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Re: query not using index for descending records?

From: Bruno Wolff III <bruno(at)wolff(dot)to>
To: Achilleus Mantzios <achill(at)matrix(dot)gatewaynet(dot)com>
Cc: email lists <lists(at)darrenmackay(dot)com>, pgsql-sql(at)postgresql(dot)org
Subject: Re: query not using index for descending records?
Date: 2004-01-29 14:06:55
Message-ID: 20040129140655.GA14885@wolff.to (view raw or flat)
Thread:
Lists: pgsql-sql
On Thu, Jan 29, 2004 at 15:29:11 +0200,
  Achilleus Mantzios <achill(at)matrix(dot)gatewaynet(dot)com> wrote:
> O kyrios Bruno Wolff III egrapse stis Jan 29, 2004 :
> 
> As i see there was a thread
> http://archives.postgresql.org/pgsql-hackers/2003-05/msg00762.php
> dealing with this issue, assuming the "correct" order by
> should be "order by datetime ASC, id DESC".

So you really didn't want them in the reverse order?

> Do you know of any progress for declaring the direction of each
> column in a multicolumn index?

If you are using 7.4 you can use a functional index to get around this.
Assuming id is a numeric type, you can make an index on datetime and
(-id) and sort by datetime, -id and the index should get used.

This should still get fixed at some point, as this trick doesn't work
for types for which the - operator exists. But I haven't heard of
anyone working on it for 7.5, so don't expect a real fix any time soon.

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