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Commit8dcaea7

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Add a fudge factor to genericcostestimate() to prevent the planner from
thinking that indexes of different sizes are equally attractive. Pergripe from Jim Nasby. (I remain unconvinced that there's such a problemin existing releases, but CVS HEAD definitely has got a problem becauseof its new count-only-leaf-pages approach to indexscan costing.)
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‎src/backend/utils/adt/selfuncs.c

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

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