To: "Steinar H. Gunderson" <sgunderson@bigfoot.com>
Cc: pgsql-performance@postgresql.org
Subject: Re: Weird performance drop after VACUUM 
In-reply-to: <20050827101945.GA18155@uio.no> 
References: <1125093859.9329.30.camel@localhost.localdomain>
	<26745.1125099111@sss.pgh.pa.us> <20050827101945.GA18155@uio.no>
Comments: In-reply-to "Steinar H. Gunderson" <sgunderson@bigfoot.com>
	message dated "Sat, 27 Aug 2005 12:19:45 +0200"
Date: Sat, 27 Aug 2005 11:05:01 -0400
Message-ID: <1404.1125155101@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

"Steinar H. Gunderson" <sgunderson@bigfoot.com> writes:
> On Fri, Aug 26, 2005 at 07:31:51PM -0400, Tom Lane wrote:
>> Or you could just play with the order of the filter conditions ... for
>> example, the date condition at the end is probably far cheaper to test
>> than the text comparisons, so if that's fairly selective it'd be worth
>> putting it first.

> That's an interesting approach -- could the planner do such things itself?

It could, but it doesn't really have enough information.  We don't
currently have any model that some operators are more expensive than
others.  IIRC the only sort of reordering the current code will do
in a filter condition list is to push clauses involving sub-SELECTs
to the end.

			regards, tom lane