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 the log destination value does not contain FILE.
The server writes less information to the slow query log (and binary log) if you use the --log-short-formatoption.
To include slow administrative statements in the statements written to 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, use the --log-queries-not-using-indexes server option. See Section 5.1.3, “Server Command Options”. When such queries 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:
The query must either not be an administrative statement, or --log-slow-admin-statements must have been specified.
The query must have taken at least long_query_time seconds, or --log-queries-not-using-indexes must have been specified and the query used no indexes for row lookups.
The query must have examined at least min_examined_row_limit rows.
The server does not write queries handled by the query cache to the slow query log, nor queries that would not benefit from the presence of an index because the table has zero rows or one row.
Prior to MySQL 5.1.45, replication slaves did not write replicated queries to the slow query log, even if the same queries were written to the slow query log on the master. (Bug #23300) In MySQL 5.1.45 and later, this behavior can be overridden using the --log-slow-slave-statements option.
The slow query log should be protected because logged statements might contain passwords. See Section 6.1.2.3, “Passwords and Logging”.
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 a slow query log file using the mysqldumpslow command to summarize the queries that appear in the log. See Section 4.6.8, “mysqldumpslow — Summarize Slow Query Log Files”.