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The slow query log consists of all SQL statements that took more
than long_query_time seconds to
execute and (as of MySQL 5.1.21) required at least
min_examined_row_limit rows to be
examined. The time to acquire the initial table 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 be
different from execution order. The minimum and default values of
long_query_time are 1 and 10,
respectively. Prior to MySQL 5.1.21, the minimum value is 1, and
the value for this variable must be an integer. Beginning with
MySQL 5.1.21, the minimum is 0, and a resolution of microseconds
is supported when logging to a file. However, the microseconds
part is ignored and only integer values are written when logging
to tables.
Before 5.1.6, the slow query log destination is always a file. To
enable the slow query log file, use the
--log-slow-queries[=
option. To enable the slow query log as of MySQL 5.1.6, start
mysqld with the
file_name]--log-slow-queries[=
option, and optionally use file_name]--log-output to
specify the log destination (as described in
Section 5.2.1, “Selecting General Query and Slow Query Log Output Destinations”). As of MySQL 5.1.29,
--log-slow-queries is deprecated: Use
--slow_query_log to enable the slow query log,
and optionally
--slow_query_log_file=
to specify a log filename. file_name--slow_query_log takes
an optional argument of 1 or 0 to enable or disable the log.
If you specify no filename for the slow query log, the default
name is
host_name-slow.log
When --log-slow-queries is specified,
--slow_query_log also may be given as of MySQL
5.1.12 to specify the initial slow query log state. With no
argument or an argument of 0, the option disables the log. If
omitted or given with an argument of 1, the option enables the
log.
For runtime control of the general query log, use the global
slow_query_log and
slow_query_log_file system
variables. Set slow_query_log to
0 (or OFF) to disable the log or to 1 (or
ON) to enable it. Set
general_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.
When the slow query log is enabled, output is written to any
destinations specified by the --log-output option
or log_output system variable. If
you enable the log, the server opens the log file and writes
startup messages to it. However, logging of queries to the file
does not occur unless the FILE log destination
is selected. If the destination is NONE, no
queries are written even if the slow query log is enabled. Setting
the log filename has no effect on logging if the log destination
value does not contain FILE.
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 become a difficult task. To make this easier, you can process the slow query log using the mysqldumpslow command to summarize the queries that appear in the log. Use mysqldumpslow --help to see the options that this command supports.
In MySQL 5.1, queries that do not use indexes are
logged in the slow query log if the
--log-queries-not-using-indexes option is
specified. See Section 5.1.2, “Server Command Options”.
MySQL Enterprise. Excessive table scans are indicative of missing or poorly optimized indexes. Using an advisor specifically designed for the task, the MySQL Enterprise Monitor can identify such problems and offer advice on resolution. For more information, see http://www.mysql.com/products/enterprise/advisors.html.
In MySQL 5.1, the
--log-slow-admin-statements server option enables
you to request logging of slow administrative statements such as
OPTIMIZE TABLE,
ANALYZE TABLE, and
ALTER TABLE to the slow query log.
Queries handled by the query cache are not added to the slow query log, nor are queries that would not benefit from the presence of an index because the table has zero rows or one row.
Replication slaves do not write replicated queries to the slow query log, even if the same queries were written to the slow query log on the master. This is a known issue which we intend to fix in a future version of MySQL. (Bug#23300)
User Comments
Please note that the mysql slow query log will not show the SQL of your slow queries if your application uses prepared statement.
You may want to turn on General Query Log instead.
When I turned on the slow_query.log, and told it to also log queries that do not use indexes my log started to fill up with SHOW COLLATION; and SHOW VARIABLES; statement.
It can cause your log file to grow VERY quickly.
Here are instructions on getting mysqldumpslow to work in your environment if you have my.cnf in a non-standard (eg, non /etc) locale. Please note this is for Unix only.
1. Make sure it can find perl in /usr/local/bin/perl
2. Make sure you've got the slow log running first
3. Copy the slow log to datadir and name it specifically: servername-slow.log (eg, servername-slow.log). This assumes your actual slowlog is located somewhere else. If it is located by default in datadir then you still must make a copy of it, named hostname-slow.log in datadir directory.
4. Make a copy of mysqldumpslow in the $MYSQL_HOME/bin directory, and name it: mysqldumpslow_new and be sure it is chmod 750
5. Edit the mysqldumpslow_new as described in the next item.
6. Assuming you don't place your server's my.cnf in /etc, you must tell my_print_defaults where datadir and basedir are because they can't be set directly in mysqldumpslow nor can they be passed as a parameter on the command line to mysqldumpslow.
This is done by altering the command line option to my_print_defaults, the results of which are fed into the mysqldumpslow perl script to establish datadir, basedir, and a few other variables. The following line in
mysqldumpslow_new should be changed.
Change from:
my $defaults = `my_print_defaults mysqld`;
to:
my $defaults = `my_print_defaults -c /location/where/config_file_lives/my.cnf mysqld`;
Make sure the my.cnf pointed to in the -c option has datadir and basedir set under [mysqld] section.
7. To run the program now, simply type:
./mysqldumpslow_new -s c -t 3
8. You have now displayed output from the slow query log telling you the top 3 slow queries on the system.
Andrea Gangini posted that slow queries made via prepared statements do not post to the slow query log. This is not exactly true.
In the case of MySQL 5.0.22 when accessed through JDBC (Java 1.5.0), slow queries constructed via PreparedStatement always appear in the slow query log. Slow queries constructed normally (via Statement), sometimes appear. I suspect that PreparedStatements bypass the query cache and so are always reported in the log whereas regular Statements are checked against the query cache. If the results are taken from the query cache and not by accessing the tables, the query does not appear in slow query log. Testing regular Statements with the SQL_CACHE and SQL_NO_CACHE hints seems to confirm this. (Remember MySQL does not expend much effort trying to recognise queries it's already seen - if the query strings aren't exactly the same, don't expect to get help from the query cache.)
Chris
If you see many trivial queries being logged here, you may have log-long-format enabled.
This logs all queries not using indexes in the slow query logs, which includes queries so trivial that using the index would be slower.
Replication queries which are slow aren't saved in the slow logs. If a Slave is behaving badly and the only thing that shows up in the slow logs are queries which are not normally a problem, then replication queries may be causing problems. Check the timetamp in the replication logs and compare them to the slow logs.
Syntax for /etc/my.cnf in Red Hat 9:
[mysqld]
set-variable=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
If you are loading a large database dump created using mysqldump in the normal way, be aware that if an insert statement takes longer than long_query_time, the ENTIRE huge insert statement (with all its individual rows) will be logged to the slow query file. This can significantly extend the time it takes to load the database.
An example:
[mysqld]
log-slow-queries = slow.log
long_query_time = 20
log-queries-not-using-indexes
- The first line under [mysqld] turns on slow query log and logs all slow queries to slow.log in the MySQL data directory.
- The second line indicates that any queries that took more than 20 seconds to execute need to be logged.
- The last line tells MySQL to log *any* queries that do not use indexes regardless of the setting in the second line.
If you like you can set the long_query_time very high so that only the queries that do not use indexes are logged (and no 'general' slow queries).
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