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Commitdf62977

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Fix an old error in clause_selectivity: the default selectivity estimate
for unhandled clause types ought to be 0.5, not 1.0. I fear I introducedthis silliness due to misreading the intent of the very-poorly-structuredcode that was there when we inherited the file from Berkeley. The lackof sanity in this behavior was exposed by an example from Sim Zacks.(Arguably this is a bug fix and should be back-patched, but I'm a bithesitant to introduce a possible planner behavior change in the backbranches; it might detune queries that worked acceptably in the past.)While at it, make estimation for DistinctExpr do something marginallyrealistic, rather than just defaulting.
1 parentf3e3f2e commitdf62977

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+17
-13
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1 file changed

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‎src/backend/optimizer/path/clausesel.c

Lines changed: 17 additions & 13 deletions
Original file line numberDiff line numberDiff line change
@@ -8,7 +8,7 @@
88
*
99
*
1010
* IDENTIFICATION
11-
* $PostgreSQL: pgsql/src/backend/optimizer/path/clausesel.c,v 1.89 2008/01/01 19:45:50 momjian Exp $
11+
* $PostgreSQL: pgsql/src/backend/optimizer/path/clausesel.c,v 1.90 2008/01/11 17:00:45 tgl Exp $
1212
*
1313
*-------------------------------------------------------------------------
1414
*/
@@ -428,7 +428,7 @@ clause_selectivity(PlannerInfo *root,
428428
intvarRelid,
429429
JoinTypejointype)
430430
{
431-
Selectivitys1=1.0;/* default for any unhandled clause type */
431+
Selectivitys1=0.5;/* default for any unhandled clause type */
432432
RestrictInfo*rinfo=NULL;
433433
boolcacheable= false;
434434

@@ -450,7 +450,7 @@ clause_selectivity(PlannerInfo *root,
450450
if (rinfo->pseudoconstant)
451451
{
452452
if (!IsA(rinfo->clause,Const))
453-
returns1;
453+
return(Selectivity)1.0;
454454
}
455455

456456
/*
@@ -517,9 +517,8 @@ clause_selectivity(PlannerInfo *root,
517517
{
518518
/*
519519
* XXX not smart about subquery references... any way to do
520-
* better?
520+
* better than default?
521521
*/
522-
s1=0.5;
523522
}
524523
else
525524
{
@@ -560,8 +559,7 @@ clause_selectivity(PlannerInfo *root,
560559
}
561560
else
562561
{
563-
/* XXX any way to do better? */
564-
s1= (Selectivity)0.5;
562+
/* XXX any way to do better than default? */
565563
}
566564
}
567565
elseif (not_clause(clause))
@@ -601,7 +599,7 @@ clause_selectivity(PlannerInfo *root,
601599
s1=s1+s2-s1*s2;
602600
}
603601
}
604-
elseif (is_opclause(clause))
602+
elseif (is_opclause(clause)||IsA(clause,DistinctExpr))
605603
{
606604
Oidopno= ((OpExpr*)clause)->opno;
607605
boolis_join_clause;
@@ -642,6 +640,15 @@ clause_selectivity(PlannerInfo *root,
642640
((OpExpr*)clause)->args,
643641
varRelid);
644642
}
643+
644+
/*
645+
* DistinctExpr has the same representation as OpExpr, but the
646+
* contained operator is "=" not "<>", so we must negate the result.
647+
* This estimation method doesn't give the right behavior for nulls,
648+
* but it's better than doing nothing.
649+
*/
650+
if (IsA(clause,DistinctExpr))
651+
s1=1.0-s1;
645652
}
646653
elseif (is_funcclause(clause))
647654
{
@@ -652,18 +659,15 @@ clause_selectivity(PlannerInfo *root,
652659
*/
653660
s1= (Selectivity)0.3333333;
654661
}
662+
#ifdefNOT_USED
655663
elseif (is_subplan(clause))
656664
{
657665
/*
658666
* Just for the moment! FIX ME! - vadim 02/04/98
659667
*/
660668
s1= (Selectivity)0.5;
661669
}
662-
elseif (IsA(clause,DistinctExpr))
663-
{
664-
/* can we do better? */
665-
s1= (Selectivity)0.5;
666-
}
670+
#endif
667671
elseif (IsA(clause,ScalarArrayOpExpr))
668672
{
669673
/* First, decide if it's a join clause, same as for OpExpr */

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