Its query planner may be good (I don't know about this), in the sense that it can typically choose the best of a bad lot. However, the query executor (of stock MySQL) is awful - no hash joins? No merge joins? It is good compared to what?

This may have been true ten years ago. Not anymore. Mysql query planner is dumb as a rock. Easy example: instead of referring to a table in FROM, try using a sub query with SELECT * from the same table. It's an easy reproduction of the planner failing to process the query tree. It should result in the same execution tree. It results in a temporary table being created, as an identical copy of the original table.

Before you attack the query itself, and note that it is an example.