Commit c37f9b9e authored by sergefp@mysql.com's avatar sergefp@mysql.com
Browse files

BUG#29582: huge memory consumption with union, subselect, joins: 

- Don't call mysql_select() several times for the select that enumerates
  a temporary table with the results of the UNION. Making this call for
  every subquery execution caused O(#enumerated-rows-in-the-outer-query) 
  memory allocations.
- Instead, call join->reinit() and join->exec(), and
  = disable constant table detection for such joins,  
  = provide special handling for table-less constant subqueries.
parent b3820697

sql/sql_select.cc

+1 −1
Original line number Diff line number Diff line
@@ -2416,7 +2416,7 @@ make_join_statistics(JOIN *join, TABLE_LIST *tables, COND *conds, 


    if ((table->s->system || table->file->records <= 1) && ! s->dependent &&

	!(table->file->table_flags() & HA_NOT_EXACT_COUNT) &&

        !table->fulltext_searched)

        !table->fulltext_searched && !join->no_const_tables)

    {

      set_position(join,const_count++,s,(KEYUSE*) 0);

    }

sql/sql_select.h

+10 −0
Original line number Diff line number Diff line
@@ -277,6 +277,14 @@ class JOIN :public Sql_alloc 
  SELECT_LEX_UNIT *unit;

  // select that processed

  SELECT_LEX *select_lex;

  /* 

    TRUE <=> optimizer must not mark any table as a constant table.

    This is needed for subqueries in form "a IN (SELECT .. UNION SELECT ..):

    when we optimize the select that reads the results of the union from a

    temporary table, we must not mark the temp. table as constant because

    the number of rows in it may vary from one subquery execution to another.

  */

  bool no_const_tables; 

  

  JOIN *tmp_join; // copy of this JOIN to be used with temporary tables

  ROLLUP rollup;				// Used with rollup
@@ -397,6 +405,8 @@ class JOIN :public Sql_alloc 
    tmp_table_param.init();

    tmp_table_param.end_write_records= HA_POS_ERROR;

    rollup.state= ROLLUP::STATE_NONE;



    no_const_tables= FALSE;

  }



  int prepare(Item ***rref_pointer_array, TABLE_LIST *tables, uint wind_num,

sql/sql_union.cc

+40 −17
Original line number Diff line number Diff line
@@ -545,6 +545,10 @@ bool st_select_lex_unit::exec() 
	/*

	  allocate JOIN for fake select only once (prevent

	  mysql_select automatic allocation)

          TODO: The above is nonsense. mysql_select() will not allocate the

          join if one already exists. There must be some other reason why we

          don't let it allocate the join. Perhaps this is because we need

          some special parameter values passed to join constructor?

	*/

	if (!(fake_select_lex->join= new JOIN(thd, item_list,

					      fake_select_lex->options, result)))
@@ -552,25 +556,36 @@ bool st_select_lex_unit::exec() 
	  fake_select_lex->table_list.empty();

	  DBUG_RETURN(TRUE);

	}

        fake_select_lex->join->no_const_tables= TRUE;



	/*

	  Fake st_select_lex should have item list for correctref_array

	  allocation.

	*/

	fake_select_lex->item_list= item_list;

        saved_error= mysql_select(thd, &fake_select_lex->ref_pointer_array,

                              &result_table_list,

                              0, item_list, NULL,

                              global_parameters->order_list.elements,

                              (ORDER*)global_parameters->order_list.first,

                              (ORDER*) NULL, NULL, (ORDER*) NULL,

                              fake_select_lex->options | SELECT_NO_UNLOCK,

                              result, this, fake_select_lex);

      }

      else

      {

	JOIN_TAB *tab,*end;

	for (tab=join->join_tab, end=tab+join->tables ;

	     tab && tab != end ;

	     tab++)

        if (describe)

        {

	  delete tab->select;

	  delete tab->quick;

	}

          /*

            In EXPLAIN command, constant subqueries that do not use any

            tables are executed two times:

             - 1st time is a real evaluation to get the subquery value

             - 2nd time is to produce EXPLAIN output rows.

            1st execution sets certain members (e.g. select_result) to perform

            subquery execution rather than EXPLAIN line production. In order 

            to reset them back, we re-do all of the actions (yes it is ugly):

          */

	  join->init(thd, item_list, fake_select_lex->options, result);

      }

          saved_error= mysql_select(thd, &fake_select_lex->ref_pointer_array,

                                &result_table_list,

                                0, item_list, NULL,
@@ -579,6 +594,14 @@ bool st_select_lex_unit::exec() 
                                (ORDER*) NULL, NULL, (ORDER*) NULL,

                                fake_select_lex->options | SELECT_NO_UNLOCK,

                                result, this, fake_select_lex);

        }

        else

        {

          join->examined_rows= 0;

          join->reinit();

          saved_error= join->exec();

        }

      }



      fake_select_lex->table_list.empty();

      if (!saved_error)