Subqueries vs joins

Question

I refactored a slow section of an application we inherited from another company to use an inner join instead of a subquery like:

WHERE id IN (SELECT id FROM          


        

Answer 1

Taken from the Reference Manual (14.2.10.11 Rewriting Subqueries as Joins):

A LEFT [OUTER] JOIN can be faster than an equivalent subquery because the server might be able to optimize it better—a fact that is not specific to MySQL Server alone.

So subqueries can be slower than LEFT [OUTER] JOINS.