sql/item.h:
  optimized single value subselect (to be used as row)
sql/item_subselect.cc:
  optimized single value subselect (to be used as row)
sql/item_subselect.h:
  optimized single value subselect (to be used as row)
sql/sql_class.cc:
  optimized single value subselect (to be used as row)