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5.4.5 The Slow Query Log

The slow query log consists of SQL statements that take more than long_query_time seconds to execute and require at least min_examined_row_limit rows to be examined. The slow query log can be used to find queries that take a long time to execute and are therefore candidates for optimization. However, examining a long slow query log can be a time-consuming task. To make this easier, you can use the mysqldumpslow command to process a slow query log file and summarize its contents. See Section 4.6.8, “mysqldumpslow — Summarize Slow Query Log Files”.

The time to acquire the initial locks is not counted as execution time. mysqld writes a statement to the slow query log after it has been executed and after all locks have been released, so log order might differ from execution order.

The slow query log should be protected because logged statements might contain passwords. See Section 6.1.2.3, “Passwords and Logging”.

Slow Query Log Parameters

The minimum and default values of long_query_time are 0 and 10, respectively. The value can be specified to a resolution of microseconds. For logging to a file, times are written including the microseconds part. For logging to tables, only integer times are written; the microseconds part is ignored.

By default, administrative statements are not logged, nor are queries that do not use indexes for lookups. This behavior can be changed using --log-slow-admin-statements and log_queries_not_using_indexes, as described later.

By default, the slow query log is disabled. To specify the initial slow query log state explicitly, use --slow_query_log[={0|1}]. With no argument or an argument of 1, --slow_query_log enables the log. With an argument of 0, this option disables the log. To specify a log file name, use --slow_query_log_file=file_name. To specify the log destination, use the log_output system variable (as described in Section 5.4.1, “Selecting General Query Log and Slow Query Log Output Destinations”). The older option to enable the slow query log file, --log-slow-queries, is deprecated.

If you specify no name for the slow query log file, the default name is host_name-slow.log. The server creates the file in the data directory unless an absolute path name is given to specify a different directory.

To disable or enable the slow query log or change the log file name at runtime, use the global slow_query_log and slow_query_log_file system variables. Set slow_query_log to 0 to disable the log or to 1 to enable it. Set slow_query_log_file to specify the name of the log file. If a log file already is open, it is closed and the new file is opened.

The server writes less information to the slow query log if you use the --log-short-format option.

To include slow administrative statements in the slow query log, use the --log-slow-admin-statements server option. Administrative statements include ALTER TABLE, ANALYZE TABLE, CHECK TABLE, CREATE INDEX, DROP INDEX, OPTIMIZE TABLE, and REPAIR TABLE.

To include queries that do not use indexes for row lookups in the statements written to the slow query log, enable the log_queries_not_using_indexes system variable. (Even with that variable enabled, the server does not log queries that would not benefit from the presence of an index due to the table having fewer than two rows.) When queries that do not use an index are logged, the slow query log may grow quickly.

The server uses the controlling parameters in the following order to determine whether to write a query to the slow query log:

  1. The query must either not be an administrative statement, or --log-slow-admin-statements must have been specified.

  2. The query must have taken at least long_query_time seconds, or log_queries_not_using_indexes must be enabled and the query used no indexes for row lookups.

  3. The query must have examined at least min_examined_row_limit rows.

The server does not log queries handled by the query cache.

By default, a replication slave does not write replicated queries to the slow query log. To change this, use the --log-slow-slave-statements server option.

Slow Query Log Contents

When the slow query log is enabled, the server writes output to any destinations specified by the log_output system variable. If you enable the log, the server opens the log file and writes startup messages to it. However, further logging of queries to the file does not occur unless the FILE log destination is selected. If the destination is NONE, the server writes no queries even if the slow query log is enabled. Setting the log file name has no effect on logging if FILE is not selected as an output destination.

If the slow query log is enabled and FILE is selected as an output destination, each statement written to the log is preceded by a line that begins with a # character and has these fields (with all fields on a single line):

  • Query_time: duration

    The statement execution time in seconds.

  • Lock_time: duration

    The time to acquire locks in seconds.

  • Rows_sent: N

    The number of rows sent to the client.

  • Rows_examined:

    The number of rows examined by the optimizer.

Each statement written to the slow query log file is preceded by a SET statement that includes a timestamp indicating when the slow statement was logged (which occurs after the statement finishes executing).


User Comments
User comments in this section are, as the name implies, provided by MySQL users. The MySQL documentation team is not responsible for, nor do they endorse, any of the information provided here.
  Posted by lvaro G. Vicario on December 20, 2011
Syntax for /etc/my.cnf in Red Hat 9:

[mysqld]
long_query_time=1
log-slow-queries=/var/log/mysql/log-slow-queries.log

You must create the file manually and change owners this way:

mkdir /var/log/mysql
touch /var/log/mysql/log-slow-queries.log
chown mysql.mysql -R /var/log/mysql

  Posted by Robert Brisita on February 20, 2015
Updated example for 2015 MySQL 5.6:

slow_query_log = 1
slow_query_log_file = /var/log/mysql/slow.log
long_query_time = 10
log_queries_not_using_indexes = 1

Change permissions so that mysqld can write to the specified log file. Giving write permissions to the 'other' group should suffice.
  Posted by Jörg Brühe on March 23, 2017
Beware:
Even if "log_slow_slave_statements" is set, the slow query log on the slave will not contain events that were replicated in row format.

This is reported as bug, verified since Oct 2014.

See here: https://bugs.mysql.com/bug.php?id=74233
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