Question

I’m using Hibernate’s JPA implementation with MySQL 5.0.67. MySQL is configured to use InnoDB.

In performing a JPA query (which is translated to SQL), I’ve discovered that using the IN clause is slower than performing individual queries. Example:

SELECT p FROM Person p WHERE p.name IN ('Joe', 'Jane', 'Bob', 'Alice') 

is slower than four separate queries:

SELECT p FROM Person p WHERE p.name = 'Joe' SELECT p FROM Person p WHERE p.name = 'Jane' SELECT p FROM Person p WHERE p.name = 'Bob' SELECT p FROM Person p WHERE p.name = 'Alice' 

Why is this? Is this a MySQL performance limitation?

Answer 1

This is a known deficiency in MySQL.

It is often true that using UNION performs better than a range query like the one you show. MySQL doesn’t employ indexes very intelligently for expressions using IN (...). A similar hole exists in the optimizer for boolean expressions with OR.

See http://www.mysqlperformanceblog.com/2006/08/10/using-union-to-implement-loose-index-scan-to-mysql/ for some explanation and detailed benchmarks.

The optimizer is being improved all the time. A deficiency in one version of MySQL may be improved in a subsequent version. So it’s worth testing your queries on different versions.

It is also advantageous to use UNION ALL instead of simply UNION. Both queries use a temporary table to store results, but the difference is that UNION applies DISTINCT to the result set, which incurs an additional un-indexed sort.