Ryan Bates wrote:
I'm trying to determine why a subquery is slower than running two
separate queries. I have a simple many-to-many association using 3
tables: projects, tags and projects_tags. Here's the query I'm using to
find the projects with a given tag:
SELECT * FROM projects WHERE id IN (SELECT projects_tags.project_id FROM
tags, projects_tags WHERE tags.name='foo' AND
projects_tags.project_id=projects.id);
(0.36 sec)
As another poster said, this kind of subquery runs slowly, but just to
elaborate on it: it's not every subquery that's a problem, just IN() and
NOT IN(). Use EXPLAIN EXTENDED followed by SHOW WARNINGS to see what's
happening.
Baron
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