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Commita8fe109

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Fix thinko in hash cost estimation: average frequency

should be computed from total number of distinct values in wholerelation, not # distinct values we expect to have after restrictionclauses are applied.

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src/backend/optimizer/path
- costsize.c

1 file changed

-16

lines changed

`‎src/backend/optimizer/path/costsize.c`

Lines changed: 6 additions & 16 deletions

Original file line number	Diff line number	Diff line change
`@@ -42,7 +42,7 @@`
`42`	`42`	`* Portions Copyright (c) 1994, Regents of the University of California`
`43`	`43`	`*`
`44`	`44`	`* IDENTIFICATION`
`45`		`- * $Header: /cvsroot/pgsql/src/backend/optimizer/path/costsize.c,v 1.75 2001/06/05 05:26:04 tgl Exp $`
	`45`	`+ * $Header: /cvsroot/pgsql/src/backend/optimizer/path/costsize.c,v 1.76 2001/06/10 02:59:35 tgl Exp $`
`46`	`46`	`*`
`47`	`47`	`*-------------------------------------------------------------------------`
`48`	`48`	`*/`
`@@ -873,6 +873,9 @@ estimate_hash_bucketsize(Query root, Var var)`
`873`	`873`	`if (ndistinct<0.0)`
`874`	`874`	`ndistinct=-ndistinct*rel->tuples;`
`875`	`875`
	`876`	`+/* Also compute avg freq of all distinct data values in raw relation */`
	`877`	`+avgfreq= (1.0-stats->stanullfrac) /ndistinct;`
	`878`	`+`
`876`	`879`	`/*`
`877`	`880`	`* Adjust ndistinct to account for restriction clauses. Observe we are`
`878`	`881`	`* assuming that the data distribution is affected uniformly by the`
`@@ -883,17 +886,6 @@ estimate_hash_bucketsize(Query root, Var var)`
`883`	`886`	`*/`
`884`	`887`	`ndistinct *=rel->rows /rel->tuples;`
`885`	`888`
`886`		`-/*`
`887`		`- * Discourage use of hash join if there seem not to be very many distinct`
`888`		`- * data values. The threshold here is somewhat arbitrary, as is the`
`889`		`- * fraction used to "discourage" the choice.`
`890`		`- */`
`891`		`-if (ndistinct<50.0)`
`892`		`-{`
`893`		`-ReleaseSysCache(tuple);`
`894`		`-return0.5;`
`895`		`-}`
`896`		`-`
`897`	`889`	`/*`
`898`	`890`	`* Form initial estimate of bucketsize fraction. Here we use rel->rows,`
`899`	`891`	`* ie the number of rows after applying restriction clauses, because`
`@@ -903,8 +895,8 @@ estimate_hash_bucketsize(Query root, Var var)`
`903`	`895`	`estfract= (double)NTUP_PER_BUCKET /rel->rows;`
`904`	`896`
`905`	`897`	`/*`
`906`		`- * Adjust estimated bucketsize if too few distinct valuesto fill`
`907`		`- * all the buckets.`
	`898`	`+ * Adjust estimated bucketsize if too few distinct values(after`
	`899`	`+ *restriction clauses) to fillall the buckets.`
`908`	`900`	`*/`
`909`	`901`	`needdistinct=rel->rows / (double)NTUP_PER_BUCKET;`
`910`	`902`	`if (ndistinct<needdistinct)`
`@@ -931,8 +923,6 @@ estimate_hash_bucketsize(Query root, Var var)`
`931`	`923`	`/*`
`932`	`924`	`* Adjust estimated bucketsize upward to account for skewed distribution.`
`933`	`925`	`*/`
`934`		`-avgfreq= (1.0-stats->stanullfrac) /ndistinct;`
`935`		`-`
`936`	`926`	`if (avgfreq>0.0&&mcvfreq>avgfreq)`
`937`	`927`	`estfract *=mcvfreq /avgfreq;`
`938`	`928`

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Commita8fe109

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`‎src/backend/optimizer/path/costsize.c`

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