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To: Claudio Freire <klaussfreire@gmail.com>
cc: Kevin Grittner <Kevin.Grittner@wicourts.gov>,
 Ogden <lists@darkstatic.com>,
	Tomas Vondra <tv@fuzzy.cz>, pgsql-performance@postgresql.org
Subject: Re: Performance 
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Comments: In-reply-to Claudio Freire <klaussfreire@gmail.com>
	message dated "Wed, 13 Apr 2011 23:17:18 +0200"
Date: Wed, 13 Apr 2011 17:52:36 -0400
Message-ID: <25734.1302731556@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>
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Claudio Freire <klaussfreire@gmail.com> writes:
> On Wed, Apr 13, 2011 at 4:32 PM, Kevin Grittner
> <Kevin.Grittner@wicourts.gov> wrote:
>> If you model the costing to reflect the reality on your server, good
>> plans will be chosen.

> Wouldn't it be "better" to derive those costs from actual performance
> data measured at runtime?

> Say, pg could measure random/seq page cost, *per tablespace* even.

> Has that been tried?

Getting numbers that mean much of anything is a slow, expensive
process.  You really don't want the database trying to do that for you.
Once you've got them, you *really* don't want the database
editorializing on them.

			regards, tom lane