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Fix an oversight in the code that makes transitive-equality deductions from

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outer join clauses.  Given, say,
	... from a left join b on a.a1 = b.b1 where a.a1 = 42;
we'll deduce a clause b.b1 = 42 and then mark the original join clause
redundant (we can't remove it completely for reasons I don't feel like
squeezing into this log entry).  However the original implementation of
that wasn't bulletproof, because clause_selectivity() wouldn't honor
this_selec if given nonzero varRelid --- which in practice meant that
it worked as desired *except* when considering index scan quals.  Which
resulted in bogus underestimation of the size of the indexscan result for
an inner indexscan in an outer join, and consequently a possibly bad
choice of indexscan vs. bitmap scan.  Fix by introducing an explicit test
into clause_selectivity().  Also, to make sure we don't trigger that test
in corner cases, change the convention to be that this_selec > 1, not
this_selec = 1, means it's been marked redundant.  Per trouble report from
Scara Maccai.

Back-patch to 8.2, where the problem was introduced.
This commit is contained in:
Tom Lane
2008-12-01 21:06:31 +00:00
parent ad807ff65d
commit 4914b70847
4 changed files with 22 additions and 14 deletions
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8
src/backend/optimizer/path/clausesel.c
View File

@@ -8,7 +8,7 @@
*
*
* IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/optimizer/path/clausesel.c,v 1.82.2.1 2007/08/31 23:35:29 tgl Exp $
* $PostgreSQL: pgsql/src/backend/optimizer/path/clausesel.c,v 1.82.2.2 2008/12/01 21:06:30 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@@ -444,6 +444,12 @@ clause_selectivity(PlannerInfo *root,
return s1;
}
/*
* If the clause is marked redundant, always return 1.0.
*/
if (rinfo->this_selec > 1)
return (Selectivity) 1.0;
/*
* If possible, cache the result of the selectivity calculation for
* the clause. We can cache if varRelid is zero or the clause

20
src/backend/optimizer/path/pathkeys.c
View File

@@ -11,7 +11,7 @@
* Portions Copyright (c) 1994, Regents of the University of California
*
* IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/optimizer/path/pathkeys.c,v 1.79.2.1 2008/01/09 20:50:11 tgl Exp $
* $PostgreSQL: pgsql/src/backend/optimizer/path/pathkeys.c,v 1.79.2.2 2008/12/01 21:06:30 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@@ -428,11 +428,11 @@ sub_generate_join_implications(PlannerInfo *root,
* its sides to the same value. However, that fails in some
* corner cases where lower outer joins could cause one of the
* variables to go to NULL. (BUG in 8.2 through 8.2.6.)
* So now we just leave it in place, but mark it with selectivity
* 1.0 so that we don't underestimate the join size output ---
* So now we just leave it in place, but mark it as redundant
* so that we don't underestimate the join size output ---
* it's mostly redundant with the constant constraints.
*/
rinfo->this_selec = 1.0;
rinfo->this_selec = 2.0;
/*
* And recurse to see if we can deduce anything from INNERVAR =
@@ -471,11 +471,11 @@ sub_generate_join_implications(PlannerInfo *root,
* its sides to the same value. However, that fails in some
* corner cases where lower outer joins could cause one of the
* variables to go to NULL. (BUG in 8.2 through 8.2.6.)
* So now we just leave it in place, but mark it with selectivity
* 1.0 so that we don't underestimate the join size output ---
* So now we just leave it in place, but mark it as redundant
* so that we don't underestimate the join size output ---
* it's mostly redundant with the constant constraints.
*/
rinfo->this_selec = 1.0;
rinfo->this_selec = 2.0;
/*
* And recurse to see if we can deduce anything from INNERVAR =
@@ -551,15 +551,15 @@ sub_generate_join_implications(PlannerInfo *root,
* of its sides to the same value. However, that fails in
* some corner cases where lower outer joins could cause one
* of the variables to go to NULL. (BUG in 8.2 through
* 8.2.6.) So now we just leave it in place, but mark it with
* selectivity 1.0 so that we don't underestimate the join
* 8.2.6.) So now we just leave it in place, but mark it as
* redundant so that we don't underestimate the join
* size output --- it's mostly redundant with the constant
* constraints.
*
* Ideally we'd do that for the COALESCE() = CONSTANT rinfo,
* too, but we don't have easy access to that here.
*/
rinfo->this_selec = 1.0;
rinfo->this_selec = 2.0;
/*
* And recurse to see if we can deduce anything from LEFTVAR =