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To: "Jim C. Nasby" <jnasby@pervasive.com>
Cc: pgsql-performance@postgresql.org
Subject: Re: planner with index scan cost way off actual cost,
	advices to tweak cost constants?
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From: Guillaume Cottenceau <gc@mnc.ch>
Date: 21 Mar 2006 14:30:22 +0100
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"Jim C. Nasby" <jnasby 'at' pervasive.com> writes:

> If you feel like running some tests, you need to change
> 
>     run_cost += max_IO_cost + csquared * (min_IO_cost - max_IO_cost);
> 
> in src/backend/optimizer/path/costsize.c to something like
> 
>     run_cost += max_IO_cost + abs(indexCorrelation) * (min_IO_cost - max_IO_cost);

Short after the beginning of a discussion about planner
associating too high cost for index scan, I'm suggested to change
source-code.. I'm already frightened about the near future :)

> That might not produce a perfect cost estimate, but I'll wager that it
> will be substantially better than what's in there now. FYI, see also
> http://archives.postgresql.org/pgsql-performance/2005-04/msg00669.php

Sad that Tom didn't share his thoughts about your cost algorithm
question in this message.

-- 
Guillaume Cottenceau