From: "Matt Clark" <matt@ymogen.net>
To: "Tom Lane" <tgl@sss.pgh.pa.us>
Cc: <pgsql-admin@postgresql.org>
Subject: Re: Cost estimates consistently too high - does it matter? 
Date: Fri, 8 Aug 2003 16:38:55 +0100
Message-ID: <OAEAKHEHCMLBLIDGAFELEEGIDGAA.matt@ymogen.net>
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Well, I usually am under a misapprehension!  Thanks for the explanation about LIMIT too.

In that case then, I shall stop worrying and learn to love the planner.

M

> -----Original Message-----
> From: Tom Lane [mailto:tgl@sss.pgh.pa.us]
> Sent: 08 August 2003 16:15
> To: Matt Clark
> Cc: pgsql-admin@postgresql.org
> Subject: Re: [ADMIN] Cost estimates consistently too high - does it
> matter? 
> 
> 
> "Matt Clark" <matt@ymogen.net> writes:
> > I've noticed that the cost estimates for a lot of my queries are
> > consistently far to high.
> 
> You seem to be under a misapprehension.  The cost estimates are not
> in units of milliseconds, they are on an arbitrary scale with 1.0
> defined as one disk fetch.
> 
> LIMIT throws another monkey wrench into the mix: the estimates for the
> plan nodes underneath the limit are done as if the plan were to be
> executed to completion, which of course it won't be.
> 
> 			regards, tom lane
> 
>