If you use the EXPLAIN SELECT statement to see how your
subqueries are treated by MySQL, you may sometimes meet the
"unique_subquery" optimization. Here is how the manual describes it:
"unique_subquery: this type replaces ref for some
IN subqueries of the following form: value IN (SELECT
primary_key FROM single_table WHERE some_expr);
unique_subquery is just an index lookup function that replaces
the subquery completely for better efficiency".Few weeks ago,
while I was reviewing a patch fixing a bug in unique_subquery, I got a
"simplification" pulsion. I told myself that:
- unique_subquery is an optimization for a special case of simple subqueries (single inner …