the client's threads are put in the cache if there are fewer than thread_cache_size threads there. Requests for threads are satisfied by reusing threads taken from the cache if possible, 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 provide a notable performance improvement if you have a good thread implementation.) By examining the difference between the Connections and Threads_created status variables, you can see how efficient the thread cache is. For details, see the section called 'SERVER STATUS VARIABLES'. This variable was added in MySQL 3.23.16.
· thread_concurrency
On Solaris, mysqld calls thr_setconcurrency() with this value. This function enables applications to give the threads system a hint about the desired number of threads that should be run at the same time. This variable was added in MySQL 3.23.7.
· 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 1.4, 'The MySQL Benchmark Suite'. The default is 64KB before MySQL 4.0.10 and 192KB thereafter.
· time_format
This variable is not implemented.
· time_zone
The current time zone. This variable is used to initialize the tome zone for each client that connects. By default, the initial value of this is ´SYSTEM' (which means, 'use the value of system_time_zone'). The value can be specified explicitly at server startup with the --default-time-zone option. See Section 9.8, 'MySQL Server Time Zone Support'. This variable was added in MySQL 4.1.3.
· timezone
The time zone for the server. This is set from the TZ environment variable when mysqld is started. The time zone also can be set by giving a --timezone argument to mysqld_safe. This variable was added in MySQL 3.23.15. As of MySQL 4.1.3, it is obsolete and has been replaced by the system_time_zone variable. See Section 4.6, 'Time Zone Problems'.
· tmp_table_size
If an in-memory temporary table exceeds this size, MySQL automatically converts 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. Starting from MySQL 4.1, this variable can be set to a list of several paths that are used in round-robin fashion. Paths should be separated by colon characters (':') on Unix and semicolon characters (';') on Windows, NetWare, and OS/2.
The multiple-directory feature can be used to spread the load between several physical disks. If the MySQL server is acting as a replication slave, you should not set tmpdir to point to a directory on a memory-based filesystem or to a directory that is cleared when the server host restarts. A replication slave needs some of its temporary files to survive a machine restart so that it can replicate temporary tables or LOAD DATA INFILE operations. If files in the temporary file directory are lost when the server restarts, replication fails. However, if you are using MySQL 4.0.0 or later, you can set the slave's temporary directory using the slave_load_tmpdir variable. In that case, the slave won't use the general tmpdir value and you can set tmpdir to a non-permanent location.
This variable was added in MySQL 3.22.4.
· transaction_alloc_block_size
The amount in bytes by which to increase a per-transaction memory pool which needs memory. See the description of transaction_prealloc_size. This variable was added in MySQL 4.0.16.
· transaction_prealloc_size
There is a per-transaction memory pool from which various transaction-related allocations take memory. The initial size of the pool in bytes is transaction_prealloc_size. For every allocation that cannot be satisfied from the pool because it has insufficient memory available, the pool is increased by transaction_alloc_block_size bytes. When the transaction ends, the pool is truncated to transaction_prealloc_size bytes.
By making transaction_prealloc_size sufficiently large to contain all statements within a single transaction, you can avoid many malloc() calls. This variable was added in MySQL 4.0.16.
The system_time_zone variable differs from time_zone. Although they might have the same value, the latter variable is used to initialize the time zone for each client that connects. See Section 9.8, 'MySQL Server Time Zone Support'.
· tx_isolation
The default transaction isolation level. This variable was added in MySQL 4.0.3.
This variable is set by the SET TRANSACTION ISOLATION LEVEL statement. See Section 4.6, 'SET TRANSACTION Syntax'. If you set tx_isolation directly to an isolation level name that contains a space, the name should be enclosed within quotes, with the space replaced by a dash. For example:
SET tx_isolation = 'READ-COMMITTED';
· version
The version number for the server.
· version_bdb
The BDB storage engine version. This variable was added in MySQL 3.23.31 with the name bdb_version and renamed to version_bdb in MySQL 4.1.1.
· version_comment
The configure script has a --with-comment option that allows a comment to be specified when building MySQL. This variable contains the value of that comment. This variable was added in MySQL 4.0.17.
· version_compile_machine
The type of machine or architecture on which MySQL was built. This variable was added in MySQL 4.1.1.
· version_compile_os
The type of operating system on which MySQL was built. This variable was added in MySQL 4.0.19.
· wait_timeout
The number of seconds the server waits for activity on a non-interactive connection before closing it. This timeout applies only to TCP/IP connections, not to connections made via Unix socket files, named pipes, or shared memory.
On thread startup, the session wait_timeout value is initialized from the global wait_timeout value or from the global interactive_timeout value, depending on the type of client (as defined by the CLIENT_INTERACTIVE connect option to mysql_real_connect()). See also interactive_timeout.
USING SYSTEM VARIABLES
The mysql server maintains many system variables that indicate how it is configured. the section called 'SERVER SYSTEM VARIABLES', describes the meaning of these variables. Each system variable has a default value. System variables can be set at server startup using options on the command line or in an option file. As of MySQL 4.0.3, most of them can be changed dynamically while the server is running by means of the SET statement, which enables you to modify operation of the server without having to stop and restart it. You can refer to system variable values in expressions.
Beginning with MySQL 4.0.3, the server maintains two kinds of system variables. Global variables affect the overall operation of the server. Session variables affect its operation for individual client connections. A given system variable can have both a global and a session value. Global and session system variables are related as follows:
· When the server starts, it initializes all global variables to their default values. These defaults can be changed by options specified on the command line or in an option file. (See Section 3, 'Specifying Program Options'.)
· The server also maintains a set of session variables for each client that connects. The client's session variables are initialized at connect time using the current values of the corresponding global variables. For example, the client's SQL mode is controlled by the session sql_mode value, which is initialized when the client connects to the value of the global sql_mode value.
System variable values can be set globally at server startup by using options on the command line or in an option file. When you use a startup option to set a variable that takes a numeric value, the value can be given with a suffix of K, M, or G (either uppercase or lowercase) to indicate a multiplier of 1024, 10242 or 10243; that is, units of kilobytes, megabytes, or gigabygtes, respectively. Thus, the following command starts the server with a query cache size of 16 megabytes and a maximum packet size of one gigabyte:
mysqld --query_cache_size=16M --max_allowed_packet=1G
Before MySQL 4.0.2, use this syntax instead:
mysqld --set-variable=query_cache_size=16M \
--set-variable=max_allowed_packet=1G
Within an option file, those variables are set like this:
[mysqld]
query_cache_size=16M
max_allowed_packet=1G
Or like this before MySQL 4.0.2:
[mysqld]
set-variable=query_cache_size=16M
set-variable=max_allowed_packet=1G
The lettercase of suffix letters does not matter; 16M and 16m are equivalent, as are 1G and 1g.
If you want to restrict the maximum value to which a system variable can be set at runtime with the SET statement, you can specify this maximum by using an option of the form --maximum-var_name=value at server startup. For example, to prevent the value of query_cache_size from being increased to more than 32MB at runtime, use the option --maximum-query_cache_size=32M. This feature is available as of MySQL 4.0.2.
Many system variables are dynamic and can be changed while the server runs by using the SET statement. For a list, see the section called 'Dynamic System Variables'. To change a system variable with SET, refer to it as var_name, optionally preceded by a modifier:
· To indicate explicitly that a variable is a global variable, precede its name by GLOBAL or @@global.. The SUPER privilege is required to set global variables.
· To indicate explicitly that a variable is a session variable, precede its name by SESSION, @@session., or @@. Setting a session variable requires no special privilege, but a client can change only its own session variables, not those of any other client.
· LOCAL and @@local. are synonyms for SESSION and @@session..
· If no modifier is present, SET changes the session variable.
A SET statement can contain multiple variable assignments, separated by commas. If you set several system variables, the most recent GLOBAL or SESSION modifier in the statement is used for following variables that have no modifier specified.
Examples:
SET sort_buffer_size=10000;
SET @@local.sort_buffer_size=10000;
SET GLOBAL sort_buffer_size=1000000, SESSION sort_buffer_size=1000000;
SET @@sort_buffer_size=1000000;
SET @@global.sort_buffer_size=1000000, @@local.sort_buffer_size=1000000;
When you assign a value to a system variable with SET, you cannot use suffix letters in the value (as can be done with startup options). However, the value can take the form of an expression:
SET sort_buffer_size = 10 * 1024 * 1024;
The @@var_name syntax for system variables is supported for compatibility with some other database systems.
If you change a session system variable, the value remains in effect until your session ends or until you change the variable to a different value. The change is not visible to other clients.
If you change a global system variable, the value is remembered and used for new connections until the server restarts. (To make a global system variable setting permanent, you should set it in an option file.) The change is visible to any client that accesses that global variable. However, the change affects the corresponding session variable only for clients that connect after the change. The global variable change does not affect the session variable for any client that is currently connected (not even that of the client that issues the SET GLOBAL statement).
To prevent incorrect usage, MySQL produces an error if you use SET GLOBAL with a variable that can only be used with SET SESSION or if you do not specify GLOBAL (or @@global.) when setting a global variable.
To set a SESSION variable to the GLOBAL value or a GLOBAL value to the compiled-in MySQL default value, use the DEFAULT keyword. For example, the following two statements are identical in setting the session value of max_join_size to the global value:
SET max_join_size=DEFAULT;
SET @@session.max_join_size=@@global.max_join_size;
Not all system variables can be set to DEFAULT. In such cases, use of DEFAULT results in an error.
You can refer to the values of specific global or sesson system variables in expressions by using one of the @@-modifiers. For example, you can retrieve values in a SELECT statement like this:
SELECT @@global.sql_mode, @@session.sql_mode, @@sql_mode;
When you refer to a system variable in an expression as @@var_name (that is, when you do not specify @@global. or @@session.), MySQL returns the session value if it exists and the global value otherwise. (This differs from SET @@var_name = value, which always refers to the session value.)
Note: Some system variables can be enabled with the SET statement by setting them to ON or 1, or disabled by setting them to OFF or 0. However, to set such a variable on the command line or in an option file, you must set it to 1 or 0; setting it to ON or OFF will not work. For example, on the command line, --delay_key_write=1 works but --delay_key_write=ON does not.
To display system variable names and values, use the SHOW VARIABLES statement.

mysql> SHOW VARIABLES;
+-------------------------------+-----------------------------------+
|Variable_name                  |Value                              |
+-------------------------------+-----------------------------------+
|back_log                       |50                                 |
|basedir                        |/usr/local/mysql                   |
|bdb_cache_size                 |8388600                            |
|bdb_home                       |/usr/local/mysql                   |
|bdb_log_buffer_size            |32768                              |
|bdb_logdir                     |                                   |
|bdb_max_lock                   |10000                              |
|bdb_shared_data                |OFF                                |
|bdb_tmpdir                     |/tmp/                              |
|binlog_cache_size              |32768                              |
|bulk_insert_buffer_size        |8388608                            |
|character_set_client           |latin1                             |
|character_set_connection       |latin1                             |
|character_set_database         |latin1                             |
|character_set_results          |latin1                             |
|character_set_server           |latin1                             |
|character_set_system           |utf8                               |
|character_sets_dir             |/usr/local/mysql/share/charsets/   |
|collation_connection           |latin1_swedish_ci                  |
|collation_database             |latin1_swedish_ci                  |
|collation_server               |latin1_swedish_ci                  |
|innodb_additional_mem_pool_size|1048576                            |
|innodb_autoextend_increment    |8                                  |
|innodb_buffer_pool_awe_mem_mb  |0                                  |
|innodb_buffer_pool_size        |8388608                            |
|innodb_data_file_path          |ibdata1:10M:autoextend             |
|innodb_data_home_dir           |                                   |
|version                        |4.1.18-max-log                     |
|version_comment                |MySQL Community Edition - Max (GPL)|
|version_compile_machine        |i686                               |
|version_compile_os             |pc-linux-gnu                       |
|wait_timeout                   |28800                              |
+-------------------------------+-----------------------------------+

With a LIKE clause, the statement displays only those variables that match the pattern. To obtain a specific variable name, use a LIKE clause as shown:
SHOW VARIABLES LIKE 'max_join_size';
SHOW SESSION VARIABLES LIKE 'max_join_size';
To get a list of variables whose name match a pattern, use the '%' wildcard character in a LIKE clause:
SHOW VARIABLES LIKE '%size%';
SHOW GLOBAL VARIABLES LIKE '%size%';
Wildcard characters can be used in any position within the pattern to be matched. Strictly speaking, because '_' is a wildcard that matches any single character, you should escape it as '\_' to match it literally. In practice, this is rarely necessary.
For SHOW VARIABLES, if you specify neither GLOBAL nor SESSION, MySQL returns SESSION values.
The reason for requiring the GLOBAL keyword when setting GLOBAL-only variables but not when retrieving them is to prevent problems in the future. If we were to remove a SESSION variable that has the same name as a GLOBAL variable, a client with the SUPER privilege might accidentally change the GLOBAL variable rather than just the SESSION variable for its own connection. If we add a SESSION variable with the same name as a GLOBAL variable, a client that intends to change the GLOBAL variable might find only its own SESSION variable changed.
Structured System Variables
Structured system variables are supported beginning with MySQL 4.1.1. A structured variable differs from a regular system variable in two respects:
· Its value is a structure with components that specify server parameters considered to be closely related.
· There might be several instances of a given type of structured variable. Each one has a different name and refers to a different resource maintained by the server.
In MySQL 4.1 (4.1.1 and above), MySQL supports one structured variable type. It specifies parameters that govern the operation of key caches. A key cache structured variable has these components:
· key_buffer_size
· key_cache_block_size
· key_cache_division_limit
· key_cache_age_threshold
The purpose of this section is to describe the syntax for referring to structured variables. Key cache variables are used for syntax examples, but specific details about how key caches operate are found elsewhere, in Section 4.6, 'The MyISAM Key Cache'.
To refer to a component of a structured variable instance, you can use a compound name in instance_name.component_name format. Examples:
hot_cache.key_buffer_size
hot_cache.key_cache_block_size
cold_cache.key_cache_block_size
For each structured system variable, an instance with the name of default is always predefined. If you refer to a component of a structured variable without any instance name, the default instance is used. Thus, default.key_buffer_size and key_buffer_size both refer to the same system variable.
Structured variable instances and components follow these naming rules:
· For a given type of structured variable, each instance must have a name that is unique within variables of that type. However, instance names need not be unique across structured variable types. For example, each structured variable has an instance named default, so default is not unique across variable types.
· The names of the components of each structured variable type must be unique across all system variable names. If this were not true (that is, if two different types of structured variables could share component member names), it would not be clear which default structured variable to use for references to member names that are not qualified by an instance name.
· If a structured variable instance name is not legal as an unquoted identifier, refer to it as a quoted identifier using backticks. For example, hot-cache is not legal, but 'hot-cache' is.
· global, session, and local are not legal instance names. This avoids a conflict with notation such as @@global.var_name for referring to non-structured system variables.
At the moment, the first two rules have no possibility of being violated because the only structured variable type is the one for key caches. These rules will assume greater significance if some other type of structured variable is created in the future.
With one exception, it is allowable to refer to structured variable components using compound names in any context where simple variable names can occur. For example, you can assign a value to a structured variable using a command-line option:
shell> mysqld --hot_cache.key_buffer_size=64K
In an option file, use this syntax:
[mysqld]
hot_cache.key_buffer_size=64K
If you start the server with such an option, it creates a key cache named hot_cache with a size of 64KB in addition to the default key cache that has a default size of 8MB.
Suppose that you start the server as follows:
shell> mysqld --key_buffer_size=256K \
--extra_cache.key_buffer_size=128K \
--extra_cache.key_cache_block_size=2048
In this case, the server sets the size of the default key cache to 256KB. (You could also have written --default.key_buffer_size=256K.) In addition, the server creates a second key cache named extra_cache that has a size of 128KB, with the size of block buffers for caching table index blocks set to 2048 bytes.
The following example starts the server with three different key caches having sizes in a 3:1:1 ratio:
shell> mysqld --key_buffer_size=6M \
--hot_cache.key_buffer_size=2M \
--cold_cache.key_buffer_size=2M
Structured variable values may be set and retrieved at runtime as well. For example, to set a key cache named hot_cache to a size of 10MB, use either of these statements:
mysql> SET GLOBAL hot_cache.key_buffer_size = 10*1024*1024;
mysql> SET @@global.hot_cache.key_buffer_size = 10*1024*1024;
To retrieve the cache size, do this:
mysql> SELECT @@global.hot_cache.key_buffer_size;
However, the following statement does not work. The variable is not interpreted as a compound name, but as a simple string for a LIKE pattern-matching operation:
mysql> SHOW GLOBAL VARIABLES LIKE 'hot_cache.key_buffer_size';
This is the exception to being able to use structured variable names anywhere a simple variable name may occur.
Dynamic System Variables
Beginning with MySQL 4.0.3, many server system variables are dynamic and can be set at runtime using SET GLOBAL or SET SESSION. You can also select their values using SELECT. See the section called 'USING SYSTEM VARIABLES'.
The following table shows the full list of all dynamic system variables. The last column indicates for each variable whether GLOBAL or SESSION (or both) apply. The table also lists session options that can be set with the SET statement. Section 5.3, 'SET Syntax', discusses these options.
Variables that have a type of 'string' take a string value. Variables that have a type of 'numeric' take a numeric value. Variables that have a type of 'boolean' can be set to 0, 1, ON or OFF. (If you set them on the command line or in an option file, use the numeric values.) Variables that are marked as 'enumeration' normally should be set to one of the available values for the variable, but can also be set to the number that corresponds to the desired enumeration value. For enumerated system variables, the first enumeration value corresponds to 0. This differs from ENUM columns, for which the first enumeration value corresponds to 1. Variable NameValue TypeTypeautocommitbooleanSESSIONbig_tablesbooleanSESSIONbinlog_cache_sizenumeric GLOBALbulk_insert_buffer_sizenumericGLOBAL | SESSIONcharacter_set_clientstring GLOBAL | SESSIONcharacter_set_connectionstringGLOBAL | SESSION character_set_ resultsstringGLOBAL | SESSIONcharacter_set_serverstringGLOBAL | SESSIONcollation _connectionstringGLOBAL | SESSION collation_serverstringGLOBAL | SESSIONconcurr ent_insertbooleanGLOBALconnect_timeoutnumericGLOBALconvert_character_setstring GLOBAL | SESSIONdefault_week_formatnumericGLOBAL | SESSIONdelay_key_writeOFF | ON | ALLGLOBALdelayed_insert_limitnumericGLOBALdelayed_insert_timeoutnumeric GLOBALdelayed_queue_sizenumericGLOBALerror_countnumericSESSIONexpire_logs_days numericGLOBALflushbooleanGLOBALflush_timenumericGLOBALforeign_key_checksboolean SESSIONft_boolean_syntaxnumericGLOBALgroup_concat_max_lennumericGLOBAL | SESSION identitynumericSESSIONinnodb_autoextend_incrementnumericGLOBALinnodb_concurrency _ticketsnumericGLOBALinnodb_max_dirty_pages_pctnumericGLOBALinnodb_max_purge_ lagnumericGLOBALinnodb_sync_spin_loopsnumericGLOBALinnodb_table_locksboolean GLOBAL | SESSIONinnodb_thread_concurrencynumericGLOBALinnodb_thread_sleep_ delaynumericGLOBALinsert_idbooleanSESSIONinteractive_timeoutnumericGLOBAL | SESSIONjoin_buffer_sizenumericGLOBAL | SESSIONkey_buffer_sizenumericGLOBAL last_insert_idnumericSESSIONlocal_infilebooleanGLOBALlog_warningsnumericGLOBAL long_query_timenumericGLOBAL | SESSIONlow_priority_updatesbooleanGLOBAL | SESSION max_allowed_packetnumericGLOBAL | SESSIONmax_binlog_cache_sizenumericGLOBALmax_ binlog_sizenumericGLOBALmax_connect_errorsnumericGLOBALmax_connectionsnumeric GLOBALmax_delayed_threadsnumericGLOBALmax_error_countnumericGLOBAL | SESSION max_heap_table_sizenumericGLOBAL | SESSIONmax_insert_delayed_threadsnumeric GLOBALmax_join_sizenumericGLOBAL | SESSIONmax_prepared_stmt_countnumericGLOBAL max_relay_log_sizenumericGLOBALmax_seeks_for_keynumericGLOBAL | SESSIONmax_sort _lengthnumericGLOBAL | SESSIONmax_tmp_tablesnumericGLOBAL | SESSIONmax_user_ connectionsnumericGLOBALmax_write_lock_countnumericGLOBALmyisam_stats_methodenum GLOBAL | SESSIONmulti_read_rangenumericGLOBAL | SESSIONmyisam_data_pointer_size numericGLOBALmyisam_max_sort_file_sizenumericGLOBAL | SESSIONmyisam_repair_threads numericGLOBAL | SESSIONmyisam_sort_buffer_sizenumericGLOBAL | SESSIONnet_ buffer_lengthnumericGLOBAL | SESSIONnet_read_timeoutnumericGLOBAL | SESSION net_retry_countnumericGLOBAL | SESSIONnet_write_timeoutnumericGLOBAL | SESSION old_passwordsnumericGLOBAL | SESSIONoptimizer_prune_levelnumericGLOBAL | SESSIONoptimizer_search_depthnumericGLOBAL | SESSIONpreload_buffer_sizenumeric GLOBAL | SESSIONquery_alloc_block_sizenumericGLOBAL | SESSIONquery_cache_limit numericGLOBALquery_cache_sizenumericGLOBALquery_cache_typeenumerationGLOBAL | SESSIONquery_cache_wlock_invalidatebooleanGLOBAL | SESSIONquery_prealloc_ sizenumericGLOBAL | SESSIONrange_alloc_block_sizenumericGLOBAL | SESSIONread_ buffer_sizenumericGLOBAL | SESSIONread_onlynumericGLOBALread_rnd_buffer_ sizenumericGLOBAL | SESSIONrpl_recovery_ranknumericGLOBALsafe_show_database booleanGLOBALsecure_authbooleanGLOBALserver_idnumericGLOBALslave_compressed_ protocolbooleanGLOBALslave_net_timeoutnumericGLOBALslave_transaction_retries numericGLOBALslow_launch_timenumericGLOBALsort_buffer_sizenumericGLOBAL | SESSIONsql_auto_is_nullbooleanSESSIONsql_big_selectsbooleanSESSIONsql_big_ tablesbooleanSESSIONsql_buffer_resultbooleanSESSIONsql_log_binbooleanSESSION sql_log_offbooleanSESSIONsql_log_updatebooleanSESSIONsql_low_priority_update sbooleanGLOBAL | SESSIONsql_max_join_sizenumericGLOBAL | SESSIONsql_modee numerationGLOBAL | SESSIONsql_notesbooleanSESSIONsql_quote_show_ createbooleanSESSIONsql_safe_updatesbooleanSESSIONsql_select_limitnumeric SESSIONsql_slave_skip_counternumericGLOBALupdatable_views_with_limite numerationGLOBAL | SESSIONsql_warningsbooleanSESSIONsync_binlognumeric GLOBALsync_frmbooleanGLOBALstorage_engineenumerationGLOBAL | SESSIONtable_ cachenumericGLOBALtable_typeenumerationGLOBAL | SESSIONthread_cache_size numericGLOBALtime_zonestringGLOBAL | SESSIONtimestampbooleanSESSIONtmp_ table_sizeenumerationGLOBAL | SESSIONtransaction_alloc_block_ sizenumericGLOBAL | SESSIONtransaction_prealloc_sizenumericGLOBAL | SESSIONtx_isolationenumerationGLOBAL | SESSIONunique_checksbooleanSESSION wait_timeoutnumericGLOBAL | SESSIONwarning_countnumericSESSION.SH \" SERVER STATUS VARIABLES\"
The server maintains many status variables that provide information about its operation. You can view these variables and their values by using the SHOW STATUS statement:

mysql> SHOW STATUS;
       +--------------------------+------------+
       | Variable_name            | Value      |
       +--------------------------+------------+
       | Aborted_clients          | 0          |
       | Aborted_connects         | 0          |
       | Bytes_received           | 155372598  |
       | Bytes_sent               | 1176560426 |
       | Connections              | 30023      |

Many status variables are reset to 0 by the FLUSH STATUS statement.
The status variables have the following meanings. The Com_xxx statement counter variables were added beginning with MySQL 3.23.47. The Qcache_xxx query cache variables were added beginning with MySQL 4.0.1. Otherwise, variables with no version indicated have been present since at least MySQL 3.22.
· Aborted_clients
The number of connections that were aborted because the client died without closing the connection properly. See Section 2.10, 'Communication Errors and Aborted Connections'.
· Aborted_connects
The number of failed attempts to connect to the MySQL server. See Section 2.10, 'Communication Errors and Aborted Connections'.
· Binlog_cache_disk_use
The number of transactions that used the temporary binary log cache but that exceeded the value of binlog_cache_size and used a temporary file to store statements from the transaction. This variable was added in MySQL 4.1.2.
· Binlog_cache_use
The number of transactions that used the temporary binary log cache. This variable was added in MySQL 4.1.2.
· Bytes_received
The number of bytes received from all clients. This variable was added in MySQL 3.23.7.
· Bytes_sent
The number of bytes sent to all clients. This variable was added in MySQL 3.23.7.
· Com_xxx
The Com_xxx statement counter variables were added beginning with MySQL 3.23.47. They indicate the number of times each xxx statement has been executed. There is one status variable for each type of statement. For example, Com_delete and Com_insert count DELETE and INSERT statements.
New Com_stmt_xxx status variables have been added in MySQL 4.1.13.
· Com_stmt_prepare
· Com_stmt_execute
· Com_stmt_send_long_data
· Com_stmt_reset
· Com_stmt_close
Those variables stand for prepared statement commands. Their names refer to the COM_xxx command set used in the network layer. In other words, their values increase whenever prepared statement API calls such as mysql_stmt_prepare(), mysql_stmt_execute(), and so forth are executed. However, Com_stmt_prepare, Com_stmt_execute and Com_stmt_close also increase for PREPARE, EXECUTE, or DEALLOCATE PREPARE, respectively. Additionally, the values of the older (available since MySQL 4.1.3) statement counter variables Com_prepare_sql, Com_execute_sql, and Com_dealloc_sql increase for the PREPARE, EXECUTE, and DEALLOCATE PREPARE statements.
All of the Com_stmt_xxx variables are increased even if their argument (a prepared statement) is unknown or an error occurred during execution; in other words: Their values correspond to the number of requests issued, not to the number of requests successfully completed.
· Connections
The number of connection attempts (successful or not) to the MySQL server.
· Created_tmp_disk_tables
The number of temporary tables on disk created automatically by the server while executing statements. This variable was added in MySQL 3.23.24.
· Created_tmp_files
How many temporary files mysqld has created. This variable was added in MySQL 3.23.28.
· Created_tmp_tables
The number of in-memory temporary tables created automatically by the server while executing statements. If Created_tmp_disk_tables is large, you may want to increase the tmp_table_size value to cause temporary tables to be memory-based instead of disk-based.
· Delayed_errors
The number of rows written with INSERT DELAYED for which some error occurred (probably duplicate key).
· Delayed_insert_threads
The number of INSERT DELAYED handler threads in use.
· Delayed_writes
The number of INSERT DELAYED rows written.
· Flush_commands
The number of executed FLUSH statements.
· Handler_commit
The number of internal COMMIT statements. This variable was added in MySQL 4.0.2.
· Handler_discover
The MySQL server can ask the NDB Cluster storage engine if it knows about a table with a given name. This is called discovery. Handler_discover indicates the number of times that tables have been discovered. This variable was added in MySQL 4.1.2.
· Handler_delete
The number of times a row was deleted from a table.
· Handler_read_first
The number of times the first entry was read from an index. If this value 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
The number of requests to read a row based on a key. If this value is high, it is a good indication that your tables are properly indexed for your queries.
· Handler_read_next
The number of requests to read the next row in key order. This value is incremented if you are querying an index column with a range constraint or if you are doing an index scan.
· Handler_read_prev
The number of requests to read the previous row in key order. This read method is mainly used to optimize ORDER BY ... DESC. This variable was added in MySQL 3.23.6.
· Handler_read_rnd
The number of requests to read a row based on a fixed position. This value is high if you are doing a lot of queries that require sorting of the result. You probably have a lot of queries that require MySQL to scan entire tables or you have joins that don't use keys properly.
· Handler_read_rnd_next
The number of requests to read the next row in the data file. This value is 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_rollback
The number of internal ROLLBACK statements. This variable was added in MySQL 4.0.2.
· Handler_update
The number of requests to update a row in a table.
· Handler_write
The number of requests to insert a row in a table.
· Key_blocks_not_flushed
The number of key blocks in the key cache that have changed but have not yet been flushed to disk. This variable was added in MySQL 4.1.1. It used to be known as Not_flushed_key_blocks.
· Key_blocks_unused
The number of unused blocks in the key cache. You can use this value to determine how much of the key cache is in use; see the discussion of key_buffer_size in the section called 'SERVER SYSTEM VARIABLES'. This variable was added in MySQL 4.1.2. the section called 'SERVER SYSTEM VARIABLES'.
· Key_blocks_used
The number of used blocks in the key cache. This value is a high-water mark that indicates the maximum number of blocks that have ever been in use at one time.
· 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. If Key_reads is large, then your key_buffer_size value is probably too small. The cache miss rate can be calculated as Key_reads/Key_read_requests.
· 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 that have been in use simultaneously since the server started.
· Not_flushed_delayed_rows
The number of rows waiting to be written in INSERT DELAY queues.
· Not_flushed_key_blocks
The old name for Key_blocks_not_flushed before MySQL 4.1.1.
· Open_files
The number of files that are open.
· Open_streams
The number of streams that are open (used mainly for logging).
· Open_tables
The number of tables that are open.
· Opened_tables
The number of tables that have been opened. If Opened_tables is big, your table_cache value is probably too small.
· Qcache_free_blocks
The number of free memory blocks in the query cache.
· Qcache_free_memory
The amount of free memory for the query cache.
· Qcache_hits
The number of query cache hits.
· Qcache_inserts
The number of queries added to the query cache.
· Qcache_lowmem_prunes
The number of queries that were deleted from the query cache because of low memory.
· Qcache_not_cached
The number of non-cached queries (not cacheable, or not cached due to the query_cache_type setting).
· Qcache_queries_in_cache
The number of queries registered in the query cache.
· Qcache_total_blocks
The total number of blocks in the query cache.
· Questions
The number of statements that clients have sent to the server.
· Rpl_status
The status of fail-safe replication (not yet implemented).
· Select_full_join
The number of joins that perform table scans because they do not use indexes. If this value is not 0, you should carefully check the indexes of your tables. This variable was added in MySQL 3.23.25.
· Select_full_range_join
The number of joins that used a range search on a reference table. This variable was added in MySQL 3.23.25.
· Select_range
The number of joins that used ranges on the first table. This is normally not critical issue even if the value is quite large. This variable was added in MySQL 3.23.25.
· Select_range_check
The number of joins without keys that check for key usage after each row. (If this is not equal to 0, you should very carefully check the indexes of your tables.) This variable was added in MySQL 3.23.25.
· Select_scan
The number of joins that did a full scan of the first table. This variable was added in MySQL 3.23.25.
· Slave_open_temp_tables
The number of temporary tables that the slave SQL thread currently has open. This variable was added in MySQL 3.23.29.
· Slave_running
This is ON if this server is a slave that is connected to a master. This variable was added in MySQL 3.23.16.
· Slave_retried_transactions
Total (since startup) number of times the replication slave SQL thread has retried transactions. This variable was added in MySQL 4.1.11.
· Slow_launch_threads
The number of threads that have taken more than slow_launch_time seconds to create. This variable was added in MySQL 3.23.15.
· Slow_queries
The number of queries that have taken more than long_query_time seconds. See Section 10.5, 'The Slow Query Log'.
· Sort_merge_passes
The number of merge passes that the sort algorithm has had to do. If this value is large, you should consider increasing the value of the sort_buffer_size system variable. This variable was added in MySQL 3.23.28.
· Sort_range
The number of sorts that were done with ranges. This variable was added in MySQL 3.23.25.
· Sort_rows
The number of sorted rows. This variable was added in MySQL 3.23.25.
· Sort_scan
The number of sorts that were done by scanning the table. This variable was added in MySQL 3.23.25.
· Ssl_xxx
Variables used for SSL connections. These variables were added in MySQL 4.0.0.
· Table_locks_immediate
The number of times that a table lock was acquired immediately. This variable was added in MySQL 3.23.33.
· Table_locks_waited
The number of times that 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 or tables or use replication. This variable was added in MySQL 3.23.33.
· Threads_cached
The number of threads in the thread cache. This variable was added in MySQL 3.23.17.
· Threads_connected
The number of currently open connections.
· Threads_created
The number of threads created to handle connections. If Threads_created is big, you may want to increase the thread_cache_size value. The cache miss rate can be calculated as Threads_created divided by Connections. This variable was added in MySQL 3.23.31.
· Threads_running
The number of threads that are not sleeping.
· Uptime
The number of seconds that the server has been up.
THE SERVER SQL MODE
The MySQL server can operate in different SQL modes, and (as of MySQL 4.1) can apply these modes differentially for different clients. This capability enables each application to tailor the server's operating mode to its own requirements.
Modes define what SQL syntax MySQL should support and what kind of data validation checks it should perform. This makes it easier to use MySQL in different environments and to use MySQL together with other database servers.
You can set the default SQL mode by starting mysqld with the --sql-mode=\"modes\" option. modes is a list of different modes separated by comma (',') characters. The default value is empty (no modes set). The modes value also can be empty (--sql-mode=\"\") if you want to clear it explicitly.
Beginning with MySQL 4.1, you can change the SQL mode at runtime by using a SET [GLOBAL|SESSION] sql_mode='modes' statement to set the sql_mode system value. Setting the GLOBAL variable requires the SUPER privilege and affects the operation of all clients that connect from that time on. Setting the SESSION variable affects only the current client. Any client can change its own session sql_mode value at any time.
You can retrieve the current global or session sql_mode value with the following statements:
SELECT @@global.sql_mode;
SELECT @@session.sql_mode;
The most important sql_mode value is ANSI, which changes syntax and behavior to be more conformant to standard SQL. This mode is available beginning in MySQL 4.1.1
The following list describes all supported modes:
· ANSI_QUOTES
Treat '\"' as an identifier quote character (like the ''' quote character) and not as a string quote character. You can still use ''' to quote identifiers with this mode enabled. With ANSI_QUOTES enabled, you cannot use double quotes to quote literal strings, because it is interpreted as an identifier. (New in MySQL 4.0.0)
· IGNORE_SPACE
Allow spaces between a function name and the '(' character. This forces all function names to be treated as reserved words. As a result, if you want to access any database, table, or column name that is a reserved word, you must quote it. For example, because there is a USER() function, the name of the user table in the mysql database and the User column in that table become reserved, so you must quote them:
SELECT \"User\" FROM mysql.\"user\";
(New in MySQL 4.0.0)
See Section 1.1, 'MyISAM Startup Options'.
· NO_AUTO_VALUE_ON_ZERO
NO_AUTO_VALUE_ON_ZERO affects handling of AUTO_INCREMENT columns. Normally, you generate the next sequence number for the column by inserting either NULL or 0 into it. NO_AUTO_VALUE_ON_ZERO suppresses this behavior for 0 so that only NULL generates the next sequence number. (New in MySQL 4.1.1)
This mode can be useful if 0 has been stored in a table's AUTO_INCREMENT column. (Storing 0 is not a recommended practice, by the way.) For example, if you dump the table with mysqldump and then reload it, MySQL normally generates new sequence numbers when it encounters the 0 values, resulting in a table with contents different from the one that was dumped. Enabling NO_AUTO_VALUE_ON_ZERO before reloading the dump file solves this problem. As of MySQL 4.1.1, mysqldump automatically includes a statement in the dump output that enables NO_AUTO_VALUE_ON_ZERO, to avoid this problem..
· NO_DIR_IN_CREATE
When creating a table, ignore all INDEX DIRECTORY and DATA DIRECTORY directives. This option is useful on slave replication servers. (New in MySQL 4.0.15)
· NO_FIELD_OPTIONS
Do not print MySQL-specific column options in the output of SHOW CREATE TABLE. This mode is used by mysqldump in portability mode. (New in MySQL 4.1.1)
· NO_KEY_OPTIONS
Do not print MySQL-specific index options in the output of SHOW CREATE TABLE. This mode is used by mysqldump in portability mode. (New in MySQL 4.1.1)
· NO_TABLE_OPTIONS
Do not print MySQL-specific table options (such as ENGINE) in the output of SHOW CREATE TABLE. This mode is used by mysqldump in portability mode. (New in MySQL 4.1.1)
· NO_UNSIGNED_SUBTRACTION
In integer subtraction operations, do not mark the result as UNSIGNED if one of the operands is unsigned. Note that this makes BIGINT UNSIGNED not 100% usable in all contexts. See Section 8, 'Cast Functions and Operators'. (New in MySQL 4.0.2)
· ONLY_FULL_GROUP_BY
Do not allow queries for which the SELECT list refers to non-aggregated columns that are not named in the GROUP BY clause. (New in MySQL 4.0.0) The following query is invalid with this mode enabled because address is not named in the GROUP BY clause:
SELECT name, address, MAX(age) FROM t GROUP BY name;
· PIPES_AS_CONCAT
Treat || as a string concatenation operator (same as CONCAT()) rather than as a synonym for OR. (New in MySQL 4.0.0)
· REAL_AS_FLOAT
Treat REAL as a synonym for FLOAT. By default, MySQL treats REAL as a synonym for DOUBLE. (New in MySQL 4.0.0)
The following special modes are provided as shorthand for combinations of mode values from the preceding list. All are available as of MySQL 4.1.1.
The descriptions include all mode values that are available in the most recent version of MySQL. For older versions, a combination mode does not include individual mode values that are not available except in newer versions.
· ANSI
Equivalent to REAL_AS_FLOAT, PIPES_AS_CONCAT, ANSI_QUOTES, IGNORE_SPACE. Before MySQL 4.1.11, ANSI also includes ONLY_FULL_GROUP_BY. See Section 9.3, 'Running MySQL in ANSI Mode'.
· DB2
Equivalent to PIPES_AS_CONCAT, ANSI_QUOTES, IGNORE_SPACE, NO_KEY_OPTIONS, NO_TABLE_OPTIONS, NO_FIELD_OPTIONS.
· MAXDB
Equivalent to PIPES_AS_CONCAT, ANSI_QUOTES, IGNORE_SPACE, NO_KEY_OPTIONS, NO_TABLE_OPTIONS, NO_FIELD_OPTIONS.
· MSSQL
Equivalent to PIPES_AS_CONCAT, ANSI_QUOTES, IGNORE_SPACE, NO_KEY_OPTIONS, NO_TABLE_OPTIONS, NO_FIELD_OPTIONS.
· MYSQL323
Equivalent to NO_FIELD_OPTIONS.
· MYSQL40
Equivalent to NO_FIELD_OPTIONS.
· ORACLE
Equivalent to PIPES_AS_CONCAT, ANSI_QUOTES, IGNORE_SPACE, NO_KEY_OPTIONS, NO_TABLE_OPTIONS, NO_FIELD_OPTIONS.
· POSTGRESQL
Equivalent to PIPES_AS_CONCAT, ANSI_QUOTES, IGNORE_SPACE, NO_KEY_OPTIONS, NO_TABLE_OPTIONS, NO_FIELD_OPTIONS.
THE MYSQL SERVER SHUTDOWN PROCESS
The server shutdown process takes place as follows:
1. The shutdown process is initiated.
Server shutdown can be initiated several ways. For example, a user with the SHUTDOWN privilege can execute a mysqladmin shutdown command. mysqladmin can be used on any platform supported by MySQL. Other operating system-specific shutdown initiation methods are possible as well: The server shuts down on Unix when it receives a SIGTERM signal. A server running as a service on Windows shuts down when the services manager tells it to. (On Windows, a user with Administrator rights can also shut down the server using NET STOP service_name, where service_name is the name of the MySQL service. By default, this is MySQL.)
2. The server creates a shutdown thread if necessary.
Depending on how shutdown was initiated, the server might create a thread to handle the shutdown process. If shutdown was requested by a client, a shutdown thread is created. If shutdown is the result of receiving a SIGTERM signal, the signal thread might handle shutdown itself, or it might create a separate thread to do so. If the server tries to create a shutdown thread and cannot (for example, if memory is exhausted), it issues a diagnostic message that appears in the error log:
Error: Can't create thread to kill server
3. The server stops accepting new connections.
To prevent new activity from being initiated during shutdown, the server stops accepting new client connections. It does this by closing the network connections to which it normally listens for connections: the TCP/IP port, the Unix socket file, the Windows named pipe, and shared memory on Windows.
4. The server terminates current activity.
For each thread that is associated with a client connection, the connection to the client is broken and the thread is marked as killed. Threads die when they notice that they are so marked. Threads for idle connections die quickly. Threads that currently are processing statements check their state periodically and take longer to die. For additional information about thread termination, see Section 5.5.3, 'KILL Syntax', in particular for the instructions about killed REPAIR TABLE or OPTIMIZE TABLE operations on MyISAM tables.
For threads that have an open transaction, the transaction is rolled back. Note that if a thread is updating a non-transactional table, an operation such as a multiple-row UPDATE or INSERT may leave the table partially updated, because the operation can terminate before completion.
If the server is a master replication server, threads associated with currently connected slaves are treated like other client threads. That is, each one is marked as killed and exits when it next checks its state.
If the server is a slave replication server, the I/O and SQL threads, if active, are stopped before client threads are marked as killed. The SQL thread is allowed to finish its current statement (to avoid causing replication problems), and then stops. If the SQL thread was in the middle of a transaction at this point, the transaction is rolled back.
5. Storage engines are shut down or closed.
At this stage, the table cache is flushed and all open tables are closed.
Each storage engine performs any actions necessary for tables that it manages. For example, MyISAM flushes any pending index writes for a table. InnoDB flushes its buffer pool to disk, writes the current LSN to the tablespace, and terminates its own internal threads.
6. The server exits.

mysql>t; SET sql_mode = '';
mysql>t; SELECT CAST(0 AS UNSIGNED) - 1;
+-------------------------+
| CAST(0 AS UNSIGNED) - 1 |
+-------------------------+
|    18446744073709551615 |
+-------------------------+
mysql>t; SET sql_mode = 'NO_UNSIGNED_SUBTRACTION';
mysql>t; SELECT CAST(0 AS UNSIGNED) - 1;
+-------------------------+
| CAST(0 AS UNSIGNED) - 1 |
+-------------------------+
|                      -1 |
+-------------------------+

SEE ALSO
isamchk(1), isamlog(1), msql2mysql(1), myisam_ftdump(1), myisamchk(1), myisamlog(1), myisampack(1), mysql(1), mysql.server(1), mysql_config(1), mysql_fix_privilege_tables(1), mysql_zap(1), mysqlaccess(1), mysqladmin(1), mysqlbinlog(1), mysqlcheck(1), mysqld_multi(1), mysqld_safe(1), mysqldump(1), mysqlhotcopy(1), mysqlimport(1), mysqlshow(1), pack_isam(1), perror(1), replace(1), safe_mysqld(1)
For more information, please refer to the MySQL Reference Manual, which may already be installed locally and which is also available online at http://dev.mysql.com/doc/.
AUTHOR
MySQL AB (http://www.mysql.com/). This software comes with no warranty.