I think the parent refers to a specific optimization that MySQL doesn't perform: SELECT ... FROM ... WHERE field IN ( SELECT field2 FROM ... ) MySQL doesn't

I think the parent refers to a specific optimization that MySQL doesn't perform: SELECT ... FROM ... WHERE field IN ( SELECT field2 FROM ... ) MySQL doesn't acceptably optimize such queries (at v5.6, as far as I remember). This is a more complex case than the one you're pointing - the field2 records may be an intermediate result, logically and/or physically. For example of the latter, in a typical mysql execution with a relatively large number of records, the subquery records are likely to reside on a temporary swapped table. In this case, the optimizer would need to be able to create the intermediate table with optimal data structure[s], which (if I remember correctly) version 5.6 is not capable of.