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Commit40db52a

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committed
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 theLIMIT. Per discussion on pgsql-sql 7/15/01.
1 parent8ba3548 commit40db52a

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1 file changed

+104
-89
lines changed

1 file changed

+104
-89
lines changed

‎src/backend/optimizer/path/allpaths.c

Lines changed: 104 additions & 89 deletions
Original file line numberDiff line numberDiff line change
@@ -8,7 +8,7 @@
88
*
99
*
1010
* IDENTIFICATION
11-
* $Header: /cvsroot/pgsql/src/backend/optimizer/path/allpaths.c,v 1.76 2001/06/05 17:13:51 tgl Exp $
11+
* $Header: /cvsroot/pgsql/src/backend/optimizer/path/allpaths.c,v 1.77 2001/07/16 17:57:02 tgl Exp $
1212
*
1313
*-------------------------------------------------------------------------
1414
*/
@@ -37,6 +37,8 @@ static void set_plain_rel_pathlist(Query *root, RelOptInfo *rel,
3737
staticvoidset_inherited_rel_pathlist(Query*root,RelOptInfo*rel,
3838
Indexrti,RangeTblEntry*rte,
3939
List*inheritlist);
40+
staticvoidset_subquery_pathlist(Query*root,RelOptInfo*rel,
41+
Indexrti,RangeTblEntry*rte);
4042
staticRelOptInfo*make_one_rel_by_joins(Query*root,intlevels_needed,
4143
List*initial_rels);
4244

@@ -101,94 +103,7 @@ set_base_rel_pathlists(Query *root)
101103
if (rel->issubquery)
102104
{
103105
/* Subquery --- generate a separate plan for it */
104-
105-
/*
106-
* If there are any restriction clauses that have been
107-
* attached to the subquery relation, consider pushing them
108-
* down to become HAVING quals of the subquery itself.(Not
109-
* WHERE clauses, since they may refer to subquery outputs
110-
* that are aggregate results. But planner.c will transfer
111-
* them into the subquery's WHERE if they do not.) This
112-
* transformation is useful because it may allow us to
113-
* generate a better plan for the subquery than evaluating all
114-
* the subquery output rows and then filtering them.
115-
*
116-
* Currently, we do not push down clauses that contain
117-
* subselects, mainly because I'm not sure it will work
118-
* correctly (the subplan hasn't yet transformed sublinks to
119-
* subselects). Also, if the subquery contains set ops
120-
* (UNION/INTERSECT/EXCEPT) we do not push down any qual
121-
* clauses, since the planner doesn't support quals at the top
122-
* level of a setop. (With suitable analysis we could try to
123-
* push the quals down into the component queries of the
124-
* setop, but getting it right is not trivial.)
125-
* Non-pushed-down clauses will get evaluated as qpquals of
126-
* the SubqueryScan node.
127-
*
128-
* XXX Are there any cases where we want to make a policy
129-
* decision not to push down, because it'd result in a worse
130-
* plan?
131-
*/
132-
if (rte->subquery->setOperations==NULL)
133-
{
134-
/* OK to consider pushing down individual quals */
135-
List*upperrestrictlist=NIL;
136-
List*lst;
137-
138-
foreach(lst,rel->baserestrictinfo)
139-
{
140-
RestrictInfo*rinfo= (RestrictInfo*)lfirst(lst);
141-
Node*clause= (Node*)rinfo->clause;
142-
143-
if (contain_subplans(clause))
144-
{
145-
/* Keep it in the upper query */
146-
upperrestrictlist=lappend(upperrestrictlist,rinfo);
147-
}
148-
else
149-
{
150-
151-
/*
152-
* We need to replace Vars in the clause (which
153-
* must refer to outputs of the subquery) with
154-
* copies of the subquery's targetlist
155-
* expressions. Note that at this point, any
156-
* uplevel Vars in the clause should have been
157-
* replaced with Params, so they need no work.
158-
*/
159-
clause=ResolveNew(clause,rti,0,
160-
rte->subquery->targetList,
161-
CMD_SELECT,0);
162-
rte->subquery->havingQual=
163-
make_and_qual(rte->subquery->havingQual,
164-
clause);
165-
166-
/*
167-
* We need not change the subquery's hasAggs or
168-
* hasSublinks flags, since we can't be pushing
169-
* down any aggregates that weren't there before,
170-
* and we don't push down subselects at all.
171-
*/
172-
}
173-
}
174-
rel->baserestrictinfo=upperrestrictlist;
175-
}
176-
177-
/* Generate the plan for the subquery */
178-
rel->subplan=subquery_planner(rte->subquery,
179-
-1.0/* default case */ );
180-
181-
/* Copy number of output rows from subplan */
182-
rel->tuples=rel->subplan->plan_rows;
183-
184-
/* Mark rel with estimated output rows, width, etc */
185-
set_baserel_size_estimates(root,rel);
186-
187-
/* Generate appropriate path */
188-
add_path(rel,create_subqueryscan_path(rel));
189-
190-
/* Select cheapest path (pretty easy in this case...) */
191-
set_cheapest(rel);
106+
set_subquery_pathlist(root,rel,rti,rte);
192107
}
193108
elseif ((inheritlist=expand_inherted_rtentry(root,rti, true))
194109
!=NIL)
@@ -353,6 +268,106 @@ set_inherited_rel_pathlist(Query *root, RelOptInfo *rel,
353268
set_cheapest(rel);
354269
}
355270

271+
/*
272+
* set_subquery_pathlist
273+
*Build the (single) access path for a subquery RTE
274+
*/
275+
staticvoid
276+
set_subquery_pathlist(Query*root,RelOptInfo*rel,
277+
Indexrti,RangeTblEntry*rte)
278+
{
279+
Query*subquery=rte->subquery;
280+
281+
/*
282+
* If there are any restriction clauses that have been attached to the
283+
* subquery relation, consider pushing them down to become HAVING quals
284+
* of the subquery itself. (Not WHERE clauses, since they may refer to
285+
* subquery outputs that are aggregate results. But planner.c will
286+
* transfer them into the subquery's WHERE if they do not.) This
287+
* transformation is useful because it may allow us to generate a better
288+
* plan for the subquery than evaluating all the subquery output rows
289+
* and then filtering them.
290+
*
291+
* There are several cases where we cannot push down clauses:
292+
*
293+
* 1. If the subquery contains set ops (UNION/INTERSECT/EXCEPT) we do not
294+
* push down any qual clauses, since the planner doesn't support quals at
295+
* the top level of a setop. (With suitable analysis we could try to push
296+
* the quals down into the component queries of the setop, but getting it
297+
* right seems nontrivial. Work on this later.)
298+
*
299+
* 2. If the subquery has a LIMIT clause we must not push down any quals,
300+
* since that could change the set of rows returned.
301+
*
302+
* 3. We do not push down clauses that contain subselects, mainly because
303+
* I'm not sure it will work correctly (the subplan hasn't yet transformed
304+
* sublinks to subselects).
305+
*
306+
* Non-pushed-down clauses will get evaluated as qpquals of the
307+
* SubqueryScan node.
308+
*
309+
* XXX Are there any cases where we want to make a policy decision not to
310+
* push down, because it'd result in a worse plan?
311+
*/
312+
if (subquery->setOperations==NULL&&
313+
subquery->limitOffset==NULL&&
314+
subquery->limitCount==NULL)
315+
{
316+
/* OK to consider pushing down individual quals */
317+
List*upperrestrictlist=NIL;
318+
List*lst;
319+
320+
foreach(lst,rel->baserestrictinfo)
321+
{
322+
RestrictInfo*rinfo= (RestrictInfo*)lfirst(lst);
323+
Node*clause= (Node*)rinfo->clause;
324+
325+
if (contain_subplans(clause))
326+
{
327+
/* Keep it in the upper query */
328+
upperrestrictlist=lappend(upperrestrictlist,rinfo);
329+
}
330+
else
331+
{
332+
/*
333+
* We need to replace Vars in the clause (which must refer to
334+
* outputs of the subquery) with copies of the subquery's
335+
* targetlist expressions. Note that at this point, any
336+
* uplevel Vars in the clause should have been replaced with
337+
* Params, so they need no work.
338+
*/
339+
clause=ResolveNew(clause,rti,0,
340+
subquery->targetList,
341+
CMD_SELECT,0);
342+
subquery->havingQual=make_and_qual(subquery->havingQual,
343+
clause);
344+
/*
345+
* We need not change the subquery's hasAggs or
346+
* hasSublinks flags, since we can't be pushing
347+
* down any aggregates that weren't there before,
348+
* and we don't push down subselects at all.
349+
*/
350+
}
351+
}
352+
rel->baserestrictinfo=upperrestrictlist;
353+
}
354+
355+
/* Generate the plan for the subquery */
356+
rel->subplan=subquery_planner(subquery,
357+
-1.0/* default case */ );
358+
359+
/* Copy number of output rows from subplan */
360+
rel->tuples=rel->subplan->plan_rows;
361+
362+
/* Mark rel with estimated output rows, width, etc */
363+
set_baserel_size_estimates(root,rel);
364+
365+
/* Generate appropriate path */
366+
add_path(rel,create_subqueryscan_path(rel));
367+
368+
/* Select cheapest path (pretty easy in this case...) */
369+
set_cheapest(rel);
370+
}
356371

357372
/*
358373
* make_fromexpr_rel

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