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Fix mergejoin cost estimation so that we consider the statistical ranges of

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the two join variables at both ends: not only trailing rows that need not be
scanned because there cannot be a match on the other side, but initial rows
that will be scanned without possibly having a match.  This allows a more
realistic estimate of startup cost to be made, per recent pgsql-performance
discussion.  In passing, fix a couple of bugs that had crept into
mergejoinscansel: it was not quite up to speed for the task of estimating
descending-order scans, which is a new requirement in 8.3.
This commit is contained in:
Tom Lane
2007-12-08 21:05:11 +00:00
parent 8821612854
commit 9fd8843647
4 changed files with 347 additions and 150 deletions
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8
src/include/nodes/relation.h
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@ -7,7 +7,7 @@
* Portions Copyright (c) 1996-2007, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
* $PostgreSQL: pgsql/src/include/nodes/relation.h,v 1.150 2007/11/15 22:25:17 momjian Exp $
* $PostgreSQL: pgsql/src/include/nodes/relation.h,v 1.151 2007/12/08 21:05:11 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@ -993,8 +993,10 @@ typedef struct MergeScanSelCache
int strategy; /* sort direction (ASC or DESC) */
bool nulls_first; /* do NULLs come before normal values? */
/* Results */
Selectivity leftscansel; /* scan fraction for clause left side */
Selectivity rightscansel; /* scan fraction for clause right side */
Selectivity leftstartsel; /* first-join fraction for clause left side */
Selectivity leftendsel; /* last-join fraction for clause left side */
Selectivity rightstartsel; /* first-join fraction for clause right side */
Selectivity rightendsel; /* last-join fraction for clause right side */
} MergeScanSelCache;
/*

6
src/include/utils/selfuncs.h
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@ -8,7 +8,7 @@
* Portions Copyright (c) 1996-2007, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
* $PostgreSQL: pgsql/src/include/utils/selfuncs.h,v 1.41 2007/11/07 22:37:24 tgl Exp $
* $PostgreSQL: pgsql/src/include/utils/selfuncs.h,v 1.42 2007/12/08 21:05:11 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@ -161,8 +161,8 @@ extern Selectivity rowcomparesel(PlannerInfo *root,
extern void mergejoinscansel(PlannerInfo *root, Node *clause,
Oid opfamily, int strategy, bool nulls_first,
Selectivity *leftscan,
Selectivity *rightscan);
Selectivity *leftstart, Selectivity *leftend,
Selectivity *rightstart, Selectivity *rightend);
extern double estimate_num_groups(PlannerInfo *root, List *groupExprs,
double input_rows);