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Commit719012e

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Add some defenses against constant-FALSE outer join conditions. Since
eval_const_expressions will generally throw away anything that's ANDed withconstant FALSE, what we're left with given an example likeselect * from tenk1 a where (unique1,0) in (select unique2,1 from tenk1 b);is a cartesian product computation, which is really not acceptable.This is a regression in CVS HEAD compared to previous releases, which wereable to notice the impossible join condition in this case --- though not insome related cases that are also improved by this patch, such asselect * from tenk1 a left join tenk1 b on (a.unique1=b.unique2 and 0=1);Fix by skipping evaluation of the appropriate side of the outer join incases where it's demonstrably unnecessary.
1 parentf2689e4 commit719012e

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1 file changed

+64
-15
lines changed

1 file changed

+64
-15
lines changed

‎src/backend/optimizer/path/joinrels.c

Lines changed: 64 additions & 15 deletions
Original file line numberDiff line numberDiff line change
@@ -8,7 +8,7 @@
88
*
99
*
1010
* IDENTIFICATION
11-
* $PostgreSQL: pgsql/src/backend/optimizer/path/joinrels.c,v 1.93 2008/08/14 18:47:59 tgl Exp $
11+
* $PostgreSQL: pgsql/src/backend/optimizer/path/joinrels.c,v 1.94 2008/08/17 19:40:11 tgl Exp $
1212
*
1313
*-------------------------------------------------------------------------
1414
*/
@@ -28,7 +28,8 @@ static List *make_rels_by_clauseless_joins(PlannerInfo *root,
2828
staticboolhas_join_restriction(PlannerInfo*root,RelOptInfo*rel);
2929
staticboolhas_legal_joinclause(PlannerInfo*root,RelOptInfo*rel);
3030
staticboolis_dummy_rel(RelOptInfo*rel);
31-
staticvoidmark_dummy_join(RelOptInfo*rel);
31+
staticvoidmark_dummy_rel(RelOptInfo*rel);
32+
staticboolrestriction_is_constant_false(List*restrictlist);
3233

3334

3435
/*
@@ -558,15 +559,20 @@ make_join_rel(PlannerInfo *root, RelOptInfo *rel1, RelOptInfo *rel2)
558559
* this way since it's conceivable that dummy-ness of a multi-element
559560
* join might only be noticeable for certain construction paths.)
560561
*
562+
* Also, a provably constant-false join restriction typically means that
563+
* we can skip evaluating one or both sides of the join. We do this
564+
* by marking the appropriate rel as dummy.
565+
*
561566
* We need only consider the jointypes that appear in join_info_list,
562567
* plus JOIN_INNER.
563568
*/
564569
switch (sjinfo->jointype)
565570
{
566571
caseJOIN_INNER:
567-
if (is_dummy_rel(rel1)||is_dummy_rel(rel2))
572+
if (is_dummy_rel(rel1)||is_dummy_rel(rel2)||
573+
restriction_is_constant_false(restrictlist))
568574
{
569-
mark_dummy_join(joinrel);
575+
mark_dummy_rel(joinrel);
570576
break;
571577
}
572578
add_paths_to_joinrel(root,joinrel,rel1,rel2,
@@ -579,9 +585,12 @@ make_join_rel(PlannerInfo *root, RelOptInfo *rel1, RelOptInfo *rel2)
579585
caseJOIN_LEFT:
580586
if (is_dummy_rel(rel1))
581587
{
582-
mark_dummy_join(joinrel);
588+
mark_dummy_rel(joinrel);
583589
break;
584590
}
591+
if (restriction_is_constant_false(restrictlist)&&
592+
bms_is_subset(rel2->relids,sjinfo->syn_righthand))
593+
mark_dummy_rel(rel2);
585594
add_paths_to_joinrel(root,joinrel,rel1,rel2,
586595
JOIN_LEFT,sjinfo,
587596
restrictlist);
@@ -592,7 +601,7 @@ make_join_rel(PlannerInfo *root, RelOptInfo *rel1, RelOptInfo *rel2)
592601
caseJOIN_FULL:
593602
if (is_dummy_rel(rel1)&&is_dummy_rel(rel2))
594603
{
595-
mark_dummy_join(joinrel);
604+
mark_dummy_rel(joinrel);
596605
break;
597606
}
598607
add_paths_to_joinrel(root,joinrel,rel1,rel2,
@@ -603,9 +612,10 @@ make_join_rel(PlannerInfo *root, RelOptInfo *rel1, RelOptInfo *rel2)
603612
restrictlist);
604613
break;
605614
caseJOIN_SEMI:
606-
if (is_dummy_rel(rel1)||is_dummy_rel(rel2))
615+
if (is_dummy_rel(rel1)||is_dummy_rel(rel2)||
616+
restriction_is_constant_false(restrictlist))
607617
{
608-
mark_dummy_join(joinrel);
618+
mark_dummy_rel(joinrel);
609619
break;
610620
}
611621
add_paths_to_joinrel(root,joinrel,rel1,rel2,
@@ -632,9 +642,12 @@ make_join_rel(PlannerInfo *root, RelOptInfo *rel1, RelOptInfo *rel2)
632642
caseJOIN_ANTI:
633643
if (is_dummy_rel(rel1))
634644
{
635-
mark_dummy_join(joinrel);
645+
mark_dummy_rel(joinrel);
636646
break;
637647
}
648+
if (restriction_is_constant_false(restrictlist)&&
649+
bms_is_subset(rel2->relids,sjinfo->syn_righthand))
650+
mark_dummy_rel(rel2);
638651
add_paths_to_joinrel(root,joinrel,rel1,rel2,
639652
JOIN_ANTI,sjinfo,
640653
restrictlist);
@@ -851,10 +864,10 @@ is_dummy_rel(RelOptInfo *rel)
851864
}
852865

853866
/*
854-
* Mark ajoinrel as proven empty.
867+
* Mark arel as proven empty.
855868
*/
856869
staticvoid
857-
mark_dummy_join(RelOptInfo*rel)
870+
mark_dummy_rel(RelOptInfo*rel)
858871
{
859872
/* Set dummy size estimate */
860873
rel->rows=0;
@@ -865,10 +878,46 @@ mark_dummy_join(RelOptInfo *rel)
865878
/* Set up the dummy path */
866879
add_path(rel, (Path*)create_append_path(rel,NIL));
867880

881+
/* Set or update cheapest_total_path */
882+
set_cheapest(rel);
883+
}
884+
885+
886+
/*
887+
* restriction_is_constant_false --- is a restrictlist just FALSE?
888+
*
889+
* In cases where a qual is provably constant FALSE, eval_const_expressions
890+
* will generally have thrown away anything that's ANDed with it. In outer
891+
* join situations this will leave us computing cartesian products only to
892+
* decide there's no match for an outer row, which is pretty stupid. So,
893+
* we need to detect the case.
894+
*/
895+
staticbool
896+
restriction_is_constant_false(List*restrictlist)
897+
{
898+
ListCell*lc;
899+
868900
/*
869-
* Although set_cheapest will be done again later, we do it immediately
870-
* in order to keep is_dummy_rel as cheap as possible (ie, not have
871-
* to examine the pathlist).
901+
* Despite the above comment, the restriction list we see here might
902+
* possibly have other members besides the FALSE constant, since other
903+
* quals could get "pushed down" to the outer join level. So we check
904+
* each member of the list.
872905
*/
873-
set_cheapest(rel);
906+
foreach(lc,restrictlist)
907+
{
908+
RestrictInfo*rinfo= (RestrictInfo*)lfirst(lc);
909+
910+
Assert(IsA(rinfo,RestrictInfo));
911+
if (rinfo->clause&&IsA(rinfo->clause,Const))
912+
{
913+
Const*con= (Const*)rinfo->clause;
914+
915+
/* constant NULL is as good as constant FALSE for our purposes */
916+
if (con->constisnull)
917+
return true;
918+
if (!DatumGetBool(con->constvalue))
919+
return true;
920+
}
921+
}
922+
return false;
874923
}

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