MySQL - enable slow query log

What is slow query log?

  • Slow queries can affect database and server performance.

  • The slow query log consists of SQL statements that took more than long_query_time seconds to execute.

  • This greatly simplifies the task of finding inefficient or time-consuming queries.

  • By default, the slow query log is disabled

How to enable?

  1. Login to MySQL, database abc_prod

    shell> mysql -u root -p abc_prod
  2. Enable slow query log

    mysql> SET GLOBAL slow_query_log = 'ON';
  3. Check path to log file

    mysql> SHOW VARIABLES LIKE 'slow_query_log_file';
    +---------------------+-----------------------------+
    | Variable_name | Value |
    +---------------------+-----------------------------+
    | slow_query_log_file | /var/lib/mysql/db3-slow.log |
    +---------------------+-----------------------------+
    1 row in set (0.00 sec)
  4. Change long query time to 5 seconds - default is 10 seconds

    mysql> SET GLOBAL long_query_time = 5;
  5. Logout MySQL session then login again

  6. Take a small test to ensure slow query log is enable

    mysql> SELECT SLEEP(10);
  7. Check sleep on slow query log

    [email protected]:/home/ubuntu:~$ cat /var/lib/mysql/db3-slow.log
    /usr/sbin/mysqld, Version: 5.6.35-log (MySQL Community Server (GPL)). started with:
    Tcp port: 3306 Unix socket: /var/run/mysqld/mysqld.sock
    Time Id Command Argument
    # Time: 170321 7:15:52
    # [email protected]: root[root] @ localhost [] Id: 1160
    # Query_time: 7.000249 Lock_time: 0.000000 Rows_sent: 1 Rows_examined: 0
    SET timestamp=1490080552;
    SELECT SLEEP(7);
    # Time: 170321 7:17:31
    # [email protected]: root[root] @ localhost [] Id: 1161
    # Query_time: 10.000215 Lock_time: 0.000000 Rows_sent: 1 Rows_examined: 0
    use abc_prod;
    SET timestamp=1490080651;
    SELECT SLEEP(10);

Most important:

Using mysqldumpslow tool for summarize slow query log files

[email protected]:/home/ubuntu:~$ mysqldumpslow /var/lib/mysql/db3-slow.log
Reading mysql slow query log from /var/lib/mysql/db3-slow.log
Count: 2 Time=8.50s (17s) Lock=0.00s (0s) Rows=1.0 (2), root[root]@localhost
SELECT SLEEP(N)

How to turn it off?

mysql> SET GLOBAL slow_query_log = 'OFF';