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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 21:06:13 +00:00
parent 7fb27531e8
commit a1feb90ef3
4 changed files with 16 additions and 8 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

3
src/backend/nodes/outfuncs.c
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@ -8,7 +8,7 @@
*
*
* IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/nodes/outfuncs.c,v 1.345 2008/11/15 19:43:46 tgl Exp $
* $PostgreSQL: pgsql/src/backend/nodes/outfuncs.c,v 1.346 2008/12/01 21:06:12 tgl Exp $
*
* NOTES
* Every node type that can appear in stored rules' parsetrees *must*
@ -1563,6 +1563,7 @@ _outRestrictInfo(StringInfo str, RestrictInfo *node)
WRITE_BITMAPSET_FIELD(right_relids);
WRITE_NODE_FIELD(orclause);
/* don't write parent_ec, leads to infinite recursion in plan tree dump */
WRITE_FLOAT_FIELD(this_selec, "%.4f");
WRITE_NODE_FIELD(mergeopfamilies);
/* don't write left_ec, leads to infinite recursion in plan tree dump */
/* don't write right_ec, leads to infinite recursion in plan tree dump */

8
src/backend/optimizer/path/clausesel.c
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@ -8,7 +8,7 @@
*
*
* IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/optimizer/path/clausesel.c,v 1.94 2008/10/04 21:56:53 tgl Exp $
* $PostgreSQL: pgsql/src/backend/optimizer/path/clausesel.c,v 1.95 2008/12/01 21:06:13 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@ -513,6 +513,12 @@ clause_selectivity(PlannerInfo *root,
return (Selectivity) 1.0;
}
/*
* 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

8
src/backend/optimizer/path/equivclass.c
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@ -10,7 +10,7 @@
* Portions Copyright (c) 1994, Regents of the University of California
*
* IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/optimizer/path/equivclass.c,v 1.13 2008/10/21 20:42:52 tgl Exp $
* $PostgreSQL: pgsql/src/backend/optimizer/path/equivclass.c,v 1.14 2008/12/01 21:06:13 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@ -1198,7 +1198,7 @@ reconsider_outer_join_clauses(PlannerInfo *root)
list_delete_cell(root->left_join_clauses, cell, prev);
/* we throw it back anyway (see notes above) */
/* but the thrown-back clause has no extra selectivity */
rinfo->this_selec = 1.0;
rinfo->this_selec = 2.0;
distribute_restrictinfo_to_rels(root, rinfo);
}
else
@ -1220,7 +1220,7 @@ reconsider_outer_join_clauses(PlannerInfo *root)
list_delete_cell(root->right_join_clauses, cell, prev);
/* we throw it back anyway (see notes above) */
/* but the thrown-back clause has no extra selectivity */
rinfo->this_selec = 1.0;
rinfo->this_selec = 2.0;
distribute_restrictinfo_to_rels(root, rinfo);
}
else
@ -1242,7 +1242,7 @@ reconsider_outer_join_clauses(PlannerInfo *root)
list_delete_cell(root->full_join_clauses, cell, prev);
/* we throw it back anyway (see notes above) */
/* but the thrown-back clause has no extra selectivity */
rinfo->this_selec = 1.0;
rinfo->this_selec = 2.0;
distribute_restrictinfo_to_rels(root, rinfo);
}
else

5
src/include/nodes/relation.h
View File

@ -7,7 +7,7 @@
* Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
* $PostgreSQL: pgsql/src/include/nodes/relation.h,v 1.164 2008/11/11 18:13:32 tgl Exp $
* $PostgreSQL: pgsql/src/include/nodes/relation.h,v 1.165 2008/12/01 21:06:13 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@ -991,7 +991,8 @@ typedef struct RestrictInfo
/* cache space for cost and selectivity */
QualCost eval_cost; /* eval cost of clause; -1 if not yet set */
Selectivity this_selec; /* selectivity; -1 if not yet set */
Selectivity this_selec; /* selectivity; -1 if not yet set; >1 means
* a redundant clause */
/* valid if clause is mergejoinable, else NIL */
List *mergeopfamilies; /* opfamilies containing clause operator */