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Add a little more smarts to estimate_hash_bucketsize(): if there's no

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statistics, but there is a unique index on the column, we can safely
assume it's well-distributed.
This commit is contained in:
Tom Lane
2003-10-05 22:44:25 +00:00
parent 22347dc102
commit a1dcd8f6dd
1 changed files with 9 additions and 1 deletions
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10
src/backend/optimizer/path/costsize.c
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@ -49,7 +49,7 @@
* Portions Copyright (c) 1994, Regents of the University of California * Portions Copyright (c) 1994, Regents of the University of California
* *
* IDENTIFICATION * IDENTIFICATION
* $Header: /cvsroot/pgsql/src/backend/optimizer/path/costsize.c,v 1.114 2003/08/08 21:41:44 momjian Exp $ * $Header: /cvsroot/pgsql/src/backend/optimizer/path/costsize.c,v 1.115 2003/10/05 22:44:25 tgl Exp $
* *
*------------------------------------------------------------------------- *-------------------------------------------------------------------------
*/ */
@ -64,6 +64,7 @@
#include "optimizer/clauses.h" #include "optimizer/clauses.h"
#include "optimizer/cost.h" #include "optimizer/cost.h"
#include "optimizer/pathnode.h" #include "optimizer/pathnode.h"
#include "optimizer/plancat.h"
#include "parser/parsetree.h" #include "parser/parsetree.h"
#include "utils/selfuncs.h" #include "utils/selfuncs.h"
#include "utils/lsyscache.h" #include "utils/lsyscache.h"
@ -1343,6 +1344,13 @@ estimate_hash_bucketsize(Query *root, Var *var, int nbuckets)
0, 0); 0, 0);
if (!HeapTupleIsValid(tuple)) if (!HeapTupleIsValid(tuple))
{ {
/*
* If the attribute is known unique because of an index,
* we can treat it as well-distributed.
*/
if (has_unique_index(rel, var->varattno))
return 1.0 / (double) nbuckets;
/* /*
* Perhaps the Var is a system attribute; if so, it will have no * Perhaps the Var is a system attribute; if so, it will have no
* entry in pg_statistic, but we may be able to guess something * entry in pg_statistic, but we may be able to guess something