Message-Id: <1168986053.16393.1169598917@webmail.messagingengine.com>
From: "Jeremy Haile" <jhaile@fastmail.fm>
To: "Tom Lane" <tgl@sss.pgh.pa.us>
Cc: pgsql-performance@postgresql.org
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	<26412.1168983547@sss.pgh.pa.us>
Subject: Re: PG8.2.1 choosing slow seqscan over idx scan
In-Reply-To: <26412.1168983547@sss.pgh.pa.us>
Date: Tue, 16 Jan 2007 17:20:53 -0500

Thanks Tom!  Reducing random_page_cost to 2 did the trick for this
query.  It now favors the index scan.

Even if this is a cached situation, I wouldn't expect a difference of 3
min vs 3 seconds.   

Even if unrelated, do you think disk fragmentation would have negative
effects?  Is it worth trying to defragment the drive on a regular basis
in Windows?

Jeremy Haile


On Tue, 16 Jan 2007 16:39:07 -0500, "Tom Lane" <tgl@sss.pgh.pa.us> said:
> "Jeremy Haile" <jhaile@fastmail.fm> writes:
> > Running PostgreSQL 8.2.1 on Win32.   The query planner is choosing a seq
> > scan over index scan even though index scan is faster (as shown by
> > disabling seqscan).  Table is recently analyzed and row count estimates
> > seem to be in the ballpark.
> 
> Try reducing random_page_cost a bit.  Keep in mind that you are probably
> measuring a fully-cached situation here, if you repeated the test case.
> If your database fits into memory reasonably well then that's fine and
> you want to optimize for that case ... but otherwise you may find
> yourself pessimizing the actual behavior.
> 
> 			regards, tom lane