I recently worked on an uncommon slow query: less than 100 rows were read and returned, the whole dataset was fitting in memory but the query took several seconds to run. Long story short: the query was a join involving 21 tables, running on MySQL 5.1. But by default MySQL 5.1 is not good at handling joins with such a large number of tables. The good news is that MySQL 5.6 brings welcome improvements.
Isolating the problem
As always with a slow query, finding the execution plan with
EXPLAIN is the 1st step to understand where time is
spent. Here the plan was very good with almost all joins using
the primary key or a unique …