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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 whole 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 
 PostgreSQL Training, Services and Support

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