Comments to the single-select optimization algorithm.

}

/* Rewrite a single-column IN/ALL/ANY subselect. */

Item_subselect::trans_res

Item_in_subselect::single_value_transformer(JOIN *join,

					    Comp_creator *func)

  if (arena->is_stmt_prepare())

    thd->set_n_backup_item_arena(arena, &backup);

  /*

    Check that the right part of the subselect contains no more than one

    column. E.g. in SELECT 1 IN (SELECT * ..) the right part is (SELECT * ...)

  */

  if (select_lex->item_list.elements > 1)

  {

    my_error(ER_OPERAND_COLUMNS, MYF(0), 1);

    goto err;

  }

  /*

    If this is an ALL/ANY single-value subselect, try to rewrite it with

    a MIN/MAX subselect. We can do that if a possible NULL result of the

    subselect can be ignored.

    E.g. SELECT * FROM t1 WHERE b > ANY (SELECT a FROM t2) can be rewritten

    with SELECT * FROM t1 WHERE b > (SELECT MAX(a) FROM t2).

    We can't check that this optimization is safe if it's not a top-level

    item of the WHERE clause (e.g. because the WHERE clause can contain IS

    NULL/IS NOT NULL functions). If so, we rewrite ALL/ANY with NOT EXISTS

    later in this method.

  */

  if ((abort_on_null || (upper_not && upper_not->top_level())) &&

      !select_lex->master_unit()->uncacheable && !func->eqne_op())

  {

  Item *item;

  item= (Item*) select_lex->item_list.head();

  /*

    Add the left part of a subselect to a WHERE or HAVING clause of

    the right part, e.g. SELECT 1 IN (SELECT a FROM t1)  =>

    SELECT Item_in_optimizer(1, SELECT a FROM t1 WHERE a=1)

    HAVING is used only if the right part contains a SUM function, a GROUP

    BY or a HAVING clause.

  */

  if (join->having || select_lex->with_sum_func ||

      select_lex->group_list.elements)

  {