MYSQLD(8) MySQL Database System MYSQLD(8)
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
Note: Not all storage engines are supported by all MySQL server binaries and configurations. To find out how
to determine which storage engines are supported by your MySQL server installation, see Section 5.4.10,
"SHOW ENGINES Syntax".
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".
MySQL Enterprise. For expert advice on setting command options, subscribe to the MySQL Network
Monitoring and Advisory Service. For more information see
http://www.mysql.com/products/enterprise/advisors.html
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 brief summary, execute mysqld --help. 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 4.3, "Security-Related mysqld Options".
SSL-related options: See Section 6.7.3, "SSL Command Options".
Binary log control options: See Section 9.3, "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 2.4, "InnoDB Startup Options and System Variables",
and Section 5.2.1, "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. Use both the --verbose and --help options to see the full message.
--abort-slave-event-count
This option is used internally by the MySQL test suite for replication testing and debugging.
--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 version 5.0.3. 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 "SQL MODES".
--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.
This option is unavailable if MySQL was configured with the --disable-grant-options option. See
Section 4.14.2, "Typical configure Options".
--character-sets-dir=path
The directory where character sets are installed. See Section 8.1, "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 behave like
MySQL 4.0.
--character-set-filesystem=charset_name
The filesystem character set. This option sets the character_set_filesystem system variable. It was added
in MySQL 5.0.19.
--character-set-server=charset_name, -C charset_name
Use charset_name as the default server character set. See Section 8.1, "The Character Set Used for Data
and Sorting". If you use this option to specify a non-default character set, you should also use
--collation-server to specify the collation.
--chroot=path
Put the mysqld server in a closed environment during startup by using the chroot() system call. This is a
recommended security measure. 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. See Section 8.1, "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
[1]MySQL Internals: Porting.
As of MySQL 5.0.25, using --with-debug to configure MySQL with debugging support enables you to use the
--debug="d,parser_debug" option when you start the server. This causes the Bison parser that is used to
process SQL statements to dump a parser trace to the server's standard error output. Typically, this
output is written to the error log.
--default-character-set=charset_name (DEPRECATED)
Use charset_name as the default character set. This option is deprecated in favor of
--character-set-server. See Section 8.1, "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.
See Section 8.1, "The Character Set Used for Data and Sorting".
--default-storage-engine=type
Set the default storage engine (table type) for tables. See Chapter 14, Storage Engines.
--default-table-type=type
This option is a synonym for --default-storage-engine.
--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.
--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.
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.
--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.
--disconnect-slave-event-count
This option is used internally by the MySQL test suite for replication testing and debugging.
--enable-named-pipe
Enable support for named pipes. This option can be used only with the mysqld-nt and mysqld-debug 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 previously was named --enable-locking.
For more information about external locking, including conditions under which it can and cannot be used,
see Section 3.4, "External Locking".
--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 1.4.2, "What to Do If MySQL Keeps Crashing".
--enable-pstack
Print a symbolic stack trace on failure.
--gdb
Install an interrupt handler for SIGINT (needed to stop mysqld with ^C to set breakpoints) and disable
stack tracing and core file handling. See [1]MySQL Internals: Porting.
--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.
This option is unavailable if MySQL was configured with the --disable-grant-options option. See
Section 4.14.2, "Typical configure Options".
--innodb-safe-binlog
Adds consistency guarantees between the content of InnoDB tables and the binary log. See Section 9.3,
"The Binary Log". This option was removed in MySQL 5.0.3, having been made obsolete by the introduction
of XA transaction support.
--innodb-xxx
The InnoDB options are listed in Section 2.4, "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 8.2, "Setting the
Error Message Language".
--large-pages
Some hardware/operating system architectures support memory pages greater than the default (usually 4KB).
The actual implementation of this support depends on the underlying hardware and OS. Applications that
perform a lot of memory accesses may obtain performance improvements by using large pages due to reduced
Translation Lookaside Buffer (TLB) misses.
Currently, MySQL supports only the Linux implementation of large pages support (which is called HugeTLB
in Linux). We have plans to extend this support to FreeBSD, Solaris and possibly other platforms.
Before large pages can be used on Linux, it is necessary to configure the HugeTLB memory pool. For
reference, consult the hugetlbpage.txt file in the Linux kernel source.
This option is disabled by default. It was added in MySQL 5.0.3.
--log[=file_name], -l [file_name]
Log connections and SQL statements received from clients to this file. See Section 9.2, "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 9.3, "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 1.8.1, "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 9.3, "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-bin-trust-function-creators[={0|1}]
With no argument or an argument of 1, this option sets the log_bin_trust_function_creators system
variable to 1. With an argument of 0, this option sets the system variable to 0.
log_bin_trust_function_creators affects how MySQL enforces restrictions on stored function creation. See
Section 4, "Binary Logging of Stored Routines and Triggers".
This option was added in MySQL 5.0.16.
--log-bin-trust-routine-creators[={0|1}]
This is the old name for --log-bin-trust-function-creators. Before MySQL 5.0.16, it also applies to
stored procedures, not just stored functions and sets the log_bin_trust_routine_creators system variable.
As of 5.0.16, this option is deprecated. It is recognized for backward compatibility but its use results
in a warning.
This option was added in MySQL 5.0.6.
--log-error[=file_name]
Log errors and startup messages to this file. See Section 9.1, "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 MyISAM changes to this file (used only when debugging MyISAM).
--log-long-format (DEPRECATED)
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 all queries. This option is deprecated,
as it now represents the default logging behavior. (See the description for --log-short-format.) 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 are logged to the
slow query log. See Section 9.4, "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.
--log-slow-admin-statements
Log slow administrative statements such as OPTIMIZE TABLE, ANALYZE TABLE, and ALTER TABLE to the slow
query log.
--log-slow-queries[=file_name]
Log all queries that have taken more than long_query_time seconds to execute to this file. See
Section 9.4, "The Slow Query Log". See the descriptions of the --log-long-format and --log-short-format
options for details.
--log-tc=file_name
The name of the memory-mapped transaction coordinator log file (for XA transactions that affect multiple
storage engines when the binary log is disabled). The default name is tc.log. The file is created under
the data directory if not given as a full pathname. Currently, this option is unused. Added in MySQL
5.0.3.
--log-tc-size=size
The size in bytes of the memory-mapped transaction coordinator log. The default size is 24KB. Added in
MySQL 5.0.3.
--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 (1) by default, and the default level
value if omitted is 1. To disable this option, use --log-warnings=0. Aborted connections are not logged
to the error log unless the value is greater than 1. See Section 1.2.10, "Communication Errors and
Aborted Connections".
--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. This affects only storage
engines that use only table-level locking (MyISAM, MEMORY, MERGE). See Section 3.2, "Table Locking
Issues".
--max-binlog-dump-events
This option is used internally by the MySQL test suite for replication testing and debugging.
--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 4.5, "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:
+--------+----------------------------------------------+
|Option | Description |
+--------+----------------------------------------------+
|DEFAULT | The same as not giving any option to |
| | --myisam-recover. |
+--------+----------------------------------------------+
|BACKUP | If the data file was changed during |
| | recovery, save a backup of the |
| | tbl_name.MYD |
| | file as |
| | tbl_name-datetime.BAK. |
+--------+----------------------------------------------+
|FORCE | Run recovery even if we would lose more than |
| | one row from the |
| | .MYD file. |
+--------+----------------------------------------------+
|QUICK | Don't check the rows in the table if there |
| | aren't 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.
See Section 1.1, "MyISAM Startup Options".
--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 3.4.2, "The Cluster
Connectstring", for syntax.
--ndbcluster
If the binary includes support for the NDB Cluster storage engine, this option enables the engine, which
is disabled by default. See Chapter 15, MySQL Cluster.
--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 5.9, "Password Hashing as
of MySQL 4.1".
--one-thread
Only use one thread (for debugging under Linux). This option is available only if the server is built
with debugging enabled. See [1]MySQL Internals: Porting.
--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_connectionsx5 or max_connections + table_open_cachex2 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.
--port-open-timeout=num
On some systems, when the server is stopped, the TCP/IP port might not become available immediately. If
the server is restarted quickly afterward, its attempt to reopen the port can fail. This option indicates
how many seconds the server should wait for the TCP/IP port to become free if it cannot be opened. The
default is not to wait. This option was added in MySQL 5.0.19.
--safe-mode
Skip some optimization stages.
--safe-show-database (DEPRECATED)
See Section 5.3, "Privileges Provided by MySQL".
--safe-user-create
If this option is enabled, a user cannot create new MySQL users by using the GRANT statement unless the
user has the INSERT privilege for the mysql.user table or any column in the table. If you want a user to
have the ability to create new users that have those privileges that the user has the right to grant, you
should grant the user the following privilege:
GRANT INSERT(user) ON mysql.user TO 'user_name'@'host_name';
This ensures that the user cannot change any privilege columns directly, but has to use the GRANT statement
to give privileges to other users.
--secure-auth
Disallow authentication by clients that attempt to use accounts that have old (pre-4.1) passwords.
--secure-file-priv=path
This option limits the effect of the LOAD_FILE() function and the LOAD DATA and SELECT ... INTO OUTFILE
statements to work only with files in the specified directory.
This option was added in MySQL 5.0.38.
--shared-memory
Enable shared-memory connections by local clients. This option is available only on Windows.
--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.
--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-external-locking
Do not use external locking (system locking). For more information about external locking, including
conditions under which it can and cannot be used, see Section 3.4, "External 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).
This option is unavailable if MySQL was configured with the --disable-grant-options option. See
Section 4.14.2, "Typical configure Options".
--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 5.9, "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-merge
Disable the MERGE storage engine. This option was added in MySQL 5.0.24. It can be used if the following
behavior is undesirable: If a user has access to MyISAM table t, that user can create a MERGE table m
that accesses t. However, if the user's privileges on t are subsequently revoked, the user can continue
to access t by doing so through m.
--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 5.9, "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 3.3, "Quick Test Setup of MySQL Cluster", for an
example of usage.
--skip-networking
Don't 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 5.9, "How MySQL Uses DNS".
--sporadic-binlog-dump-fail
This option is used internally by the MySQL test suite for replication testing and debugging.
--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 6.7.3, "SSL Command Options".
--standalone
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".
--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 don't 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
Don't 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 [1]MySQL Internals: Porting.
--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 "SQL MODES".
--sysdate-is-now
As of MySQL 5.0.13, SYSDATE() by default returns the time at which it executes, not the time at which the
statement in which it occurs begins executing. This differs from the behavior of NOW(). This option
causes SYSDATE() to be an alias for NOW(). For information about the implications for binary logging and
replication, see the description for SYSDATE() in Section 6, "Date and Time Functions" and for SET
TIMESTAMP in Section 5.3, "SET Syntax".
This option was added in MySQL 5.0.20.
--tc-heuristic-recover={COMMIT|ROLLBACK}
The type of decision to use in the heuristic recovery process. Currently, this option is unused. Added in
MySQL 5.0.3.
--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. 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 1.4.4, "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 4.1, "General
Security Guidelines".
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.
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 -O var_name=value syntax.
This syntax is deprecated.
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 "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.
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. 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:
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:
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 are
present in all MySQL 5.0 releases. For historical information concerning their implementation, please
see MySQL 3.23, 4.0, 4.1 Reference Manual.
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 system variables can be found in Section 5.2, "Tuning Server Parameters".
Section 2.4, "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.
auto_increment_increment
auto_increment_increment and auto_increment_offset are intended for use with master-to-master
replication, and can be used to control the operation of AUTO_INCREMENT columns. Both variables
can be set globally or locally, and each can assume an integer value between 1 and 65,535
inclusive. Setting the value of either of these two variables to 0 causes its value to be set to 1
instead. Attempting to set the value of either of these two variables to an integer greater than
65,535 or less than 0 causes its value to be set to 65,535 instead. Attempting to set the value of
auto_increment_increment or auto_increment_offset to a non-integer value gives rise to an error,
and the actual value of the variable remains unchanged.
These two variables affect AUTO_INCREMENT column behavior as follows:
auto_increment_increment controls the interval between successive column values. For example:
mysql> SHOW VARIABLES LIKE 'auto_inc%';
+--------------------------+-------+
| Variable_name | Value |
+--------------------------+-------+
| auto_increment_increment | 1 |
| auto_increment_offset | 1 |
+--------------------------+-------+
2 rows in set (0.00 sec)
mysql> CREATE TABLE autoinc1
-> (col INT NOT NULL AUTO_INCREMENT PRIMARY KEY);
Query OK, 0 rows affected (0.04 sec)
mysql> SET @@auto_increment_increment=10;
Query OK, 0 rows affected (0.00 sec)
mysql> SHOW VARIABLES LIKE 'auto_inc%';
+--------------------------+-------+
| Variable_name | Value |
+--------------------------+-------+
| auto_increment_increment | 10 |
| auto_increment_offset | 1 |
+--------------------------+-------+
2 rows in set (0.01 sec)
mysql> INSERT INTO autoinc1 VALUES (NULL), (NULL), (NULL), (NULL);
Query OK, 4 rows affected (0.00 sec)
Records: 4 Duplicates: 0 Warnings: 0
mysql> SELECT col FROM autoinc1;
+-----+
| col |
+-----+
| 1 |
| 11 |
| 21 |
| 31 |
+-----+
4 rows in set (0.00 sec)
(Note how SHOW VARIABLES is used here to obtain the current values for these variables.)
auto_increment_offset determines the starting point for the AUTO_INCREMENT column value.
Consider the following, assuming that these statements are executed during the same session as
the example given in the description for auto_increment_increment:
mysql> SET @@auto_increment_offset=5;
Query OK, 0 rows affected (0.00 sec)
mysql> SHOW VARIABLES LIKE 'auto_inc%';
+--------------------------+-------+
| Variable_name | Value |
+--------------------------+-------+
| auto_increment_increment | 10 |
| auto_increment_offset | 5 |
+--------------------------+-------+
2 rows in set (0.00 sec)
mysql> CREATE TABLE autoinc2
-> (col INT NOT NULL AUTO_INCREMENT PRIMARY KEY);
Query OK, 0 rows affected (0.06 sec)
mysql> INSERT INTO autoinc2 VALUES (NULL), (NULL), (NULL), (NULL);
Query OK, 4 rows affected (0.00 sec)
Records: 4 Duplicates: 0 Warnings: 0
mysql> SELECT col FROM autoinc2;
+-----+
| col |
+-----+
| 5 |
| 15 |
| 25 |
| 35 |
+-----+
4 rows in set (0.02 sec)
If the value of auto_increment_offset is greater than that of auto_increment_increment, the value
of auto_increment_offset is ignored.
Should one or both of these variables be changed and then new rows inserted into a table
containing an AUTO_INCREMENT column, the results may seem counterintuitive because the series of
AUTO_INCREMENT values is calculated without regard to any values already present in the column,
and the next value inserted is the least value in the series that is greater than the maximum
existing value in the AUTO_INCREMENT column. In other words, the series is calculated like so:
auto_increment_offset + N x auto_increment_increment
where N is a positive integer value in the series [1, 2, 3, ...]. For example:
mysql> SHOW VARIABLES LIKE 'auto_inc%';
+--------------------------+-------+
| Variable_name | Value |
+--------------------------+-------+
| auto_increment_increment | 10 |
| auto_increment_offset | 5 |
+--------------------------+-------+
2 rows in set (0.00 sec)
mysql> SELECT col FROM autoinc1;
+-----+
| col |
+-----+
| 1 |
| 11 |
| 21 |
| 31 |
+-----+
4 rows in set (0.00 sec)
mysql> INSERT INTO autoinc1 VALUES (NULL), (NULL), (NULL), (NULL);
Query OK, 4 rows affected (0.00 sec)
Records: 4 Duplicates: 0 Warnings: 0
mysql> SELECT col FROM autoinc1;
+-----+
| col |
+-----+
| 1 |
| 11 |
| 21 |
| 31 |
| 35 |
| 45 |
| 55 |
| 65 |
+-----+
8 rows in set (0.00 sec)
The values shown for auto_increment_increment and auto_increment_offset generate the series 5 + N x
10, that is, [5, 15, 25, 35, 45, ...]. The greatest value present in the col column prior to the
INSERT is 31, and the next available value in the AUTO_INCREMENT series is 35, so the inserted values
for col begin at that point and the results are as shown for the SELECT query.
It is important to remember that it is not possible to confine the effects of these two variables to
a single table, and thus they do not take the place of the sequences offered by some other database
management systems; these variables control the behavior of all AUTO_INCREMENT columns in all tables
on the MySQL server. If one of these variables is set globally, its effects persist until the global
value is changed or overridden by setting them locally, or until mysqld is restarted. If set locally,
the new value affects AUTO_INCREMENT columns for all tables into which new rows are inserted by the
current user for the duration of the session, unless the values are changed during that session.
The auto_increment_increment variable was added in MySQL 5.0.2. Its default value is 1. See
Section 13, "Auto-Increment in Multiple-Master Replication".
auto_increment_increment is supported for use with NDB tables beginning with MySQL 5.0.46.
Previously, setting it when using MySQL Cluster tables produced unpredictable results.
auto_increment_offset
This variable was introduced in MySQL 5.0.2. Its default value is 1. For particulars, see the
description for auto_increment_increment.
auto_increment_offset is supported for use with NDB tables beginning with MySQL 5.0.46.
Previously, setting it when using MySQL Cluster tables produced unpredictable results.
automatic_sp_privileges
When this variable has a value of 1 (the default), the server automatically grants the EXECUTE and
ALTER ROUTINE privileges to the creator of a stored routine, if the user cannot already execute
and alter or drop the routine. (The ALTER ROUTINE privileges is required to drop the routine.) The
server also automatically drops those privileges when the creator drops the routine. If
automatic_sp_privileges is 0, the server does not automatically add and drop these privileges.
This variable was added in MySQL 5.0.3.
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 don't
use BDB tables, you should start mysqld with --skip-bdb to not allocate memory for this cache.
bdb_home
The base directory for BDB tables. This should be assigned the same value as the datadir variable.
bdb_log_buffer_size
The size of the buffer that is allocated for caching indexes and rows for BDB tables. If you don't
use BDB tables, you should set this to 0 or start mysqld with --skip-bdb to not allocate memory
for this cache.
bdb_logdir
The directory where the BDB storage engine writes its log files. This variable can be set with the
--bdb-logdir option.
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 ...
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.)
bdb_tmpdir
The BDB temporary file directory.
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 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. See Section 9.3, "The Binary Log".
MySQL Enterprise. For recommendations on the optimum setting for binlog_cache_size subscribe to the
MySQL Network Monitoring and Advisory Service. For more information see
http://www.mysql.com/products/enterprise/advisors.html
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.
character_set_client
The character set for statements that arrive from the client.
character_set_connection
The character set used for literals that do not have a character set introducer and for
number-to-string conversion.
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.
character_set_filesystem
The filesystem character set. This variable is used to interpret string literals that refer to
filenames, such as in the LOAD DATA INFILE and SELECT ... INTO OUTFILE statements and the
LOAD_FILE() function. Such filenames are converted from character_set_client to
character_set_filesystem before the file opening attempt occurs. The default value is binary,
which means that no conversion occurs. For systems on which multi-byte filenames are allowed, a
different value may be more appropriate. For example, if the system represents filenames using
UTF-8, set character_set_filesystem to 'utf8'. This variable was added in MySQL 5.0.19.
character_set_results
The character set used for returning query results to the client.
character_set_server
The server's default character set.
character_set_system
The character set used by the server for storing identifiers. The value is always utf8.
character_sets_dir
The directory where character sets are installed.
collation_connection
The collation of the connection character set.
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.
collation_server
The server's default collation.
completion_type
The transaction completion type:
If the value is 0 (the default), COMMIT and ROLLBACK are unaffected.
If the value is 1, COMMIT and ROLLBACK are equivalent to COMMIT AND CHAIN and ROLLBACK AND
CHAIN, respectively. (A new transaction starts immediately with the same isolation level as the
just-terminated transaction.)
If the value is 2, COMMIT and ROLLBACK are equivalent to COMMIT RELEASE and ROLLBACK RELEASE,
respectively. (The server disconnects after terminating the transaction.)
This variable was added in MySQL 5.0.3
concurrent_insert
If 1 (the default), MySQL allows INSERT and SELECT statements to run concurrently for MyISAM
tables that have no free blocks in the middle of the data file. You can turn this option off by
starting mysqld with --safe or --skip-new.
In MySQL 5.0.6, this variable was changed to take three integer values:
+------+--------------------------------------+
|Value | Description |
+------+--------------------------------------+
|0 | Off |
+------+--------------------------------------+
|1 | (Default) Enables concurrent insert |
| | for MyISAM tables |
| | that don't |
| | have holes |
+------+--------------------------------------+
|2 | Enables concurrent inserts for all |
| | MyISAM tables, |
| | even those |
| | that have holes. For a table with a |
| | hole, new rows |
| | are inserted at the end of the |
| | table if it is |
| | in use by another thread. |
| | Otherwise, |
| | MySQL acquires a normal write lock |
| | and |
| | inserts the |
| | row into the hole. |
+------+--------------------------------------+
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.
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 6, "Date and Time Functions".
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.
+-------+---------------------------------------+
|Option | Description |
+-------+---------------------------------------+
|OFF | DELAY_KEY_WRITE is ignored. |
+-------+---------------------------------------+
|ON | MySQL honors any DELAY_KEY_WRITE |
| | option specified in |
| | CREATE TABLE |
| | statements. This |
| | is the default |
| | value. |
+-------+---------------------------------------+
|ALL | All 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 "COMMAND OPTIONS", and Section 1.1, "MyISAM Startup Options".
Note that if you enable external locking with --external-locking, there is no protection against
index corruption for tables that use delayed key writes.
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.
div_precision_increment
This variable indicates the number of digits of precision by which to increase the result of
division operations performed with the / operator. The default value is 4. The minimum and maximum
values are 0 and 30, respectively. The following example illustrates the effect of increasing the
default value.
mysql> SELECT 1/7;
+--------+
| 1/7 |
+--------+
| 0.1429 |
+--------+
mysql> SET div_precision_increment = 12;
mysql> SELECT 1/7;
+----------------+
| 1/7 |
+----------------+
| 0.142857142857 |
+----------------+
This variable was added in MySQL 5.0.6.
engine_condition_pushdown
This variable applies to NDB. By default it is 0 (OFF): If you execute a query such as SELECT *
FROM t WHERE mycol = 42, where mycol is a non-indexed column, the query is executed as a full
table scan on every NDB node. Each node sends every row to the MySQL server, which applies the
WHERE condition. If engine_condition_pushdown is set to 1 (ON), the condition is "pushed down" to
the storage engine and sent to the NDB nodes. Each node uses the condition to perform the scan,
and only sends back to the MySQL server the rows that match the condition.
This variable was added in MySQL 5.0.3. Before that, the default NDB behavior is the same as for a
value of OFF.
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.
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 1.4.2, "What to Do If MySQL Keeps Crashing".
This variable is set to ON if you start mysqld with the --flush option.
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
systems with minimal resources.
ft_boolean_syntax
The list of operators supported by boolean full-text searches performed using IN BOOLEAN MODE. See
Section 8.1, "Boolean Full-Text Searches".
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.
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.
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.
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.
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.
have_archive
YES if mysqld supports ARCHIVE tables, NO if not.
have_bdb
YES if mysqld supports BDB tables. DISABLED if --skip-bdb is used.
have_blackhole_engine
YES if mysqld supports BLACKHOLE tables, NO if not.
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.
have_crypt
YES if the crypt() system call is available to the server, NO if not. If not, the ENCRYPT()
function cannot be used.
have_csv
YES if mysqld supports CSV tables, NO if not.
have_example_engine
YES if mysqld supports EXAMPLE tables, NO if not.
have_federated_engine
YES if mysqld supports FEDERATED tables, NO if not. This variable was added in MySQL 5.0.3.
have_geometry
YES if the server supports spatial data types, NO if not.
have_innodb
YES if mysqld supports InnoDB tables. DISABLED if --skip-innodb is used.
have_isam
In MySQL 5.0, this variable appears only for reasons of backward compatibility. It is always NO
because ISAM tables are no longer supported.
have_merge_engine
YES if mysqld supports MERGE tables. DISABLED if --skip-merge is used. This variable was added in
MySQL 5.0.24.
have_ndbcluster
YES if mysqld supports NDB Cluster tables. DISABLED if --skip-ndbcluster is used.
have_openssl
YES if mysqld supports SSL connections, NO if not.
have_query_cache
YES if mysqld supports the query cache, NO if not.
have_raid
In MySQL 5.0, this variable appears only for reasons of backward compatibility. It is always NO
because RAID tables are no longer supported.
have_rtree_keys
YES if RTREE indexes are available, NO if not. (These are used for spatial indexes in MyISAM
tables.)
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.
hostname
The server sets this variable to the server hostname at startup. This variable was added in MySQL
5.0.38.
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.
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.
Note that the --init-file option is unavailable if MySQL was configured with the
--disable-grant-options option. See Section 4.14.2, "Typical configure Options".
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.
innodb_xxx
InnoDB system variables are listed in Section 2.4, "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 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 2.17,
"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, which is available from the
key_cache_block_size system variable:
1 - ((Key_blocks_unused x 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.
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. 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. 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. 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.
large_pages
Whether large page support is enabled. This variable was added in MySQL 5.0.3.
lc_time_names
This variable specifies the locale that controls the language used to display day and month names
and abbreviations. This variable affects the output from the DATE_FORMAT(), DAYNAME() and
MONTHNAME() functions. Locale names are POSIX-style values such as 'ja_JP' or 'pt_BR'. The default
value is 'en_US' regardless of your system's locale setting. For further information, see
Section 8.9, "MySQL Server Locale Support". This variable was added in MySQL 5.0.25.
license
The type of license the server has.
local_infile
Whether LOCAL is supported for LOAD DATA INFILE statements. See Section 4.4, "Security Issues with
LOAD DATA LOCAL".
locked_in_memory
Whether mysqld was locked in memory with --memlock.
log
Whether logging of all statements to the general query log is enabled. See Section 9.2, "The
General Query Log".
log_bin
Whether the binary log is enabled. See Section 9.3, "The Binary Log".
log_bin_trust_function_creators
This variable applies when binary logging is enabled. It controls whether stored function creators
can be trusted not to create stored functions that will cause unsafe events to be written to the
binary log. If set to 0 (the default), users are not allowed to create or alter stored functions
unless they have the SUPER privilege in addition to the CREATE ROUTINE or ALTER ROUTINE privilege.
A setting of 0 also enforces the restriction that a function must be declared with the
DETERMINISTIC characteristic, or with the READS SQL DATA or NO SQL characteristic. If the variable
is set to 1, MySQL does not enforce these restrictions on stored function creation. See Section 4,
"Binary Logging of Stored Routines and Triggers".
This variable was added in MySQL 5.0.16.
log_bin_trust_routine_creators
This is the old name for log_bin_trust_function_creators. Before MySQL 5.0.16, it also applies to
stored procedures, not just stored functions. As of 5.0.16, this variable is deprecated. It is
recognized for backward compatibility but its use results in a warning.
This variable was added in MySQL 5.0.6.
log_error
The location of the error log.
log_queries_not_using_indexes
Whether queries that do not use indexes are logged to the slow query log. See Section 9.4, "The
Slow Query Log". This variable was added in MySQL 5.0.23.
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.
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. See Section 9.4, "The Slow Query Log".
log_warnings
Whether to produce additional warning messages. It is enabled (1) by default and can be disabled
by setting it to 0. 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 9.4, "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 affects only storage engines that
use only table-level locking (MyISAM, MEMORY, MERGE). This variable previously was named
sql_low_priority_updates.
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.
lower_case_table_names
If set to 1, table names are stored in lowercase on disk and table name comparisons are not case
sensitive. If set to 2 table names are stored as given but compared in lowercase. This option also
applies to database names and table aliases. See Section 2.2, "Identifier Case Sensitivity".
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). 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 1GB.
max_binlog_cache_size
If a multiple-statement transaction requires more than this many bytes of memory, the server
generates a Multi-statement transaction required more than 'max_binlog_cache_size' bytes of
storage error. The minimum value is 4096, the maximum and default values are 4GB.
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 default value is 1GB.
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_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. By default, this is 100. See Section 1.2.6,
"Too many connections", for more information.
MySQL Enterprise. For notification that the maximum number of connections is getting dangerously high
and for advice on setting the optimum value for max_connections subscribe to the MySQL Network
Monitoring and Advisory Service. For more information see
http://www.mysql.com/products/enterprise/advisors.html 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.
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, this disables DELAYED entirely.
max_error_count
The maximum number of error, warning, and note messages to be stored for display by the SHOW
ERRORS and SHOW WARNINGS statements.
max_heap_table_size
This variable sets the maximum size to which MEMORY 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.
MySQL Enterprise. Subscribers to the MySQL Network Monitoring and Advisory Service receive
recommendations for the optimum setting for max_heap_table_size. For more information see
http://www.mysql.com/products/enterprise/advisors.html
max_insert_delayed_threads
This variable is a synonym for max_delayed_threads.
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 2.11, "ORDER BY Optimization".
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 million. 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 5.0.21.
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. 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 5.4.13, "SHOW INDEX Syntax"). By setting this to a low value (say, 100), you can force
MySQL to prefer indexes instead of table scans.
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_sp_recursion_depth
The number of times that a stored procedure may call itself. The default value for this option is
0, which completely disallows recursion in stored procedures. The maximum value is 255.
This variable can be set globally and per session.
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."
Before MySQL 5.0.3, this variable has only global scope. Beginning with MySQL 5.0.3, it also has a
read-only session scope. The session variable has the same value as the global variable unless the
current account has a non-zero MAX_USER_CONNECTIONS resource limit. In that case, the session
value reflects the account limit.
max_write_lock_count
After this many write locks, allow some pending read lock requests to be processed in between.
multi_range_count
The maximum number of ranges to send to a table handler at once during range selects. The default
value is 256. Sending multiple ranges to a handler at once can improve the performance of certain
selects dramatically. This especially true for the NDB Cluster table handler, which needs to send
the range requests to all nodes. Sending a batch of those requests at once reduces the
communication costs significantly. This variable was added in MySQL 5.0.3.
myisam_block_size
The block size to be used for MyISAM index pages.
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 6
(4 before MySQL 5.0.6). This variable was added in MySQL 4.1.2. See Section 1.2.11, "The table is
full".
myisam_max_extra_sort_file_size (DEPRECATED)
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. The value
is given in bytes.
Note: This variable was removed in MySQL 5.0.6.
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. The value is
given in bytes.
The default value is 2GB. If MyISAM index files exceed this size and disk space is available,
increasing the value may help performance.
myisam_recover_options
The value of the --myisam-recover option. See the section called "COMMAND OPTIONS".
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.
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.
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 5.0.14. For older versions, the statistics collection method is
equivalent to nulls_equal.
multi_read_range
Specifies the maximum number of ranges to send to a storage engine during range selects. The
default value is 256. Sending multiple ranges to an engine is a feature that can improve the
performance of certain selects dramatically, particularly for NDBCLUSTER. This engine needs to
send the range requests to all nodes, and sending many of those requests at once reduces the
communication costs significantly. This variable was added in MySQL 5.0.3.
named_pipe
(Windows only.) Indicates whether the server supports connections over named pipes.
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 milliseconds 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 on every
query.
The recommended maximum value for this variable is 1000, which means that the query cache is
checked once per second. A larger value means the NDB query cache is less often checked and
invalidated due to updates on a different mysqld. It is generally not desirable to set this to a
value greater than 2000.
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 queried 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_optimized_node_selection
Causes an SQL node to contact the nearest data node in the cluster. Enabled by default. Set to 0
or OFF to disable, in which case the SQL node attempts to contact data nodes in succession.
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 a 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
Each client thread is associated with a connection buffer and result buffer. Both begin with a
size given by net_buffer_length but are dynamically enlarged up to max_allowed_packet bytes as
needed. The result buffer shrinks to net_buffer_length after each SQL statement.
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
connection buffer is automatically enlarged. The maximum value to which net_buffer_length can be
set is 1MB.
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.
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.
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.
new
This variable was used in MySQL 4.0 to turn on some 4.1 behaviors, and is retained for backward
compatibility. In MySQL 5.0, its value is always OFF.
old_passwords
Whether the server should use pre-4.1-style passwords for MySQL user accounts. See Section 1.2.3,
"Client does not support authentication protocol".
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.
optimizer_prune_level
Controls the heuristics applied during query optimization to prune less-promising partial plans
from the optimizer search space. A value of 0 disables heuristics so that the optimizer performs
an exhaustive search. A value of 1 causes the optimizer to prune plans based on the number of rows
retrieved by intermediate plans. This variable was added in MySQL 5.0.1.
optimizer_search_depth
The maximum depth of search performed by the query optimizer. Values larger than the number of
relations in a query result in better query plans, but take longer to generate an execution plan
for a query. Values smaller than the number of relations in a query return an execution plan
quicker, but the resulting plan may be far from being optimal. If set to 0, the system
automatically picks a reasonable value. If set to the maximum number of tables used in a query
plus 2, the optimizer switches to the algorithm used in MySQL 5.0.0 (and previous versions) for
performing searches. This variable was added in MySQL 5.0.1.
pid_file
The pathname of the process ID (PID) file. This variable can be set with the --pid-file option.
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.
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 5.0.21. In MySQL
5.0.32, it was converted to the global Prepared_stmt_count status variable.
protocol_version
The version of the client/server protocol used by the MySQL server.
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.
query_cache_limit
Don't cache results that are larger than this number of bytes. The default value is 1MB.
query_cache_min_res_unit
The minimum size (in bytes) for blocks allocated by the query cache. The default value is 4096
(4KB). Tuning information for this variable is given in Section 5.4.3, "Query Cache
Configuration".
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
query_cache_type is set to 0. See Section 5.4.3, "Query Cache Configuration", for more
information.
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. Possible values are shown in the following table:
+------------+----------------------------------------+
|Option | Description |
+------------+----------------------------------------+
|0 or OFF | Don'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 ON | Cache all query results except for |
| | those that begin with SELECT |
| | SQL_NO_CACHE. |
+------------+----------------------------------------+
|2 or DEMAND | Cache results only for queries that |
| | begin with SELECT |
| | SQL_CACHE. |
+------------+----------------------------------------+
This variable defaults to ON.
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.
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.
range_alloc_block_size
The size of blocks that are allocated when doing range optimization.
read_buffer_size
Each thread that does a sequential scan allocates a buffer of this size (in bytes) for each table
it scans. If you do many sequential scans, you might want to increase this value, which defaults
to 131072.
read_buffer_size and read_rnd_buffer_size are not specific to any storage engine and apply in a
general manner for optimization. See Section 5.7, "How MySQL Uses Memory", for example.
read_only
When this variable is set to ON, the server allows no updates except from users that have the
SUPER privilege or (on a slave server) from updates performed by slave threads. On a slave server,
this can be useful to ensure that the slave accepts updates only from its master server and not
from clients. As of MySQL 5.0.16, this variable does not apply to TEMPORARY tables.
read_only exists only as a GLOBAL variable, so changes to its value require the SUPER privilege.
Changes to read_only on a master server are not replicated to slave servers. The value can be set
on a slave server independent of the setting on the master.
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.
read_buffer_size and read_rnd_buffer_size are not specific to any storage engine and apply in a
general manner for optimization. See Section 5.7, "How MySQL Uses Memory", for example.
relay_log_purge
Disables or enables automatic purging of relay log files as soon as they are not needed any more.
The default value is 1 (ON).
rpl_recovery_rank
This variable is unused.
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 employing the old format
(and hence insecure communication over the network).
Server startup fails with an error if this option is enabled and the privilege tables are in
pre-4.1 format. See Section 1.2.3, "Client does not support authentication protocol".
secure_file_priv
By default, this variable is empty. If set to the name of a directory, it limits the effect of the
LOAD_FILE() function and the LOAD DATA and SELECT ... INTO OUTFILE statements to work only with
files in that directory.
This variable was added in MySQL 5.0.38.
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.
shared_memory
(Windows only.) Whether the server allows shared-memory connections.
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. The default name is MYSQL. The
name is case sensitive.
skip_external_locking
This is OFF if mysqld uses external locking, ON if external locking is disabled.
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.
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 have concerns about users being able to see
databases belonging to other users. Its effect 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 the names of only those databases for which the
user has the SHOW DATABASES or other privilege.
slave_compressed_protocol
Whether to use compression of the slave/master protocol if both the slave and the master support
it.
slave_load_tmpdir
The name of the directory where the slave creates temporary files for replicating LOAD DATA INFILE
statements.
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.
slave_skip_errors
The replication errors that the slave should skip (ignore).
slave_transaction_retries
If a replication slave SQL thread fails to execute a transaction because of an InnoDB deadlock or
exceeded InnoDB's innodb_lock_wait_timeout or NDBCluster's TransactionDeadlockDetectionTimeout or
TransactionInactiveTimeout, it automatically retries slave_transaction_retries times before
stopping with an error. The default priot to MySQL 4.0.3 is 0. You must explicitly set the value
greater than 0 to enable the "retry" behavior, which is probably a good idea. In MySQL 5.0.3 or
newer, the default is 10.
slow_launch_time
If creating a thread takes longer than this many seconds, the server increments the
Slow_launch_threads status variable.
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 1.4.4, "Where MySQL Stores Temporary Files".
sql_mode
The current server SQL mode, which can be set dynamically. See the section called "SQL MODES".
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".
ssl_ca
The path to a file with a list of trusted SSL CAs. This variable was added in MySQL 5.0.23.
ssl_capath
The path to a directory that contains trusted SSL CA certificates in PEM format. This variable was
added in MySQL 5.0.23.
ssl_cert
The name of the SSL certificate file to use for establishing a secure connection. This variable
was added in MySQL 5.0.23.
ssl_cipher
A list of allowable ciphers to use for SSL encryption. The cipher list has the same format as the
openssl ciphers command. This variable was added in MySQL 5.0.23.
ssl_key
The name of the SSL key file to use for establishing a secure connection. This variable was added
in MySQL 5.0.23.
storage_engine
The default storage engine (table type). To set the storage engine at server startup, use the
--default-storage-engine option. See the section called "COMMAND OPTIONS".
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).
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.
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.
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 8.8, "MySQL Server Time Zone Support".
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 "STATUS VARIABLES". If the
value of Opened_tables is large and you don't 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_lock_wait_timeout
Specifies a wait timeout for table-level locks, in seconds. The default timeout is 50 seconds. The
timeout is active only if the connection has open cursors. This variable can also be set globally
at runtime (you need the SUPER privilege to do this). It's available as of MySQL 5.0.10.
table_type
This variable is a synonym for storage_engine. In MySQL 5.0, storage_engine is the preferred name.
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 "STATUS VARIABLES".
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.
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 192KB.
time_format
This variable is not implemented.
time_zone
The current time zone. This variable is used to initialize the time 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 8.8, "MySQL Server Time Zone Support".
timed_mutexes
This variable controls whether InnoDB mutexes are timed. If this variable is set to 0 or OFF (the
default), mutex timing is disabled. If the variable is set to 1 or ON, mutex timing is enabled.
With timing enabled, the os_wait_times value in the output from SHOW ENGINE INNODB MUTEX indicates
the amount of time (in ms) spent in operating system waits. Otherwise, the value is 0. This
variable was added in MySQL 5.0.3.
tmp_table_size
The maximum size of in-memory temporary tables. (The actual limit is determined as the smaller of
max_heap_table_size and tmp_table_size.) If an in-memory temporary table exceeds the limit, MySQL
automatically converts it to an on-disk MyISAM table. Increase the value of tmp_table_size (and
max_heap_table_size if necessary) if you do many advanced GROUP BY queries and you have lots of
memory.
tmpdir
The directory used for temporary files and temporary tables. 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.
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.
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.
tx_isolation
The default transaction isolation level. Defaults to REPEATABLE-READ.
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';
updatable_views_with_limit
This variable controls whether updates to a view can be made when the view does not contain all
columns of the primary key defined in the underlying table, if the update statement contains a
LIMIT clause. (Such updates often are generated by GUI tools.) An update is an UPDATE or DELETE
statement. Primary key here means a PRIMARY KEY, or a UNIQUE index in which no column can contain
NULL.
The variable can have two values:
1 or YES: Issue a warning only (not an error message). This is the default value.
0 or NO: Prohibit the update.
This variable was added in MySQL 5.0.2.
version
The version number for the server.
Starting with MySQL 5.0.24, the version number will also indicate whether the server is a standard
release (Community) or Enterprise release (for example, 5.0.28-enterprise-gpl-nt).
version_bdb
The BDB storage engine version.
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.
For precompiled binaries, this variable will hold the server version and license information.
Starting with MySQL 5.0.24, version_comment will include the full server type and license. For
community users this will appear as MySQL Community Edition - Standard (GPL). For Enterprise
users, the version might be displayed as MySQL Enterprise Server (GPL). The corresponding license
for your MySQL binary is shown in parentheses. For server compiled from source, the default value
will be the same as that for Community releases.
version_compile_machine
The type of machine or architecture on which MySQL was built.
version_compile_os
The type of operating system on which MySQL was built.
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 and Unix socket file connections, not to connections made
via 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.
MySQL Enterprise. Expert use of server system variables is part of the service offered by the MySQL
Network Monitoring and Advisory Service. To subscribe see
http://www.mysql.com/products/enterprise/advisors.html
USING SYSTEM VARIABLES
The mysql server maintains many system variables that indicate how it is configured. the section
called "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. 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.
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 gigabytes,
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
Within an option file, those variables are set like this:
[mysqld]
query_cache_size=16M
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.
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 |
+--------+--------------------------------------------------------------+
| auto_increment_increment | 1 |
| auto_increment_offset | 1 |
| automatic_sp_privileges | ON |
| back_log | 50 |
| basedir | / |
| bdb_cache_size | 8388600 |
| bdb_home | /var/lib/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/share/mysql/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_checksums | ON |
| innodb_commit_concurrency | 0 |
| innodb_concurrency_tickets | 500 |
| innodb_data_file_path | ibdata1:10M:autoextend |
| innodb_data_home_dir | |
| version | 5.0.19 |
| version_comment | MySQL Community Edition - (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
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.
MySQL 5.0 supports one structured variable type, which specifies parameters governing 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
This section describes 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.
Currently, 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, you can 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 this 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
Many server system variables are dynamic and can be set at runtime using SET GLOBAL or SET SESSION.
You can also obtain 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 Name | Value Type | Type |
+--------------------------------+----------------+------------------+
|character_set_results | string | GLOBAL | SESSION |
+--------------------------------+----------------+------------------+
|server_id | numeric | GLOBAL |
+--------------------------------+----------------+------------------+
|slave_compressed_protocol | boolean | GLOBAL |
+--------------------------------+----------------+------------------+
|slave_net_timeout | numeric | GLOBAL |
+--------------------------------+----------------+------------------+
|slave_transaction_retries | numeric | GLOBAL |
+--------------------------------+----------------+------------------+
|slow_launch_time | numeric | GLOBAL |
+--------------------------------+----------------+------------------+
|sort_buffer_size | numeric | GLOBAL | SESSION |
+--------------------------------+----------------+------------------+
|sql_auto_is_null | boolean | SESSION |
+--------------------------------+----------------+------------------+
|sql_big_selects | boolean | SESSION |
+--------------------------------+----------------+------------------+
|sql_big_tables | boolean | SESSION |
+--------------------------------+----------------+------------------+
|sql_buffer_result | boolean | SESSION |
+--------------------------------+----------------+------------------+
|character_set_server | string | GLOBAL | SESSION |
+--------------------------------+----------------+------------------+
|sql_log_bin | boolean | SESSION |
+--------------------------------+----------------+------------------+
|sql_log_off | boolean | SESSION |
+--------------------------------+----------------+------------------+
|sql_log_update | boolean | SESSION |
+--------------------------------+----------------+------------------+
|sql_low_priority_updates | boolean | GLOBAL | SESSION |
+--------------------------------+----------------+------------------+
|sql_max_join_size | numeric | GLOBAL | SESSION |
+--------------------------------+----------------+------------------+
|sql_mode | enumeration | GLOBAL | SESSION |
+--------------------------------+----------------+------------------+
|sql_notes | boolean | SESSION |
+--------------------------------+----------------+------------------+
|sql_quote_show_create | boolean | SESSION |
+--------------------------------+----------------+------------------+
|sql_safe_updates | boolean | SESSION |
+--------------------------------+----------------+------------------+
|sql_select_limit | numeric | SESSION |
+--------------------------------+----------------+------------------+
|collation_connection | string | GLOBAL | SESSION |
+--------------------------------+----------------+------------------+
|sql_slave_skip_counter | numeric | GLOBAL |
+--------------------------------+----------------+------------------+
|updatable_views_with_limit | enumeration | GLOBAL | SESSION |
+--------------------------------+----------------+------------------+
|sql_warnings | boolean | SESSION |
+--------------------------------+----------------+------------------+
|sync_binlog | numeric | GLOBAL |
+--------------------------------+----------------+------------------+
|sync_frm | boolean | GLOBAL |
+--------------------------------+----------------+------------------+
|storage_engine | enumeration | GLOBAL | SESSION |
+--------------------------------+----------------+------------------+
|table_cache | numeric | GLOBAL |
+--------------------------------+----------------+------------------+
|table_type | enumeration | GLOBAL | SESSION |
+--------------------------------+----------------+------------------+
|thread_cache_size | numeric | GLOBAL |
+--------------------------------+----------------+------------------+
|time_zone | string | GLOBAL | SESSION |
+--------------------------------+----------------+------------------+
|collation_server | string | GLOBAL | SESSION |
+--------------------------------+----------------+------------------+
|timestamp | boolean | SESSION |
+--------------------------------+----------------+------------------+
|tmp_table_size | enumeration | GLOBAL | SESSION |
+--------------------------------+----------------+------------------+
|transaction_alloc_block_size | numeric | GLOBAL | SESSION |
+--------------------------------+----------------+------------------+
|transaction_prealloc_size | numeric | GLOBAL | SESSION |
+--------------------------------+----------------+------------------+
|tx_isolation | enumeration | GLOBAL | SESSION |
+--------------------------------+----------------+------------------+
|unique_checks | boolean | SESSION |
+--------------------------------+----------------+------------------+
|wait_timeout | numeric | GLOBAL | SESSION |
+--------------------------------+----------------+------------------+
|warning_count | numeric | SESSION |
+--------------------------------+----------------+------------------+
|completion_type | numeric | GLOBAL | SESSION |
+--------------------------------+----------------+------------------+
|concurrent_insert | numeric | GLOBAL |
+--------------------------------+----------------+------------------+
|connect_timeout | numeric | GLOBAL |
+--------------------------------+----------------+------------------+
|default_week_format | numeric | GLOBAL | SESSION |
+--------------------------------+----------------+------------------+
|delay_key_write | OFF | ON | ALL | GLOBAL |
+--------------------------------+----------------+------------------+
|delayed_insert_limit | numeric | GLOBAL |
+--------------------------------+----------------+------------------+
|autocommit | boolean | SESSION |
+--------------------------------+----------------+------------------+
|delayed_insert_timeout | numeric | GLOBAL |
+--------------------------------+----------------+------------------+
|delayed_queue_size | numeric | GLOBAL |
+--------------------------------+----------------+------------------+
|div_precision_increment | numeric | GLOBAL | SESSION |
+--------------------------------+----------------+------------------+
|engine_condition_pushdown | boolean | GLOBAL | SESSION |
+--------------------------------+----------------+------------------+
|error_count | numeric | SESSION |
+--------------------------------+----------------+------------------+
|expire_logs_days | numeric | GLOBAL |
+--------------------------------+----------------+------------------+
|flush | boolean | GLOBAL |
+--------------------------------+----------------+------------------+
|flush_time | numeric | GLOBAL |
+--------------------------------+----------------+------------------+
|foreign_key_checks | boolean | SESSION |
+--------------------------------+----------------+------------------+
|ft_boolean_syntax | string | GLOBAL |
+--------------------------------+----------------+------------------+
|automatic_sp_privileges | boolean | GLOBAL |
+--------------------------------+----------------+------------------+
|group_concat_max_len | numeric | GLOBAL | SESSION |
+--------------------------------+----------------+------------------+
|identity | numeric | SESSION |
+--------------------------------+----------------+------------------+
|innodb_autoextend_increment | numeric | GLOBAL |
+--------------------------------+----------------+------------------+
|innodb_commit_concurrency | numeric | GLOBAL |
+--------------------------------+----------------+------------------+
|innodb_concurrency_tickets | numeric | GLOBAL |
+--------------------------------+----------------+------------------+
|innodb_max_dirty_pages_pct | numeric | GLOBAL |
+--------------------------------+----------------+------------------+
|innodb_max_purge_lag | numeric | GLOBAL |
+--------------------------------+----------------+------------------+
|innodb_support_xa | boolean | GLOBAL | SESSION |
+--------------------------------+----------------+------------------+
|innodb_sync_spin_loops | numeric | GLOBAL |
+--------------------------------+----------------+------------------+
|innodb_table_locks | boolean | GLOBAL | SESSION |
+--------------------------------+----------------+------------------+
|big_tables | boolean | SESSION |
+--------------------------------+----------------+------------------+
|innodb_thread_concurrency | numeric | GLOBAL |
+--------------------------------+----------------+------------------+
|innodb_thread_sleep_delay | numeric | GLOBAL |
+--------------------------------+----------------+------------------+
|insert_id | numeric | SESSION |
+--------------------------------+----------------+------------------+
|interactive_timeout | numeric | GLOBAL | SESSION |
+--------------------------------+----------------+------------------+
|join_buffer_size | numeric | GLOBAL | SESSION |
+--------------------------------+----------------+------------------+
|key_buffer_size | numeric | GLOBAL |
+--------------------------------+----------------+------------------+
|last_insert_id | numeric | SESSION |
+--------------------------------+----------------+------------------+
|lc_time_names | string | GLOBAL | SESSION |
+--------------------------------+----------------+------------------+
|local_infile | boolean | GLOBAL |
+--------------------------------+----------------+------------------+
|log_queries_not_using_indexes | boolean | GLOBAL |
+--------------------------------+----------------+------------------+
|binlog_cache_size | numeric | GLOBAL |
+--------------------------------+----------------+------------------+
|log_warnings | numeric | GLOBAL |
+--------------------------------+----------------+------------------+
|long_query_time | numeric | GLOBAL | SESSION |
+--------------------------------+----------------+------------------+
|low_priority_updates | boolean | GLOBAL | SESSION |
+--------------------------------+----------------+------------------+
|max_allowed_packet | numeric | GLOBAL | SESSION |
+--------------------------------+----------------+------------------+
|max_binlog_cache_size | numeric | GLOBAL |
+--------------------------------+----------------+------------------+
|max_binlog_size | numeric | GLOBAL |
+--------------------------------+----------------+------------------+
|max_connect_errors | numeric | GLOBAL |
+--------------------------------+----------------+------------------+
|max_connections | numeric | GLOBAL |
+--------------------------------+----------------+------------------+
|max_delayed_threads | numeric | GLOBAL |
+--------------------------------+----------------+------------------+
|max_error_count | numeric | GLOBAL | SESSION |
+--------------------------------+----------------+------------------+
|bulk_insert_buffer_size | numeric | GLOBAL | SESSION |
+--------------------------------+----------------+------------------+
|max_heap_table_size | numeric | GLOBAL | SESSION |
+--------------------------------+----------------+------------------+
|max_insert_delayed_threads | numeric | GLOBAL |
+--------------------------------+----------------+------------------+
|max_join_size | numeric | GLOBAL | SESSION |
+--------------------------------+----------------+------------------+
|max_prepared_stmt_count | numeric | GLOBAL |
+--------------------------------+----------------+------------------+
|max_relay_log_size | numeric | GLOBAL |
+--------------------------------+----------------+------------------+
|max_seeks_for_key | numeric | GLOBAL | SESSION |
+--------------------------------+----------------+------------------+
|max_sort_length | numeric | GLOBAL | SESSION |
+--------------------------------+----------------+------------------+
|max_tmp_tables | numeric | GLOBAL | SESSION |
+--------------------------------+----------------+------------------+
|max_user_connections | numeric | GLOBAL |
+--------------------------------+----------------+------------------+
|max_write_lock_count | numeric | GLOBAL |
+--------------------------------+----------------+------------------+
|character_set_client | string | GLOBAL | SESSION |
+--------------------------------+----------------+------------------+
|multi_range_count | numeric | GLOBAL | SESSION |
+--------------------------------+----------------+------------------+
|myisam_data_pointer_size | numeric | GLOBAL |
+--------------------------------+----------------+------------------+
|log_bin_trust_function_creators | boolean | GLOBAL |
+--------------------------------+----------------+------------------+
|myisam_max_sort_file_size | numeric | GLOBAL | SESSION |
+--------------------------------+----------------+------------------+
|myisam_repair_threads | numeric | GLOBAL | SESSION |
+--------------------------------+----------------+------------------+
|myisam_sort_buffer_size | numeric | GLOBAL | SESSION |
+--------------------------------+----------------+------------------+
|myisam_stats_method | enum | GLOBAL | SESSION |
+--------------------------------+----------------+------------------+
|net_buffer_length | numeric | GLOBAL | SESSION |
+--------------------------------+----------------+------------------+
|net_read_timeout | numeric | GLOBAL | SESSION |
+--------------------------------+----------------+------------------+
|net_retry_count | numeric | GLOBAL | SESSION |
+--------------------------------+----------------+------------------+
|character_set_connection | string | GLOBAL | SESSION |
+--------------------------------+----------------+------------------+
|net_write_timeout | numeric | GLOBAL | SESSION |
+--------------------------------+----------------+------------------+
|old_passwords | numeric | GLOBAL | SESSION |
+--------------------------------+----------------+------------------+
|optimizer_prune_level | numeric | GLOBAL | SESSION |
+--------------------------------+----------------+------------------+
|optimizer_search_depth | numeric | GLOBAL | SESSION |
+--------------------------------+----------------+------------------+
|preload_buffer_size | numeric | GLOBAL | SESSION |
+--------------------------------+----------------+------------------+
|profiling | boolean | SESSION |
+--------------------------------+----------------+------------------+
|profiling_history_size | numeric | SESSION |
+--------------------------------+----------------+------------------+
|query_alloc_block_size | numeric | GLOBAL | SESSION |
+--------------------------------+----------------+------------------+
|query_cache_limit | numeric | GLOBAL |
+--------------------------------+----------------+------------------+
|query_cache_size | numeric | GLOBAL |
+--------------------------------+----------------+------------------+
|character_set_filesystem | string | GLOBAL | SESSION |
+--------------------------------+----------------+------------------+
|query_cache_type | enumeration | GLOBAL | SESSION |
+--------------------------------+----------------+------------------+
|query_cache_wlock_invalidate | boolean | GLOBAL | SESSION |
+--------------------------------+----------------+------------------+
|query_prealloc_size | numeric | GLOBAL | SESSION |
+--------------------------------+----------------+------------------+
|range_alloc_block_size | numeric | GLOBAL | SESSION |
+--------------------------------+----------------+------------------+
|read_buffer_size | numeric | GLOBAL | SESSION |
+--------------------------------+----------------+------------------+
|read_only | numeric | GLOBAL |
+--------------------------------+----------------+------------------+
|read_rnd_buffer_size | numeric | GLOBAL | SESSION |
+--------------------------------+----------------+------------------+
|rpl_recovery_rank | numeric | GLOBAL |
+--------------------------------+----------------+------------------+
|safe_show_database | boolean | GLOBAL |
+--------------------------------+----------------+------------------+
|secure_auth | boolean | GLOBAL |
+--------------------------------+----------------+------------------+
MySQL Enterprise. Improper configuration of system variables can adversely affect performance and
security. The MySQL Network Monitoring and Advisory Service continually monitors system variables and
provides expert advice about appropriate settings. For more information see
http://www.mysql.com/products/enterprise/advisors.html
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 [GLOBAL] STATUS statement. The optional GLOBAL
keyword aggregates the values over all connections.
mysql> SHOW GLOBAL STATUS;
+-----------------------------------+------------+
| Variable_name | Value |
+-----------------------------------+------------+
| Aborted_clients | 0 |
| Aborted_connects | 0 |
| Bytes_received | 155372598 |
| Bytes_sent | 1176560426 |
| Connections | 30023 |
| Created_tmp_disk_tables | 0 |
| Created_tmp_files | 3 |
| Created_tmp_tables | 2 |
| Threads_created | 217 |
| Threads_running | 88 |
| Uptime | 1389872 |
+-----------------------------------+------------+
Note: Before MySQL 5.0.2, SHOW STATUS returned global status values. Because the default as of 5.0.2
is to return session values, this is incompatible with previous versions. To issue a SHOW STATUS
statement that will retrieve global status values for all versions of MySQL, write it like this:
SHOW /*!50002 GLOBAL */ STATUS;
Many status variables are reset to 0 by the FLUSH STATUS statement.
MySQL Enterprise. For expert advice on using status variables, subscribe to the MySQL Network
Monitoring and Advisory Service. For more information see
http://www.mysql.com/products/enterprise/advisors.html
The status variables have the following meanings. Variables with no version indicated were already
present prior to MySQL 5.0. For information regarding their implementation history, see MySQL 3.23,
4.0, 4.1 Reference Manual.
Aborted_clients
The number of connections that were aborted because the client died without closing the connection
properly. See Section 1.2.10, "Communication Errors and Aborted Connections".
Aborted_connects
The number of failed attempts to connect to the MySQL server. See Section 1.2.10, "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.
Binlog_cache_use
The number of transactions that used the temporary binary log cache.
Bytes_received
The number of bytes received from all clients.
Bytes_sent
The number of bytes sent to all clients.
Com_xxx
The Com_xxx statement counter variables 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, respectively. However, if a query result is
returned from query cache, the server increments the Qcache_hits status variable, not Com_select.
See Section 5.4.4, "Query Cache Status and Maintenance".
All of the Com_stmt_xxx variables are increased even if a prepared statement argument 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.
The Com_stmt_xxx status variables were added in 5.0.8:
Com_stmt_prepare
Com_stmt_execute
Com_stmt_fetch
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. Com_stmt_fetch stands for the total
number of network round-trips issued when fetching from cursors.
Compression
Whether the client connection uses compression in the client/server protocol. Added in MySQL
5.0.16.
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.
Created_tmp_files
How many temporary files mysqld has created.
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.
Handler_delete
The number of times that rows have been deleted from tables.
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 via this mechanism.
Handler_prepare
A counter for the prepare phase of two-phase commit operations. Added in MySQL 5.0.3.
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.
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 requests for a storage engine to perform a rollback operation.
Handler_savepoint
The number of requests for a storage engine to place a savepoint. Added in MySQL 5.0.3.
Handler_savepoint_rollback
The number of requests for a storage engine to roll back to a savepoint. Added in MySQL 5.0.3.
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.
Innodb_buffer_pool_pages_data
The number of pages containing data (dirty or clean). Added in MySQL 5.0.2.
Innodb_buffer_pool_pages_dirty
The number of pages currently dirty. Added in MySQL 5.0.2.
Innodb_buffer_pool_pages_flushed
The number of buffer pool page-flush requests. Added in MySQL 5.0.2.
Innodb_buffer_pool_pages_free
The number of free pages. Added in MySQL 5.0.2.
Innodb_buffer_pool_pages_latched
The number of latched pages in InnoDB buffer pool. These are pages currently being read or written
or that cannot be flushed or removed for some other reason. Added in MySQL 5.0.2.
Innodb_buffer_pool_pages_misc
The number of pages that are busy because they have been allocated for administrative overhead
such as row locks or the adaptive hash index. This value can also be calculated as
Innodb_buffer_pool_pages_total - Innodb_buffer_pool_pages_free - Innodb_buffer_pool_pages_data.
Added in MySQL 5.0.2.
Innodb_buffer_pool_pages_total
The total size of buffer pool, in pages. Added in MySQL 5.0.2.
Innodb_buffer_pool_read_ahead_rnd
The number of "random" read-aheads initiated by InnoDB. This happens when a query scans a large
portion of a table but in random order. Added in MySQL 5.0.2.
Innodb_buffer_pool_read_ahead_seq
The number of sequential read-aheads initiated by InnoDB. This happens when InnoDB does a
sequential full table scan. Added in MySQL 5.0.2.
Innodb_buffer_pool_read_requests
The number of logical read requests InnoDB has done. Added in MySQL 5.0.2.
Innodb_buffer_pool_reads
The number of logical reads that InnoDB could not satisfy from the buffer pool and had to do a
single-page read. Added in MySQL 5.0.2.
Innodb_buffer_pool_wait_free
Normally, writes to the InnoDB buffer pool happen in the background. However, if it is necessary
to read or create a page and no clean pages are available, it is also necessary to wait for pages
to be flushed first. This counter counts instances of these waits. If the buffer pool size has
been set properly, this value should be small. Added in MySQL 5.0.2.
Innodb_buffer_pool_write_requests
The number writes done to the InnoDB buffer pool. Added in MySQL 5.0.2.
Innodb_data_fsyncs
The number of fsync() operations so far. Added in MySQL 5.0.2.
Innodb_data_pending_fsyncs
The current number of pending fsync() operations. Added in MySQL 5.0.2.
Innodb_data_pending_reads
The current number of pending reads. Added in MySQL 5.0.2.
Innodb_data_pending_writes
The current number of pending writes. Added in MySQL 5.0.2.
Innodb_data_read
The amount of data read so far, in bytes. Added in MySQL 5.0.2.
Innodb_data_reads
The total number of data reads. Added in MySQL 5.0.2.
Innodb_data_writes
The total number of data writes. Added in MySQL 5.0.2.
Innodb_data_written
The amount of data written so far, in bytes. Added in MySQL 5.0.2.
Innodb_dblwr_writes, Innodb_dblwr_pages_written
The number of doublewrite operations that have been performed and the number of pages that have
been written for this purpose. Added in MySQL 5.0.2. See Section 2.14.1, "InnoDB Disk I/O".
Innodb_log_waits
The number of times that the log buffer was too small and a wait was required for it to be flushed
before continuing. Added in MySQL 5.0.2.
Innodb_log_write_requests
The number of log write requests. Added in MySQL 5.0.2.
Innodb_log_writes
The number of physical writes to the log file. Added in MySQL 5.0.2.
Innodb_os_log_fsyncs
The number of fsync() writes done to the log file. Added in MySQL 5.0.2.
Innodb_os_log_pending_fsyncs
The number of pending log file fsync() operations. Added in MySQL 5.0.2.
Innodb_os_log_pending_writes
The number of pending log file writes. Added in MySQL 5.0.2.
Innodb_os_log_written
The number of bytes written to the log file. Added in MySQL 5.0.2.
Innodb_page_size
The compiled-in InnoDB page size (default 16KB). Many values are counted in pages; the page size
allows them to be easily converted to bytes. Added in MySQL 5.0.2.
Innodb_pages_created
The number of pages created. Added in MySQL 5.0.2.
Innodb_pages_read
The number of pages read. Added in MySQL 5.0.2.
Innodb_pages_written
The number of pages written. Added in MySQL 5.0.2.
Innodb_row_lock_current_waits
The number of row locks currently being waited for. Added in MySQL 5.0.3.
Innodb_row_lock_time
The total time spent in acquiring row locks, in milliseconds. Added in MySQL 5.0.3.
Innodb_row_lock_time_avg
The average time to acquire a row lock, in milliseconds. Added in MySQL 5.0.3.
Innodb_row_lock_time_max
The maximum time to acquire a row lock, in milliseconds. Added in MySQL 5.0.3.
Innodb_row_lock_waits
The number of times a row lock had to be waited for. Added in MySQL 5.0.3.
Innodb_rows_deleted
The number of rows deleted from InnoDB tables. Added in MySQL 5.0.2.
Innodb_rows_inserted
The number of rows inserted into InnoDB tables. Added in MySQL 5.0.2.
Innodb_rows_read
The number of rows read from InnoDB tables. Added in MySQL 5.0.2.
Innodb_rows_updated
The number of rows updated in InnoDB tables. Added in MySQL 5.0.2.
Key_blocks_not_flushed
The number of key blocks in the key cache that have changed but have not yet been flushed to disk.
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 "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.
Last_query_cost
The total cost of the last compiled query as computed by the query optimizer. This is useful for
comparing the cost of different query plans for the same query. The default value of 0 means that
no query has been compiled yet. This variable was added in MySQL 5.0.1, with a default value of
-1. In MySQL 5.0.7, the default was changed to 0; also in version 5.0.7, the scope of
Last_query_cost was changed to session rather than global.
Prior to MySQL 5.0.16, this variable was not updated for queries served from the query cache.
Max_used_connections
The maximum number of connections that have been in use simultaneously since the server started.
Ndb_cluster_node_id
If the server is acting as a MySQL Cluster node, then the value of this variable its node ID in
the cluster.
If the server is not part of a MySQL Cluster, then the value of this variable is 0.
Ndb_config_from_host
If the server is part of a MySQL Cluster, the value of this variable is the hostname or IP address
of the Cluster management server from which it gets its configuration data.
If the server is not part of a MySQL Cluster, then the value of this variable is an empty string.
Prior to MySQL 5.0.23, this variable was named Ndb_connected_host.
Ndb_config_from_port
If the server is part of a MySQL Cluster, the value of this variable is the number of the port
through which it is connected to the Cluster management server from which it gets its
configuration data.
If the server is not part of a MySQL Cluster, then the value of this variable is 0.
Prior to MySQL 5.0.23, this variable was named Ndb_connected_port.
Ndb_number_of_data_nodes
If the server is part of a MySQL Cluster, the value of this variable is the number of data nodes
in the cluster.
If the server is not part of a MySQL Cluster, then the value of this variable is 0.
Prior to MySQL 5.0.29, this variable was named Ndb_number_of_storage_nodes.
Not_flushed_delayed_rows
The number of rows waiting to be written in INSERT DELAY queues.
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.
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 5.0.32.
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.
Select_full_range_join
The number of joins that used a range search on a reference table.
Select_range
The number of joins that used ranges on the first table. This is normally not a critical issue
even if the value is quite large.
Select_range_check
The number of joins without keys that check for key usage after each row. If this is not 0, you
should carefully check the indexes of your tables.
Select_scan
The number of joins that did a full scan of the first table.
Slave_open_temp_tables
The number of temporary tables that the slave SQL thread currently has open.
Slave_running
This is ON if this server is a slave that is connected to a master.
Slave_retried_transactions
The total number of times since startup that the replication slave SQL thread has retried
transactions. This variable was added in version 5.0.4.
Slow_launch_threads
The number of threads that have taken more than slow_launch_time seconds to create.
Slow_queries
The number of queries that have taken more than long_query_time seconds. See Section 9.4, "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.
Sort_range
The number of sorts that were done using ranges.
Sort_rows
The number of sorted rows.
Sort_scan
The number of sorts that were done by scanning the table.
Ssl_xxx
Variables used for SSL connections.
Table_locks_immediate
The number of times that a table lock was acquired immediately.
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.
Tc_log_max_pages_used
For the memory-mapped implementation of the log that is used by mysqld when it acts as the
transaction coordinator for recovery of internal XA transactions, this variable indicates the
largest number of pages used for the log since the server started. If the product of
Tc_log_max_pages_used and Tc_log_page_size is always significantly less than the log size, the
size is larger than necessary and can be reduced. (The size is set by the --log-tc-size option.
Currently, this variable is unused: It is unneeded for binary log-based recovery, and the
memory-mapped recovery log method is not used unless the number of storage engines capable of
two-phase commit is greater than one. (InnoDB is the only applicable engine.) Added in MySQL
5.0.3.
Tc_log_page_size
The page size used for the memory-mapped implementation of the XA recovery log. The default value
is determined using getpagesize(). Currently, this variable is unused for the same reasons as
described for Tc_log_max_pages_used. Added in MySQL 5.0.3.
Tc_log_page_waits
For the memory-mapped implementation of the recovery log, this variable increments each time the
server was not able to commit a transaction and had to wait for a free page in the log. If this
value is large, you might want to increase the log size (with the --log-tc-size option). For
binary log-based recovery, this variable increments each time the binary log cannot be closed
because there are two-phase commits in progress. (The close operation waits until all such
transactions are finished.) Added in MySQL 5.0.3.
Threads_cached
The number of threads in the thread cache.
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/Connections.
Threads_running
The number of threads that are not sleeping.
Uptime
The number of seconds that the server has been up.
SQL MODES
The MySQL server can operate in different SQL modes, and can apply these modes differently for
different clients. This capability enables each application to tailor the server's operating mode to
its own requirements.
For answers to some questions that are often asked about server SQL modes in MySQL, see Section 3,
"MySQL 5.0 FAQ -- Server SQL Mode".
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, or by using
sql-mode="modes" in my.cnf (Unix operating systems) or my.ini (Windows). 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="" on the command line, or sql-mode="" in my.cnf on Unix
systems or in my.ini on Windows) if you want to clear it explicitly.
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 values are probably these:
ANSI
This mode changes syntax and behavior to conform more closely to standard SQL.
STRICT_TRANS_TABLES
If a value could not be inserted as given into a transactional table, abort the statement. For a
non-transactional table, abort the statement if the value occurs in a single-row statement or the
first row of a multiple-row statement. More detail is given later in this section. (Implemented in
MySQL 5.0.2)
TRADITIONAL
Make MySQL behave like a "traditional" SQL database system. A simple description of this mode is
"give an error instead of a warning" when inserting an incorrect value into a column. Note: The
INSERT/UPDATE aborts as soon as the error is noticed. This may not be what you want if you are
using a non-transactional storage engine, because data changes made prior to the error may not be
rolled back, resulting in a "partially done" update. (Added in MySQL 5.0.2)
When this manual refers to "strict mode," it means a mode where at least one of STRICT_TRANS_TABLES
or STRICT_ALL_TABLES is enabled.
The following list describes all supported modes:
ALLOW_INVALID_DATES
Don't do full checking of dates. Check only that the month is in the range from 1 to 12 and the
day is in the range from 1 to 31. This is very convenient for Web applications where you obtain
year, month, and day in three different fields and you want to store exactly what the user
inserted (without date validation). This mode applies to DATE and DATETIME columns. It does not
apply TIMESTAMP columns, which always require a valid date.
This mode is implemented in MySQL 5.0.2. Before 5.0.2, this was the default MySQL date-handling
mode. As of 5.0.2, the server requires that month and day values be legal, and not merely in the
range 1 to 12 and 1 to 31, respectively. With strict mode disabled, invalid dates such as
'2004-04-31' are converted to '0000-00-00' and a warning is generated. With strict mode enabled,
invalid dates generate an error. To allow such dates, enable ALLOW_INVALID_DATES.
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.
ERROR_FOR_DIVISION_BY_ZERO
Produce an error in strict mode (otherwise a warning) when a division by zero (or MOD(X,0)) occurs
during an INSERT or UPDATE. If this mode is not enabled, MySQL instead returns NULL for divisions
by zero. For INSERT IGNORE or UPDATE IGNORE, MySQL generates a warning for divisions by zero, but
the result of the operation is NULL. (Implemented in MySQL 5.0.2)
HIGH_NOT_PRECEDENCE
From MySQL 5.0.2 on, the precedence of the NOT operator is such that expressions such as NOT a
BETWEEN b AND c are parsed as NOT (a BETWEEN b AND c). Before MySQL 5.0.2, the expression is
parsed as (NOT a) BETWEEN b AND c. The old higher-precedence behavior can be obtained by enabling
the HIGH_NOT_PRECEDENCE SQL mode. (Added in MySQL 5.0.2)
mysql> SET sql_mode = '';
mysql> SELECT NOT 1 BETWEEN -5 AND 5;
-> 0
mysql> SET sql_mode = 'HIGH_NOT_PRECEDENCE';
mysql> SELECT NOT 1 BETWEEN -5 AND 5;
-> 1
IGNORE_SPACE
Allow spaces between a function name and the `(' character. This causes built-in function names to
be treated as reserved words. As a result, identifiers that are the same as function names must be
quoted as described in Section 2, "Database, Table, Index, Column, and Alias Names". For example,
because there is a COUNT() function, the use of count as a table name in the following statement
causes an error:
mysql> CREATE TABLE count (i INT);
ERROR 1064 (42000): You have an error in your SQL syntax
The table name should be quoted:
mysql> CREATE TABLE `count` (i INT);
Query OK, 0 rows affected (0.00 sec)
The IGNORE_SPACE SQL mode applies to built-in functions, not to user-defined functions or stored
functions. It is always allowable to have spaces after a UDF or stored function name, regardless of
whether IGNORE_SPACE is enabled.
For further discussion of IGNORE_SPACE, see Section 2.3, "Function Name Parsing and Resolution".
NO_AUTO_CREATE_USER
Prevent the GRANT statement from automatically creating new users if it would otherwise do so,
unless a non-empty password also is specified. (Added in MySQL 5.0.2)
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.
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. mysqldump now
automatically includes in its output a statement that enables NO_AUTO_VALUE_ON_ZERO, to avoid this
problem.
NO_BACKSLASH_ESCAPES
Disable the use of the backslash character (`\') as an escape character within strings. With this
mode enabled, backslash becomes an ordinary character like any other. (Implemented in MySQL 5.0.1)
NO_DIR_IN_CREATE
When creating a table, ignore all INDEX DIRECTORY and DATA DIRECTORY directives. This option is
useful on slave replication servers.
NO_ENGINE_SUBSTITUTION
Control automatic substitution of the default storage engine when a statement such as CREATE TABLE
or ALTER TABLE specifies a storage engine that is disabled or not compiled in. (Implemented in
MySQL 5.0.8)
With NO_ENGINE_SUBSTITUTION disabled, the default engine is used and a warning occurs if the
desired engine is known but disabled or not compiled in. If the desired engine is invalid (not a
known engine name), an error occurs and the table is not created or altered.
With NO_ENGINE_SUBSTITUTION enabled, an error occurs and the table is not created or altered if
the desired engine is unavailable for any reason (whether disabled or invalid).
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.
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.
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.
NO_UNSIGNED_SUBTRACTION
In integer subtraction operations, do not mark the result as UNSIGNED if one of the operands is
unsigned. In other words, the result of a subtraction is always signed whenever this mode is in
effect, even if one of the operands is unsigned. For example, compare the type of column c2 in
table t1 with that of column c2 in table t2:
mysql> SET SQL_MODE='';
mysql> CREATE TABLE test (c1 BIGINT UNSIGNED NOT NULL);
mysql> CREATE TABLE t1 SELECT c1 - 1 AS c2 FROM test;
mysql> DESCRIBE t1;
+-------+---------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------------------+------+-----+---------+-------+
| c2 | bigint(21) unsigned | | | 0 | |
+-------+---------------------+------+-----+---------+-------+
mysql> SET SQL_MODE='NO_UNSIGNED_SUBTRACTION';
mysql> CREATE TABLE t2 SELECT c1 - 1 AS c2 FROM test;
mysql> DESCRIBE t2;
+-------+------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+------------+------+-----+---------+-------+
| c2 | bigint(21) | | | 0 | |
+-------+------------+------+-----+---------+-------+
Note that this means that BIGINT UNSIGNED is not 100% usable in all contexts. See Section 9, "Cast
Functions and Operators".
mysql> SET SQL_MODE = '';
mysql> SELECT CAST(0 AS UNSIGNED) - 1;
+-------------------------+
| CAST(0 AS UNSIGNED) - 1 |
+-------------------------+
| 18446744073709551615 |
+-------------------------+
mysql> SET SQL_MODE = 'NO_UNSIGNED_SUBTRACTION';
mysql> SELECT CAST(0 AS UNSIGNED) - 1;
+-------------------------+
| CAST(0 AS UNSIGNED) - 1 |
+-------------------------+
| -1 |
+-------------------------+
NO_ZERO_DATE
In strict mode, don't allow '0000-00-00' as a valid date. You can still insert zero dates with the
IGNORE option. When not in strict mode, the date is accepted but a warning is generated. (Added in
MySQL 5.0.2)
NO_ZERO_IN_DATE
In strict mode, don't accept dates where the month or day part is 0. If used with the IGNORE
option, MySQL inserts a '0000-00-00' date for any such date. When not in strict mode, the date is
accepted but a warning is generated. (Added in MySQL 5.0.2)
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. 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;
As of MySQL 5.0.23, this mode also restricts references to non-aggregated columns in the HAVING
clause that are not named in the GROUP BY clause.
PIPES_AS_CONCAT
Treat || as a string concatenation operator (same as CONCAT()) rather than as a synonym for OR.
REAL_AS_FLOAT
Treat REAL as a synonym for FLOAT. By default, MySQL treats REAL as a synonym for DOUBLE.
STRICT_ALL_TABLES
Enable strict mode for all storage engines. Invalid data values are rejected. Additional detail
follows. (Added in MySQL 5.0.2)
STRICT_TRANS_TABLES
Enable strict mode for transactional storage engines, and when possible for non-transactional
storage engines. Additional details follow. (Implemented in MySQL 5.0.2)
Strict mode controls how MySQL handles input values that are invalid or missing. A value can be
invalid for several reasons. For example, it might have the wrong data type for the column, or it
might be out of range. A value is missing when a new row to be inserted does not contain a value for
a non-NULL column that has no explicit DEFAULT clause in its definition. (For a NULL column, NULL is
inserted if the value is missing.)
For transactional tables, an error occurs for invalid or missing values in a statement when either of
the STRICT_ALL_TABLES or STRICT_TRANS_TABLES modes are enabled. The statement is aborted and rolled
back.
For non-transactional tables, the behavior is the same for either mode, if the bad value occurs in
the first row to be inserted or updated. The statement is aborted and the table remains unchanged. If
the statement inserts or modifies multiple rows and the bad value occurs in the second or later row,
the result depends on which strict option is enabled:
For STRICT_ALL_TABLES, MySQL returns an error and ignores the rest of the rows. However, in this
case, the earlier rows still have been inserted or updated. This means that you might get a
partial update, which might not be what you want. To avoid this, it's best to use single-row
statements because these can be aborted without changing the table.
For STRICT_TRANS_TABLES, MySQL converts an invalid value to the closest valid value for the column
and insert the adjusted value. If a value is missing, MySQL inserts the implicit default value for
the column data type. In either case, MySQL generates a warning rather than an error and continues
processing the statement. Implicit defaults are described in Section 1.4, "Data Type Default
Values".
Strict mode disallows invalid date values such as '2004-04-31'. It does not disallow dates with zero
parts such as '2004-04-00' or "zero" dates. To disallow these as well, enable the NO_ZERO_IN_DATE and
NO_ZERO_DATE SQL modes in addition to strict mode.
If you are not using strict mode (that is, neither STRICT_TRANS_TABLES nor STRICT_ALL_TABLES is
enabled), MySQL inserts adjusted values for invalid or missing values and produces warnings. In
strict mode, you can produce this behavior by using INSERT IGNORE or UPDATE IGNORE. See
Section 5.4.28, "SHOW WARNINGS Syntax".
The following special modes are provided as shorthand for combinations of mode values from the
preceding list. All are available in MySQL 5.0 beginning with version 5.0.0, except for TRADITIONAL,
which was implemented in MySQL 5.0.2.
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 5.0.3, ANSI
also includes ONLY_FULL_GROUP_BY.
As of MySQL 5.0.40, ANSI mode also causes the server to return an error for queries where a set
function S with an outer reference S(outer_ref) cannot be aggregated in the outer query against
which the outer reference has been resolved. This is such a query:
SELECT * FROM t1 WHERE t1.a IN (SELECT MAX(t1.b) FROM t2 WHERE ...);
Here, MAX(t1.b) cannot aggregated in the outer query because it appears in the WHERE clause of that
query. Standard SQL requires an error in this situation. If ANSI mode is not enabled, the server
treats S(outer_ref) in such queries the same way that it would interpret S(const), as was always done
prior to 5.0.40.
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, NO_AUTO_CREATE_USER.
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, HIGH_NOT_PRECEDENCE.
MYSQL40
Equivalent to NO_FIELD_OPTIONS, HIGH_NOT_PRECEDENCE.
ORACLE
Equivalent to PIPES_AS_CONCAT, ANSI_QUOTES, IGNORE_SPACE, NO_KEY_OPTIONS, NO_TABLE_OPTIONS,
NO_FIELD_OPTIONS, NO_AUTO_CREATE_USER.
POSTGRESQL
Equivalent to PIPES_AS_CONCAT, ANSI_QUOTES, IGNORE_SPACE, NO_KEY_OPTIONS, NO_TABLE_OPTIONS,
NO_FIELD_OPTIONS.
TRADITIONAL
Equivalent to STRICT_TRANS_TABLES, STRICT_ALL_TABLES, NO_ZERO_IN_DATE, NO_ZERO_DATE,
ERROR_FOR_DIVISION_BY_ZERO, NO_AUTO_CREATE_USER.
THE 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.
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 (starting
from 5.0.5: unless innodb_fast_shutdown is 2), writes the current LSN to the tablespace, and
terminates its own internal threads.
6. The server exits.
SERVER-SIDE HELP
MySQL Server supports a HELP statement that returns online information from the MySQL Reference
manual (see Section 3.2, "HELP Syntax"). The proper operation of this statement requires that the
help tables in the mysql database be initialized with help topic information, which is done by
processing the contents of the fill_help_tables.sql script.
For a MySQL binary distribution on Unix, help table setup occurs when you run mysql_install_db. For
an RPM distribution on Linux or binary distribution on Windows, help table setup occurs as part of
the MySQL installation process.
For a MySQL source distribution, you can find the fill_help_tables.sql file in the scripts directory.
To load the file manually, make sure that you have initialized the mysql database by running
mysql_install_db, and then process the file with the mysql client as follows:
shell> mysql -u root mysql < fill_help_tables.sql
If you are working with BitKeeper and a MySQL development source tree, the tree doesn't contain
fill_help_tables.sql. You can download the proper file for your version of MySQL from
http://dev.mysql.com/doc/. After downloading and uncompressing the file, process it with mysql as
just described.
COPYRIGHT
Copyright 1997-2007 MySQL AB
This documentation is NOT distributed under a GPL license. Use of this documentation is subject to
the following terms: You may create a printed copy of this documentation solely for your own personal
use. Conversion to other formats is allowed as long as the actual content is not altered or edited in
any way. You shall not publish or distribute this documentation in any form or on any media, except
if you distribute the documentation in a manner similar to how MySQL disseminates it (that is,
electronically for download on a Web site with the software) or on a CD-ROM or similar medium,
provided however that the documentation is disseminated together with the software on the same
medium. Any other use, such as any dissemination of printed copies or use of this documentation, in
whole or in part, in another publication, requires the prior written consent from an authorized
representative of MySQL AB. MySQL AB reserves any and all rights to this documentation not expressly
granted above.
Please email <docs@mysql.com> for more information.
REFERENCES
1. MySQL Internals: Porting
http://forge.mysql.com/wiki/MySQL_Internals_Porting
SEE ALSO
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.
MySQL 5.0 07/04/2007 MYSQLD(8)
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