From: Gaetano Mendola <mendola@gmail.com>
Subject: Useless sort by
Date: Mon, 13 Sep 2010 11:47:24 +0200
Organization: Hub.Org Networking Services
Lines: 27
Message-ID: <i6ks04$2jet$1@news.hub.org>
Mime-Version: 1.0
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: 7bit
User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US;
	rv:1.9.2.10pre) Gecko/20100903 Shredder/3.1.4pre
To: pgsql-performance@postgresql.org

Hi all,
I have a view v_table defined as following:

select a,b,c,d,e,f
from  t_table
sort by  a,b,c;

the usage pattern of this view is the following:

select distinct(a) from v_table;
select distinct(b) from v_table where a = "XXX";
select distinct(c) from v_table where a = "XXX" and b = "YYYY";

because of that sort in the view definition the first query above
takes not less than 3 seconds. I have solved this performance issue
removing the sort from the view definition and putting it in the
select reducing the time from > 3secons to < 150ms.

Can not the optimizer take rid of that useless sort on those
kind of queries ?


Regards
Gaetano Mendola