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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 introduced
this silliness due to misreading the intent of the very-poorly-structured
code that was there when we inherited the file from Berkeley.  The lack
of 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 bit
hesitant to introduce a possible planner behavior change in the back
branches; it might detune queries that worked acceptably in the past.)

While at it, make estimation for DistinctExpr do something marginally
realistic, rather than just defaulting.
This commit is contained in:
Tom Lane 2008-01-11 17:00:45 +00:00
parent f3e3f2e158
commit df62977d00
1 changed files with 17 additions and 13 deletions
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30
src/backend/optimizer/path/clausesel.c
View File

@ -8,7 +8,7 @@
*
*
* IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/optimizer/path/clausesel.c,v 1.89 2008/01/01 19:45:50 momjian Exp $
* $PostgreSQL: pgsql/src/backend/optimizer/path/clausesel.c,v 1.90 2008/01/11 17:00:45 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@ -428,7 +428,7 @@ clause_selectivity(PlannerInfo *root,
int varRelid,
JoinType jointype)
{
Selectivity s1 = 1.0; /* default for any unhandled clause type */
Selectivity s1 = 0.5; /* default for any unhandled clause type */
RestrictInfo *rinfo = NULL;
bool cacheable = false;
@ -450,7 +450,7 @@ clause_selectivity(PlannerInfo *root,
if (rinfo->pseudoconstant)
{
if (!IsA(rinfo->clause, Const))
return s1;
return (Selectivity) 1.0;
}
/*
@ -517,9 +517,8 @@ clause_selectivity(PlannerInfo *root,
{
/*
* XXX not smart about subquery references... any way to do
* better?
* better than default?
*/
s1 = 0.5;
}
else
{
@ -560,8 +559,7 @@ clause_selectivity(PlannerInfo *root,
}
else
{
/* XXX any way to do better? */
s1 = (Selectivity) 0.5;
/* XXX any way to do better than default? */
}
}
else if (not_clause(clause))
@ -601,7 +599,7 @@ clause_selectivity(PlannerInfo *root,
s1 = s1 + s2 - s1 * s2;
}
}
else if (is_opclause(clause))
else if (is_opclause(clause) || IsA(clause, DistinctExpr))
{
Oid opno = ((OpExpr *) clause)->opno;
bool is_join_clause;
@ -642,6 +640,15 @@ clause_selectivity(PlannerInfo *root,
((OpExpr *) clause)->args,
varRelid);
}
/*
* DistinctExpr has the same representation as OpExpr, but the
* contained operator is "=" not "<>", so we must negate the result.
* This estimation method doesn't give the right behavior for nulls,
* but it's better than doing nothing.
*/
if (IsA(clause, DistinctExpr))
s1 = 1.0 - s1;
}
else if (is_funcclause(clause))
{
@ -652,6 +659,7 @@ clause_selectivity(PlannerInfo *root,
*/
s1 = (Selectivity) 0.3333333;
}
#ifdef NOT_USED
else if (is_subplan(clause))
{
/*
@ -659,11 +667,7 @@ clause_selectivity(PlannerInfo *root,
*/
s1 = (Selectivity) 0.5;
}
else if (IsA(clause, DistinctExpr))
{
/* can we do better? */
s1 = (Selectivity) 0.5;
}
#endif
else if (IsA(clause, ScalarArrayOpExpr))
{
/* First, decide if it's a join clause, same as for OpExpr */