Date: Wed, 18 Oct 2006 17:35:51 -0500
From: "Jim C. Nasby" <jim@nasby.net>
To: Jeff Davis <pgsql@j-davis.com>
Cc: Dimitri Fontaine <dim@dalibo.com>, pgsql-performance@postgresql.org,
	Ioana Danes <ioanasoftware@yahoo.ca>
Subject: Re: Postgresql 8.1.4 - performance issues for select on
Message-ID: <20061018223551.GD56874@nasby.net>
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	<200610182319.41920.dim@dalibo.com>
	<1161207230.31645.266.camel@dogma.v10.wvs>
	<20061018221046.GG85041@nasby.net>
	<1161210735.31645.277.camel@dogma.v10.wvs>
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On Wed, Oct 18, 2006 at 03:32:15PM -0700, Jeff Davis wrote:
> On Wed, 2006-10-18 at 17:10 -0500, Jim C. Nasby wrote:
> > Sorry, don't have the earlier part of this thread, but what about...
> > 
> > SELECT greatest(max(a), max(b)) ...
> > 
> > ?
> 
> To fill you in, we're trying to get the max of a union (a view across
> two physical tables).

UNION or UNION ALL? You definitely don't want to do a plain UNION if you
can possibly avoid it.

> It can be done if you're creative with the query; I suggested a query
> that selected the max of the max()es of the individual tables. Your
> query could work too. However, the trick would be getting postgresql to
> recognize that it can transform "SELECT max(x) FROM foo" into that,
> where foo is a view of a union.
> 
> If PostgreSQL could sort the result of a union by merging the results of
> two index scans, I think the problem would be solved. Is there something
> preventing this, or is it just something that needs to be added to the
> planner?

Hrm... it'd be worth trying the old ORDER BY ... LIMIT 1 trick just to
see if that worked in this case, but I don't have much hope for that.
-- 
Jim Nasby                                            jim@nasby.net
EnterpriseDB      http://enterprisedb.com      512.569.9461 (cell)