Subject: Re: Searching for the cause of a bad plan
From: Simon Riggs <simon@2ndquadrant.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Csaba Nagy <nagy@ecircle-ag.com>,
	postgres performance list <pgsql-performance@postgresql.org>
In-Reply-To: <6175.1190390913@sss.pgh.pa.us>
References: <1190369024.4661.173.camel@PCD12478>
	<6175.1190390913@sss.pgh.pa.us>
Content-Type: text/plain
Date: Fri, 21 Sep 2007 18:39:12 +0100
Message-Id: <1190396352.4202.109.camel@ebony.site>
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On Fri, 2007-09-21 at 12:08 -0400, Tom Lane wrote:
> Csaba Nagy <nagy@ecircle-ag.com> writes:
> > Looking at Plan 2, it looks like the "limit" step is estimating wrongly
> > it's cost.
> 
> The reason you get a bad plan is that this rowcount estimate is so far
> off:

That's true, but its not relevant, since the query would still be fast
even if that estimate was exactly right. With LIMIT 10, it wouldn't
matter how many rows were there as long as there were more than 10. The
true execution cost is limited, the cost model is not.

-- 
  Simon Riggs
  2ndQuadrant  http://www.2ndQuadrant.com