Date: Tue, 28 Jun 2005 17:16:41 +0000
From: "Karl O. Pinc" <kop@meme.com>
Subject: Re: Poor index choice -- multiple indexes of the same
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: pgsql-performance@postgresql.org
References: <1119913766l.4428l.2l@mofo> <11497.1119940856@sss.pgh.pa.us>
In-Reply-To: <11497.1119940856@sss.pgh.pa.us> (from tgl@sss.pgh.pa.us on Tue
	Jun 28 01:40:56 2005)
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On 06/28/2005 01:40:56 AM, Tom Lane wrote:
> "Karl O. Pinc" <kop@meme.com> writes:
> > I have a query
> 
> > select 1
> >   from census
> >   where date < '1975-9-21' and sname = 'RAD' and status != 'A'
> >   limit 1;
> 
> > Explain analyze says it always uses the index made by:
> 
> >    CREATE INDEX census_date_sname ON census (date, sname);
> 
> > this is even after I made the index:
> 
> >    CREATE INDEX census_sname_date ON census (sname, date);
> 
> I don't believe that any existing release can tell the difference
> between these two indexes as far as costs go.  I just recently
> added some code to btcostestimate that would cause it to prefer
> the index on (sname, date) but of course that's not released yet.
> 
> However: isn't the above query pretty seriously underspecified?
> With a LIMIT and no ORDER BY, you are asking for a random one
> of the rows matching the condition.  I realize that with
> "select 1" you may not care much, but adding a suitable ORDER BY
> would help push the planner towards using the right index.  In
> this case "ORDER BY sname DESC, date DESC" would probably do the
> trick.

Yes, that works.  I'd already tried "ORDER BY date DESC", before
I first wrote, and that did not work.  (I started with no LIMIT
either, and tried adding specifications until I gave up.  It's
very good that the new planner will figure out things by itself.)
"ORDER BY sname DESC" works as well.  This is a
bit odd, as with the constant in the = comparison "ORDER BY date
DESC" is the same as "ORDER BY sname DESC, date DESC".
I guess that's why I gave up on my attempts to get the planner
to use the (sname, date) index before I got to your solution.

Thanks everybody for the help.


Karl <kop@meme.com>
Free Software:  "You don't pay back, you pay forward."
                  -- Robert A. Heinlein