If you have a slow web app or website it might be because of unoptimized sql queries. You want to track and fix those and the mysql slow query log is very helpful here.
Enabling the slow query log
MySQL prior to 5.1.0 requires adding a setting to the MySQL my.cnf file and restart the mysql daemon in order to log slow queries; from MySQL 5.1.0 onwards you can also change this dynamically without having to restart.
To make the setting permanent every time you start MySQL (or you have a version prior to 5.1.0) add (ow uncomment) the following in my.cnf (usually
log_slow_queries = /var/log/mysql/mysql-slow.log
You can skip the path or leave it blank completely and mysql will create the log in the default location. The default is to log the queries into a file in the MySQL data directory.
To enable or disable the setting dynamically in MySQL 5.1.0 run the following query to enable it:
set log_slow_queries = ON;
and to disable it:
set log_slow_queries = OFF;
Changing the long query time
You can also set how long a query needs to take before it’s considered a “long” query. The default is 10 seconds. I usually like to set it lower:
To change it to 5 seconds add in my.cnf:
long_query_time = 5
This can be changed dynamically in MySQL 5.0.0+ (and possibly earlier versions) by running the following query:
set global long_query_time = 5;
This will only work for new connections; any connections which have already been established will continue to use the old setting. Once the user disconnects and reconnects their new connection will use the new setting.
Errors you can get applying these settings dynamically
If the following error message appears when attempting to change the log_slow_queries setting dynamically means you are using a version of MySQL that does not support changing the setting dynamically:
ERROR 1193 (HY000): Unknown system variable 'log_slow_queries'
The long_query_time value must be integer; if it’s not (e.g. you set long_query_time = 2.5;) then you’ll see this error:
#1232 - Incorrect argument type to variable 'long_query_time'
Note also that if you set the long_query_time to 0 it will not fail, but the actual setting applied will be 1 and not 0.