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subselect.c |
Subselect notes from Vadim. From owner-pgsql-hackers@hub.org Fri Feb 13 09:01:19 1998 Received: from renoir.op.net (root@renoir.op.net [209.152.193.4]) by candle.pha.pa.us (8.8.5/8.8.5) with ESMTP id JAA11576 for <maillist@candle.pha.pa.us>; Fri, 13 Feb 1998 09:01:17 -0500 (EST) Received: from hub.org (hub.org [209.47.148.200]) by renoir.op.net (o1/$Revision: 1.1 $) with ESMTP id IAA09761 for <maillist@candle.pha.pa.us>; Fri, 13 Feb 1998 08:41:22 -0500 (EST) Received: from localhost (majordom@localhost) by hub.org (8.8.8/8.7.5) with SMTP id IAA08135; Fri, 13 Feb 1998 08:40:17 -0500 (EST) Received: by hub.org (TLB v0.10a (1.23 tibbs 1997/01/09 00:29:32)); Fri, 13 Feb 1998 08:38:42 -0500 (EST) Received: (from majordom@localhost) by hub.org (8.8.8/8.7.5) id IAA06646 for pgsql-hackers-outgoing; Fri, 13 Feb 1998 08:38:35 -0500 (EST) Received: from dune.krasnet.ru (dune.krasnet.ru [193.125.44.86]) by hub.org (8.8.8/8.7.5) with ESMTP id IAA04568 for <hackers@postgreSQL.org>; Fri, 13 Feb 1998 08:37:16 -0500 (EST) Received: from sable.krasnoyarsk.su (dune.krasnet.ru [193.125.44.86]) by dune.krasnet.ru (8.8.7/8.8.7) with ESMTP id UAA13717 for <hackers@postgreSQL.org>; Fri, 13 Feb 1998 20:51:03 +0700 (KRS) (envelope-from vadim@sable.krasnoyarsk.su) Message-ID: <34E44FBA.D64E7997@sable.krasnoyarsk.su> Date: Fri, 13 Feb 1998 20:50:50 +0700 From: "Vadim B. Mikheev" <vadim@sable.krasnoyarsk.su> Organization: ITTS (Krasnoyarsk) X-Mailer: Mozilla 4.04 [en] (X11; I; FreeBSD 2.2.5-RELEASE i386) MIME-Version: 1.0 To: PostgreSQL Developers List <hackers@postgreSQL.org> Subject: [HACKERS] Subselects are in CVS... Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Sender: owner-pgsql-hackers@hub.org Precedence: bulk Status: OR This is some implementation notes and opened issues... First, implementation uses new type of parameters - PARAM_EXEC - to deal with correlation Vars. When query_planner() is called, it first tries to replace all upper queries Var referenced in current query with Param of this type. Some global variables are used to keep mapping of Vars to Params and Params to Vars. After this, all current query' SubLinks are processed: for each SubLink found in query' qual union_planner() (old planner() function) will be called to plan corresponding subselect (union_planner() calls query_planner() for "simple" query and supports UNIONs). After subselect are planned, optimizer knows about is this correlated, un-correlated or _undirect_ correlated (references some grand-parent Vars but no parent ones: uncorrelated from the parent' point of view) query. For uncorrelated and undirect correlated subqueries of EXPRession or EXISTS type SubLinks will be replaced with "normal" clauses from SubLink->Oper list (I changed this list to be list of EXPR nodes, not just Oper ones). Right sides of these nodes are replaced with PARAM_EXEC parameters. This is second use of new parameter type. At run-time these parameters get value from result of subquery evaluation (i.e. - from target list of subquery). Execution plan of subquery itself becomes init plan of parent query. InitPlan knows what parameters are to get values from subquery' results and will be executed "on-demand" (for query select * from table where x > 0 and y > (select max(a) from table_a) subquery will not be executed at all if there are no tuples with x > 0 _and_ y is not used in index scan). SubLinks for subqueries of all other types are transformed into new type of Expr node - SUBPLAN_EXPR. Expr->args are just correlation variables from _parent_ query. Expr->oper is new SubPlan node. This node is used for InitPlan too. It keeps subquery range table, indices of Params which are to get value from _parent_ query Vars (i.e. - from Expr->args), indices of Params into which subquery' results are to be substituted (this is for InitPlans), SubLink and subquery' execution plan. Plan node was changed to know about dependencies on Params from parent queries and InitPlans, to keep list of changed Params (from the above) and so be re-scanned if this list is not NULL. Also, added list of InitPlans (actually, all of them for current query are in topmost plan node now) and other SubPlans (from plan->qual) - to initialize them and let them know about changed Params (from the list of their "interests"). After all SubLinks are processed, query_planner() calls qual' canonificator and does "normal" work. By using Params optimizer is mostly unchanged. Well, Executor. To get subplans re-evaluated without ExecutorStart() and ExecutorEnd() (without opening and closing relations and indices and without many palloc() and pfree() - this is what SQL-funcs does on each call) ExecReScan() now supports most of Plan types... Explanation of EXPLAIN. vac=> explain select * from tmp where x >= (select max(x2) from test2 where y2 = y and exists (select * from tempx where tx = x)); NOTICE: QUERY PLAN: Seq Scan on tmp (cost=40.03 size=101 width=8) SubPlan ^^^^^^^ subquery is in Seq Scan' qual, its plan is below -> Aggregate (cost=2.05 size=0 width=0) InitPlan ^^^^^^^^ EXISTS subsubquery is InitPlan of subquery -> Seq Scan on tempx (cost=4.33 size=1 width=4) -> Result (cost=2.05 size=0 width=0) ^^^^^^ EXISTS subsubquery was transformed into Param and so we have Result node here -> Index Scan on test2 (cost=2.05 size=1 width=4) Opened issues. 1. No read permissions checking (easy, just not done yet). 2. readfuncs.c can't read subplan-s (easy, not critical, because of we currently nowhere use ascii representation of execution plans). 3. ExecReScan() doesn't support all plan types. At least support for MergeJoin has to be implemented. 4. Memory leaks in ExecReScan(). 5. I need in advice: if subquery introduced with NOT IN doesn't return any tuples then qualification is failed, yes ? 6. Regression tests !!!!!!!!!!!!!!!!!!!! (Could we use data/queries from MySQL' crash.me ? Copyright-ed ? Could they give us rights ?) 7. Performance. - Should be good when subquery is transformed into InitPlan. - Something should be done for uncorrelated subqueries introduced with ANY/ALL - keep thinking. Currently, subplan will be re-scanned for each parent tuple - very slow... Results of some test. TMP is table with x,y (int4-s), x in 0-9, y = 100 - x, 1000 tuples (10 duplicates of each tuple). TEST2 is table with x2, y2 (int4-s), x2 in 1-99, y2 = 100 -x2, 10000 tuples (100 dups). Trying select * from tmp where x >= (select max(x2) from test2 where y2 = y); and begin; select y as ty, max(x2) as mx into table tsub from test2, tmp where y2 = y group by ty; vacuum tsub; select x, y from tmp, tsub where x >= mx and y = ty; drop table tsub; end; Without index on test2(y2): SubSelect -> 320 sec Using temp table -> 32 sec Having index SubSelect -> 17 sec (2M of memory) Using temp table -> 32 sec (12M of memory: -S 8192) Vadim