NotificationsYou must be signed in to change notification settings
Fork6
Star31

Commita8fe109

committed

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.

1 parent26c94c5 commita8fe109Copy full SHA for a8fe109

File tree

1 file changed

-16

lines changed

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`

0 commit comments

Comments

(0)

Movatterモバイル変換

Navigation Menu

Search code, repositories, users, issues, pull requests...

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

Uh oh!

Commita8fe109

File tree

1 file changed

1 file changed

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

0 commit comments