Enable MySQL’s slow query log without a restart

Log on to mysql as root
Set the log-file location for the Slow Query log. Make sure mysql has write permission.

mysql> SET GLOBAL slow_query_log_file = '/path/to/slow_query.log';

Determine what makes a query "slow", by setting the limit (in seconds) after which a query is logged to the slow query log. The example below logs every query that exceeds 10 seconds in duration.

mysql> SET GLOBAL long_query_time = 10;