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Re: simple join uses indexes, very slow

From: "Jim C(dot) Nasby" <jnasby(at)pervasive(dot)com>
To: pgsql-performance(at)postgresql(dot)org
Subject: Re: simple join uses indexes, very slow
Date: 2006-03-28 17:30:47
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-performance
On Tue, Mar 28, 2006 at 06:29:08PM +0200, Steinar H. Gunderson wrote:
> On Tue, Mar 28, 2006 at 10:18:25AM -0600, Dave Dutcher wrote:
> >>     "parameters_idx" btree (run, opset_num, step_num, opset,
> > opset_ver,
> >> step, step_ver, name, split, wafers)
> >>     "parameters_opset_idx" btree (opset, step, name)
> >>     "parameters_step_idx" btree (step, name)
> > Have you tried creating some different indexes on parameters?  I don't
> > know if it should matter or not, but I would try some indexes like:
> > 
> > (run, opset_num) //Without all the other columns
> > (opset_num, run) //Backwards
> > (opset_num)
> An index on (A,B,C) can be used for a query on (A,B) or (A), so it doesn't
> really matter. It isn't usable for a query on (B), (C) or (B,C), though. (The
> index rows will get bigger, of course, so you'll need more I/O if you want to
> scan large parts of it, but I guess that's beside the point.)

Note that given how statistics currenly work, there are many situations
where the planner will refuse to use a multi-column index. This probably
won't change until there's some concept of multi-column statistics, at
least for multi-column indexes.
Jim C. Nasby, Sr. Engineering Consultant      jnasby(at)pervasive(dot)com
Pervasive Software    work: 512-231-6117
vcard:       cell: 512-569-9461

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