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

The subquery was probably executing a "full table scan". In other words, not using the index and returning way too many rows that the Where from the main query were needing to filter out.

Just a guess without details of course but that's the common situation.