But the optimiser is really bad at doing sensible things with them. Worse still, subqueries in the FROM clause are (documented to be) implemented as an unindex

But the optimiser is really bad at doing sensible things with them. Worse still, subqueries in the FROM clause are (documented to be) implemented as an unindexed temp table. Google apparently compiles subquery support out of their mysql instances so people don't mistakenly think they're usable.