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Do not push down quals into subqueries that have LIMIT/OFFSET clauses,
since the added qual could change the set of rows that get past the LIMIT. Per discussion on pgsql-sql 7/15/01.
This commit is contained in:
Tom Lane
parent
8ba3548ebf
commit
40db52af34
@ -8,7 +8,7 @@
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*
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*
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*
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*
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* IDENTIFICATION
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* IDENTIFICATION
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* $Header: /cvsroot/pgsql/src/backend/optimizer/path/allpaths.c,v 1.76 2001/06/05 17:13:51 tgl Exp $
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* $Header: /cvsroot/pgsql/src/backend/optimizer/path/allpaths.c,v 1.77 2001/07/16 17:57:02 tgl Exp $
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*
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*
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*-------------------------------------------------------------------------
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*-------------------------------------------------------------------------
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*/
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*/
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@ -37,6 +37,8 @@ static void set_plain_rel_pathlist(Query *root, RelOptInfo *rel,
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static void set_inherited_rel_pathlist(Query *root, RelOptInfo *rel,
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static void set_inherited_rel_pathlist(Query *root, RelOptInfo *rel,
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Index rti, RangeTblEntry *rte,
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Index rti, RangeTblEntry *rte,
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List *inheritlist);
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List *inheritlist);
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static void set_subquery_pathlist(Query *root, RelOptInfo *rel,
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Index rti, RangeTblEntry *rte);
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static RelOptInfo *make_one_rel_by_joins(Query *root, int levels_needed,
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static RelOptInfo *make_one_rel_by_joins(Query *root, int levels_needed,
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List *initial_rels);
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List *initial_rels);
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@ -101,94 +103,7 @@ set_base_rel_pathlists(Query *root)
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if (rel->issubquery)
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if (rel->issubquery)
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{
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{
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/* Subquery --- generate a separate plan for it */
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/* Subquery --- generate a separate plan for it */
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set_subquery_pathlist(root, rel, rti, rte);
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/*
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* If there are any restriction clauses that have been
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* attached to the subquery relation, consider pushing them
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* down to become HAVING quals of the subquery itself. (Not
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* WHERE clauses, since they may refer to subquery outputs
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* that are aggregate results. But planner.c will transfer
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* them into the subquery's WHERE if they do not.) This
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* transformation is useful because it may allow us to
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* generate a better plan for the subquery than evaluating all
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* the subquery output rows and then filtering them.
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*
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* Currently, we do not push down clauses that contain
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* subselects, mainly because I'm not sure it will work
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* correctly (the subplan hasn't yet transformed sublinks to
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* subselects). Also, if the subquery contains set ops
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* (UNION/INTERSECT/EXCEPT) we do not push down any qual
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* clauses, since the planner doesn't support quals at the top
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* level of a setop. (With suitable analysis we could try to
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* push the quals down into the component queries of the
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* setop, but getting it right is not trivial.)
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* Non-pushed-down clauses will get evaluated as qpquals of
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* the SubqueryScan node.
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*
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* XXX Are there any cases where we want to make a policy
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* decision not to push down, because it'd result in a worse
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* plan?
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*/
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if (rte->subquery->setOperations == NULL)
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{
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/* OK to consider pushing down individual quals */
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List *upperrestrictlist = NIL;
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List *lst;
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foreach(lst, rel->baserestrictinfo)
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{
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RestrictInfo *rinfo = (RestrictInfo *) lfirst(lst);
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Node *clause = (Node *) rinfo->clause;
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if (contain_subplans(clause))
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{
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/* Keep it in the upper query */
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upperrestrictlist = lappend(upperrestrictlist, rinfo);
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}
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else
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{
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/*
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* We need to replace Vars in the clause (which
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* must refer to outputs of the subquery) with
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* copies of the subquery's targetlist
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* expressions. Note that at this point, any
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* uplevel Vars in the clause should have been
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* replaced with Params, so they need no work.
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*/
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clause = ResolveNew(clause, rti, 0,
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rte->subquery->targetList,
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CMD_SELECT, 0);
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rte->subquery->havingQual =
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make_and_qual(rte->subquery->havingQual,
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clause);
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/*
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* We need not change the subquery's hasAggs or
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* hasSublinks flags, since we can't be pushing
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* down any aggregates that weren't there before,
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* and we don't push down subselects at all.
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*/
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}
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}
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rel->baserestrictinfo = upperrestrictlist;
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}
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/* Generate the plan for the subquery */
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rel->subplan = subquery_planner(rte->subquery,
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-1.0 /* default case */ );
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/* Copy number of output rows from subplan */
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rel->tuples = rel->subplan->plan_rows;
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/* Mark rel with estimated output rows, width, etc */
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set_baserel_size_estimates(root, rel);
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/* Generate appropriate path */
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add_path(rel, create_subqueryscan_path(rel));
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/* Select cheapest path (pretty easy in this case...) */
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set_cheapest(rel);
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}
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}
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else if ((inheritlist = expand_inherted_rtentry(root, rti, true))
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else if ((inheritlist = expand_inherted_rtentry(root, rti, true))
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!= NIL)
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!= NIL)
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@ -353,6 +268,106 @@ set_inherited_rel_pathlist(Query *root, RelOptInfo *rel,
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set_cheapest(rel);
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set_cheapest(rel);
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}
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}
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/*
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* set_subquery_pathlist
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* Build the (single) access path for a subquery RTE
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*/
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static void
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set_subquery_pathlist(Query *root, RelOptInfo *rel,
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Index rti, RangeTblEntry *rte)
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{
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Query *subquery = rte->subquery;
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/*
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* If there are any restriction clauses that have been attached to the
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* subquery relation, consider pushing them down to become HAVING quals
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* of the subquery itself. (Not WHERE clauses, since they may refer to
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* subquery outputs that are aggregate results. But planner.c will
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* transfer them into the subquery's WHERE if they do not.) This
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* transformation is useful because it may allow us to generate a better
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* plan for the subquery than evaluating all the subquery output rows
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* and then filtering them.
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*
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* There are several cases where we cannot push down clauses:
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*
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* 1. If the subquery contains set ops (UNION/INTERSECT/EXCEPT) we do not
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* push down any qual clauses, since the planner doesn't support quals at
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* the top level of a setop. (With suitable analysis we could try to push
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* the quals down into the component queries of the setop, but getting it
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* right seems nontrivial. Work on this later.)
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*
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* 2. If the subquery has a LIMIT clause we must not push down any quals,
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* since that could change the set of rows returned.
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*
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* 3. We do not push down clauses that contain subselects, mainly because
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* I'm not sure it will work correctly (the subplan hasn't yet transformed
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* sublinks to subselects).
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*
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* Non-pushed-down clauses will get evaluated as qpquals of the
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* SubqueryScan node.
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*
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* XXX Are there any cases where we want to make a policy decision not to
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* push down, because it'd result in a worse plan?
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*/
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if (subquery->setOperations == NULL &&
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subquery->limitOffset == NULL &&
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subquery->limitCount == NULL)
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{
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/* OK to consider pushing down individual quals */
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List *upperrestrictlist = NIL;
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List *lst;
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foreach(lst, rel->baserestrictinfo)
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{
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RestrictInfo *rinfo = (RestrictInfo *) lfirst(lst);
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Node *clause = (Node *) rinfo->clause;
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if (contain_subplans(clause))
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{
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/* Keep it in the upper query */
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upperrestrictlist = lappend(upperrestrictlist, rinfo);
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}
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else
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{
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/*
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* We need to replace Vars in the clause (which must refer to
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* outputs of the subquery) with copies of the subquery's
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* targetlist expressions. Note that at this point, any
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* uplevel Vars in the clause should have been replaced with
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* Params, so they need no work.
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*/
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clause = ResolveNew(clause, rti, 0,
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subquery->targetList,
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CMD_SELECT, 0);
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subquery->havingQual = make_and_qual(subquery->havingQual,
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clause);
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/*
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* We need not change the subquery's hasAggs or
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* hasSublinks flags, since we can't be pushing
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* down any aggregates that weren't there before,
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* and we don't push down subselects at all.
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*/
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}
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}
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rel->baserestrictinfo = upperrestrictlist;
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}
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/* Generate the plan for the subquery */
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rel->subplan = subquery_planner(subquery,
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-1.0 /* default case */ );
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/* Copy number of output rows from subplan */
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rel->tuples = rel->subplan->plan_rows;
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/* Mark rel with estimated output rows, width, etc */
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set_baserel_size_estimates(root, rel);
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/* Generate appropriate path */
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add_path(rel, create_subqueryscan_path(rel));
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/* Select cheapest path (pretty easy in this case...) */
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set_cheapest(rel);
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}
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/*
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/*
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* make_fromexpr_rel
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* make_fromexpr_rel
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