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Adjust cost_nonsequential_access() to have more reasonable behavior

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when random_page_cost has a small value.  Per Manfred Koizar, though
I didn't use his equation exactly.
This commit is contained in:
Tom Lane 2004-06-10 21:02:00 +00:00
parent 387b38bec4
commit 3485cc3a7c
1 changed files with 9 additions and 8 deletions
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17
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
* $PostgreSQL: pgsql/src/backend/optimizer/path/costsize.c,v 1.130 2004/06/05 01:55:04 tgl Exp $ * $PostgreSQL: pgsql/src/backend/optimizer/path/costsize.c,v 1.131 2004/06/10 21:02:00 tgl Exp $
* *
*------------------------------------------------------------------------- *-------------------------------------------------------------------------
*/ */
@ -189,11 +189,12 @@ cost_seqscan(Path *path, Query *root,
* for now by assuming we are given an effective_cache_size parameter. * for now by assuming we are given an effective_cache_size parameter.
* *
* Given a guesstimated cache size, we estimate the actual I/O cost per page * Given a guesstimated cache size, we estimate the actual I/O cost per page
* with the entirely ad-hoc equations: * with the entirely ad-hoc equations (writing relsize for
* if relpages >= effective_cache_size: * relpages/effective_cache_size):
* random_page_cost * (1 - (effective_cache_size/relpages)/2) * if relsize >= 1:
* if relpages < effective_cache_size: * random_page_cost - (random_page_cost-1)/2 * (1/relsize)
* 1 + (random_page_cost/2-1) * (relpages/effective_cache_size) ** 2 * if relsize < 1:
* 1 + ((random_page_cost-1)/2) * relsize ** 2
* These give the right asymptotic behavior (=> 1.0 as relpages becomes * These give the right asymptotic behavior (=> 1.0 as relpages becomes
* small, => random_page_cost as it becomes large) and meet in the middle * small, => random_page_cost as it becomes large) and meet in the middle
* with the estimate that the cache is about 50% effective for a relation * with the estimate that the cache is about 50% effective for a relation
@ -213,9 +214,9 @@ cost_nonsequential_access(double relpages)
relsize = relpages / effective_cache_size; relsize = relpages / effective_cache_size;
if (relsize >= 1.0) if (relsize >= 1.0)
return random_page_cost * (1.0 - 0.5 / relsize); return random_page_cost - (random_page_cost - 1.0) * 0.5 / relsize;
else else
return 1.0 + (random_page_cost * 0.5 - 1.0) * relsize * relsize; return 1.0 + (random_page_cost - 1.0) * 0.5 * relsize * relsize;
} }
/* /*