Slow queries

Recently, we were reviewing the performance of a large site that has a significant portion of its traffic from logged in users. The site was suffering from a high load average during peak times.

We enabled slow query logging on the site for a entire week, using the following in my.cnf:

For sites that have lots of slow queries, disk access is often the bottleneck. For these slow queries, MySQL writes temporary tables to disk, populates them with intermediate results, then query them again for the final result.

We all know that the disk is the slowest part in a computer, because it is limited by being mechanical, rather than electronic. One way of mitigating this is to tell MySQL to use memory rather than disk for temporary tables.

A client site was facing intermittent outages, specially at peak hours.

We investigated the issue over a few days, and narrowed down the cause to certain slow queries, described in more details below.

They had tagadelic on every page, displaying a tag cloud, and from that, crawlers hit every term to paths like taxonomy/term/1581.

Slow queries causing server outage