Received: from localhost (unknown [200.46.204.183])
	by mail.postgresql.org (Postfix) with ESMTP id BB27D650452
	for <pgsql-performance-postgresql.org@mail.postgresql.org>;
	Tue, 11 Nov 2008 22:34:57 -0400 (AST)
Received: from mail.postgresql.org ([200.46.204.86])
	by localhost (mx1.hub.org [200.46.204.183]) (amavisd-maia, port 10024)
	with ESMTP id 27203-03
	for <pgsql-performance-postgresql.org@mail.postgresql.org>;
	Tue, 11 Nov 2008 22:34:54 -0400 (AST)
X-Greylist: from auto-whitelisted by SQLgrey-1.7.6
Received: from sss.pgh.pa.us (sss.pgh.pa.us [66.207.139.130])
	by mail.postgresql.org (Postfix) with ESMTP id F386D64FC2D
	for <pgsql-performance@postgresql.org>;
	Tue, 11 Nov 2008 22:34:54 -0400 (AST)
Received: from sss2.sss.pgh.pa.us (tgl@localhost [127.0.0.1])
	by sss.pgh.pa.us (8.14.2/8.14.2) with ESMTP id mAC2YoSc021847;
	Tue, 11 Nov 2008 21:34:50 -0500 (EST)
To: "Jim 'Decibel!' Nasby" <jnasby@cashnetusa.com>
cc: Richard Huxton <dev@archonet.com>,
	postgresql performance list <pgsql-performance@postgresql.org>
Subject: Re: Oddity with view (now with test case) 
In-reply-to: <1F9C31B6-6F89-4BD9-8799-2DA4DF254A48@cashnetusa.com> 
References: <3CF0CFC1-FDBA-4A61-ACAA-6AF556C4C6F4@cashnetusa.com>
	<3128.1226322412@sss.pgh.pa.us>
	<7B598355-07C0-450A-9914-B5979F48A5B3@cashnetusa.com>
	<49187BA7.8020403@archonet.com>
	<1E5A4D7D-3B95-440B-A377-B6B9F8768F7A@cashnetusa.com>
	<12776.1226345499@sss.pgh.pa.us>
	<C20FA82B-9FF8-4384-BC98-3D3D84001E17@cashnetusa.com>
	<9474.1226373609@sss.pgh.pa.us>
	<03B6DDDD-C1E9-4897-864B-46892FCBE379@cashnetusa.com>
	<10664.1226430946@sss.pgh.pa.us>
	<1F9C31B6-6F89-4BD9-8799-2DA4DF254A48@cashnetusa.com>
Comments: In-reply-to "Jim 'Decibel!' Nasby" <jnasby@cashnetusa.com>
	message dated "Tue, 11 Nov 2008 16:42:57 -0600"
Date: Tue, 11 Nov 2008 21:34:50 -0500
Message-ID: <21846.1226457290@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>
X-Virus-Scanned: Maia Mailguard 1.0.1
X-Spam-Status: No, hits=0 tagged_above=0 required=5 tests=none
X-Spam-Level: 
X-Archive-Number: 200811/85
X-Sequence-Number: 31816

"Jim 'Decibel!' Nasby" <jnasby@cashnetusa.com> writes:
> So here's something odd... in both 8.3 and HEAD from a while ago it  
> gives a better plan for SELECT * than for SELECT count(*):

The short answer is that the Subquery Scan nodes can be dropped out
when they are no-ops, which is to say producing the same set of columns
their input produces (and not testing any filter conditions, but that's
not relevant here).  SELECT count(*) doesn't want to know about any
columns so the output of the UNION arm doesn't match ...

			regards, tom lane