Performance
IN/NOT IN subqueries are now much more efficient
+
+ In previous releases, IN/NOT IN subqueries were joined to the
+ upper query by sequentially scanning the subquery looking for
+ a join. The 7.4 code uses the same sophisticated techniques
+ used by ordinary joins and so is much faster, and is now faster
+ than EXISTS subqueries.
+
+
Improved GROUP BY processing by using hash buckets