Identifying slow MySQL queries

yolau — Fri, 09 Nov 2007 12:14:53 +0000

MySQL can sometimes create big problems on a server when you have users abusing it.
This article will teach you how to correctly identify the queries that are creating a problem for your server.

MySQL can log those queries that are taking longer then X seconds but this future is not turned on by default.
Here’s how you turn it on:

Login to your server as root
Open my.cnf with your favorite editor. Example:

pico /etc/my.cnf

Into the [mysqld] section add the fallowing lines

log-slow-queries = /var/log/mysql-slow.log
long_query_time = 3

This is just an example. You can use any file name that you want and you can modify the long_query_time to any value. In this example I will be logging to /var/log/mysql-slow.log any queries that are taking longer then 3 seconds.

Go ahead and save the configuration.

For pico: CTRL+X and YES

Now we have to actually create the log file.

touch /var/log/mysql-slow.log

Now we are changing the owner of the file so that mysql and actually write to it.

chown mysql.root /var/log/mysql-slow.log

Now we restart mysql

service mysql restart

It should restart successfully. If it doesn’t check that you didn’t brake my.cnf

Wait a few minutes and then examine the slow queries log

A few examples on how to do it:

cat /var/log/mysql-slow.log
tail /var/log/mysql-slow.log
tail -50 /var/log/mysql-slow.log

After you have identified the offending query go ahead and optimize or remove it.
Again test the results by looking at your server load and the mysql slow queries log.

After you fixed all the problems go ahead and comment the slow queries logging as it will slow your server a bit if you let it on. my.cnf should now look similar to this:

#log-slow-queries = /var/log/mysql-slow.log
#long_query_time = 3

And don’t forget to restart MySQL after this.

service mysql restart

Hope this helps !

I suggest you also read:

If you have any problems or suggestions don’t hesitate to leave a comment.

cPanelConfig - cPanel server configuration guide » log-slow-queries

Identifying slow MySQL queries