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Re: Any better plan for this query?..

From: Simon Riggs <simon(at)2ndQuadrant(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Dimitri <dimitrik(dot)fr(at)gmail(dot)com>, Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>, Dimitri Fontaine <dfontaine(at)hi-media(dot)com>, Alvaro Herrera <alvherre(at)commandprompt(dot)com>, Aidan Van Dyk <aidan(at)highrise(dot)ca>, Merlin Moncure <mmoncure(at)gmail(dot)com>, PostgreSQL Performance <pgsql-performance(at)postgresql(dot)org>
Subject: Re: Any better plan for this query?..
Date: 2009-05-19 13:10:29
Message-ID: 1242738629.14551.198.camel@ebony.2ndQuadrant (view raw or flat)
Thread:
Lists: pgsql-performance
On Tue, 2009-05-19 at 08:58 -0400, Tom Lane wrote:
> Simon Riggs <simon(at)2ndQuadrant(dot)com> writes:
> > Both plans for this query show an IndexScan on a two column-index, with
> > an Index Condition of equality on the leading column. The ORDER BY
> > specifies a sort by the second index column, so the top-level Sort is
> > superfluous in this case.
> 
> > My understanding is that we don't currently eliminate superfluous
> > additional sorts of this kind.
> 
> Nonsense.  The planner might think some other plan is cheaper, but
> it definitely knows how to do this, and has since at least 8.1.

Please look at Dimitri's plan. If it can remove the pointless sort, why
does it not do so?

I agree that it will remove a Sort when the data is already has the
exact same interesting sort order. In this case the sort order is not
exactly the same, but looks fully removable to me.

-- 
 Simon Riggs           www.2ndQuadrant.com
 PostgreSQL Training, Services and Support


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