for (SELECT_LEX *sl= unit->first_select(); sl; sl= sl->next_select())

  {

    JOIN *cur_join= sl->join;

    if (!cur_join)

      continue;

    /*

      Subqueries whose result is known after optimization are not expensive.

      Such subqueries have all tables optimized away, thus have no join plan.

    */

    if (cur_join->optimized &&

        (cur_join->zero_result_cause || !cur_join->tables_list))

      return false;

 (SELECT FROM t1 WHERE 1=2 

  UNION

  SELECT sum(col) FROM very_big_table

 )