The slow query log consists of SQL statements that took more than long_query_time seconds to execute and required at leastmin_examined_row_limit rows to be examined.

It is very useful in troubleshooting bad queries.

To edit a file, first make a back-up.

cp /etc/my.cnf /etc/my.cnf.old

Then Now open it in vi:

vi /etc/my.cnf

Add the following line to enable the slow query log:

log-slow-queries=/var/lib/mysql/slow.log

After that, do the following commands to create the file with the right ownership and file permissions:

vi /var/lib/mysql/slow.log
chmod 660 /var/lib/mysql/slow.log
chown mysql:mysql /var/lib/mysql/slow.log

Once you are done, restart mysql.