Subject: Re: Postgresql 8.1.4 - performance issues for select on
From: Jeff Davis <pgsql@j-davis.com>
To: "Jim C. Nasby" <jim@nasby.net>
Cc: Dimitri Fontaine <dim@dalibo.com>, pgsql-performance@postgresql.org,
	Ioana Danes <ioanasoftware@yahoo.ca>
In-Reply-To: <20061018221046.GG85041@nasby.net>
References: <20061018210201.56435.qmail@web55907.mail.re3.yahoo.com>
	<200610182319.41920.dim@dalibo.com>
	<1161207230.31645.266.camel@dogma.v10.wvs>
	<20061018221046.GG85041@nasby.net>
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Date: Wed, 18 Oct 2006 15:32:15 -0700
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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).

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?

Regards,
	Jeff Davis