Enable slow query log in MySQL

Answer:

Turn on the Slow Query log in MySQL allow you to debug the sources of slow and problematic SQL.

To activate it, edit the MySQL configurations, e.g. /etc/my.cnf

[mysqld]
...
...
log-slow-queries        = /var/log/mysqld-slow.log                                                                                
long_query_time         = 5     

Restart MySQL and you will notice the file mysqld-slow.log will be created.

Any queries take more than 5 seconds to execute will be logged in this file.