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. sql/sql_select.cc: BUG#29582: huge memory consumption with union, subselect, joins: - Don't mark tables as constant if JOIN::no_const_tables flag is set sql/sql_select.h: BUG#29582: huge memory consumption with union, subselect, joins: - Don't mark tables as constant if JOIN::no_const_tables flag is set sql/sql_union.cc: 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 UNION results. - Instead, call join->reinit() and join->exec(). - Provide special handling for table-less constant subqueries.
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@ -2416,7 +2416,7 @@ make_join_statistics(JOIN *join, TABLE_LIST *tables, COND *conds,
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if ((table->s->system || table->file->records <= 1) && ! s->dependent &&
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!(table->file->table_flags() & HA_NOT_EXACT_COUNT) &&
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!table->fulltext_searched)
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!table->fulltext_searched && !join->no_const_tables)
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{
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set_position(join,const_count++,s,(KEYUSE*) 0);
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}
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@ -277,6 +277,14 @@ public:
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SELECT_LEX_UNIT *unit;
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// select that processed
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SELECT_LEX *select_lex;
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/*
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TRUE <=> optimizer must not mark any table as a constant table.
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This is needed for subqueries in form "a IN (SELECT .. UNION SELECT ..):
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when we optimize the select that reads the results of the union from a
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temporary table, we must not mark the temp. table as constant because
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the number of rows in it may vary from one subquery execution to another.
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*/
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bool no_const_tables;
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JOIN *tmp_join; // copy of this JOIN to be used with temporary tables
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ROLLUP rollup; // Used with rollup
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@ -397,6 +405,8 @@ public:
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tmp_table_param.init();
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tmp_table_param.end_write_records= HA_POS_ERROR;
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rollup.state= ROLLUP::STATE_NONE;
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no_const_tables= FALSE;
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}
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int prepare(Item ***rref_pointer_array, TABLE_LIST *tables, uint wind_num,
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@ -545,6 +545,10 @@ bool st_select_lex_unit::exec()
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/*
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allocate JOIN for fake select only once (prevent
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mysql_select automatic allocation)
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TODO: The above is nonsense. mysql_select() will not allocate the
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join if one already exists. There must be some other reason why we
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don't let it allocate the join. Perhaps this is because we need
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some special parameter values passed to join constructor?
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*/
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if (!(fake_select_lex->join= new JOIN(thd, item_list,
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fake_select_lex->options, result)))
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@ -552,33 +556,52 @@ bool st_select_lex_unit::exec()
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fake_select_lex->table_list.empty();
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DBUG_RETURN(TRUE);
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}
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fake_select_lex->join->no_const_tables= TRUE;
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/*
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Fake st_select_lex should have item list for correctref_array
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allocation.
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*/
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fake_select_lex->item_list= item_list;
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saved_error= mysql_select(thd, &fake_select_lex->ref_pointer_array,
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&result_table_list,
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0, item_list, NULL,
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global_parameters->order_list.elements,
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(ORDER*)global_parameters->order_list.first,
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(ORDER*) NULL, NULL, (ORDER*) NULL,
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fake_select_lex->options | SELECT_NO_UNLOCK,
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result, this, fake_select_lex);
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}
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else
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{
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JOIN_TAB *tab,*end;
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for (tab=join->join_tab, end=tab+join->tables ;
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tab && tab != end ;
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tab++)
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{
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delete tab->select;
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delete tab->quick;
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}
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join->init(thd, item_list, fake_select_lex->options, result);
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if (describe)
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{
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/*
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In EXPLAIN command, constant subqueries that do not use any
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tables are executed two times:
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- 1st time is a real evaluation to get the subquery value
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- 2nd time is to produce EXPLAIN output rows.
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1st execution sets certain members (e.g. select_result) to perform
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subquery execution rather than EXPLAIN line production. In order
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to reset them back, we re-do all of the actions (yes it is ugly):
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*/
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join->init(thd, item_list, fake_select_lex->options, result);
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saved_error= mysql_select(thd, &fake_select_lex->ref_pointer_array,
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&result_table_list,
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0, item_list, NULL,
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global_parameters->order_list.elements,
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(ORDER*)global_parameters->order_list.first,
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(ORDER*) NULL, NULL, (ORDER*) NULL,
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fake_select_lex->options | SELECT_NO_UNLOCK,
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result, this, fake_select_lex);
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}
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else
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{
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join->examined_rows= 0;
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join->reinit();
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saved_error= join->exec();
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}
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}
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saved_error= mysql_select(thd, &fake_select_lex->ref_pointer_array,
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&result_table_list,
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0, item_list, NULL,
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global_parameters->order_list.elements,
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(ORDER*)global_parameters->order_list.first,
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(ORDER*) NULL, NULL, (ORDER*) NULL,
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fake_select_lex->options | SELECT_NO_UNLOCK,
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result, this, fake_select_lex);
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fake_select_lex->table_list.empty();
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if (!saved_error)
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