13.1.2 Row IN Subquery

To rewrite a row IN subquery, the method used is Item_in_subselect::row_value_transformer. It works in almost the same way as the scalar analog, but works with Item_cache_row for caching left expression and uses references for elements of Item_cache_row. To refer to the item list, it uses Item_ref_null_helper(ref_array+i).

A subquery with HAVING, SUM() function, or GROUP BY will transformed in the following way:

ROW(l1, l2, ... lN) IN (SELECT i1, i2, ... iN FROM t HAVING <having_expr>)

will become:

(SELECT i1, i2, ... iN FROM t
   HAVING <having_expr> and      <cache_l0> = <Item_ref_null_helper(ref_array[0]> AND      <cache_l1> = <Item_ref_null_helper(ref_array[1])> AND
     <cache_lN-1> = <Item_ref_null_helper(ref_array[N-1]>)

SELECT without FROM will be transformed in this way, too.

It will be the same for other subqueries, except for the WHERE clause.