From: Gregory Stark <stark@enterprisedb.com>
To: "Karl Wright" <kwright@metacarta.com>
Cc: "Tom Lane" <tgl@sss.pgh.pa.us>,  <pgsql-performance@postgresql.org>
Subject: Re: Performance query about large tables, lots of concurrent access
In-Reply-To: <4677ECA9.3000402@metacarta.com> (Karl Wright's message of "Tue,
	19 Jun 2007 10:48:09 -0400")
Organization: EnterpriseDB
References: <4676DD57.7070304@metacarta.com> <14291.1182221811@sss.pgh.pa.us>
	<4677B831.8060704@metacarta.com> <27690.1182263794@sss.pgh.pa.us>
	<4677ECA9.3000402@metacarta.com>
Date: Tue, 19 Jun 2007 16:46:20 +0100
Message-ID: <873b0o7xrn.fsf@oxford.xeocode.com>
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"Karl Wright" <kwright@metacarta.com> writes:

>> In this case it looks like the planner is afraid that that's exactly
>> what will happen --- a cost of 14177 suggests that several thousand row
>> fetches are expected to happen, and yet it's only predicting 5 rows out
>> after the filter.  It's using this plan anyway because it has no better
>> alternative, but you should think about whether a different index
>> definition would help.

Another index won't help if the reason the cost is so high isn't because the
index isn't very selective but because there are lots of dead tuples.

-- 
  Gregory Stark
  EnterpriseDB          http://www.enterprisedb.com