I was debugging the performance of a DELETE statement that contained a subquery in the FROM clause. As there is no EXPLAIN for DELETE, I converted it to a SELECT statement (and hoped the same optimizations were done). But I still had to wait for EXPLAIN to complete. EXPLAIN evaluates subqueries in the FROM clause for MySQL. This can make EXPLAIN take a long time and create load on a server. Recent versions of MySQL have had many improvements for subquery optimization, but the documentation for all versions states that this is still done. A feature request is open to change this. Feature requests are also open to get EXPLAIN for UPDATE, INSERT and DELETE.
Do other RDBMS products support EXPLAIN for subqueries in a FROM clause without evaluating the subquery?