mirror/postgresql
2
0
Fork 0
mirror of https://git.postgresql.org/git/postgresql.git synced 2024-12-27 08:39:28 +08:00
Code Issues Packages Projects Releases Wiki Activity

Add a fudge factor to genericcostestimate() to prevent the planner from

Browse Source 
thinking that indexes of different sizes are equally attractive.  Per
gripe from Jim Nasby.  (I remain unconvinced that there's such a problem
in existing releases, but CVS HEAD definitely has got a problem because
of its new count-only-leaf-pages approach to indexscan costing.)
This commit is contained in:
Tom Lane 2006-07-24 01:19:48 +00:00
parent a794fb0681
commit 8dcaea7be0
1 changed files with 19 additions and 1 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

20
src/backend/utils/adt/selfuncs.c
View File

@ -15,7 +15,7 @@
*
*
* IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/utils/adt/selfuncs.c,v 1.209 2006/07/14 14:52:24 momjian Exp $
* $PostgreSQL: pgsql/src/backend/utils/adt/selfuncs.c,v 1.210 2006/07/24 01:19:48 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@ -4657,6 +4657,24 @@ genericcostestimate(PlannerInfo *root,
*indexTotalCost = numIndexPages * random_page_cost;
}
/*
* A difficulty with the leaf-pages-only cost approach is that for
* small selectivities (eg, single index tuple fetched) all indexes
* will look equally attractive because we will estimate exactly 1
* leaf page to be fetched. All else being equal, we should prefer
* physically smaller indexes over larger ones. (An index might be
* smaller because it is partial or because it contains fewer columns;
* presumably the other columns in the larger index aren't useful to
* the query, or the larger index would have better selectivity.)
*
* We can deal with this by adding a very small "fudge factor" that
* depends on the index size. The fudge factor used here is one
* random_page_cost per 100000 index pages, which should be small
* enough to not alter index-vs-seqscan decisions, but will prevent
* indexes of different sizes from looking exactly equally attractive.
*/
*indexTotalCost += index->pages * random_page_cost / 100000.0;
/*
* CPU cost: any complex expressions in the indexquals will need to be
* evaluated once at the start of the scan to reduce them to runtime keys