Message-ID: <4382E15C.5070206@archonet.com>
Date: Tue, 22 Nov 2005 09:14:04 +0000
From: Richard Huxton <dev@archonet.com>
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To: gry@ll.mit.edu
Cc: pgsql-sql@postgresql.org
Subject: Re: tid_le comparison for tuple id (ctid) values?
References: <20051121160016.282c66c2.gry@ll.mit.edu>
	<4271.1132607968@sss.pgh.pa.us>
	<20051121203527.5378122b.gry@ll.mit.edu>
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george young wrote:
> Well, I don't have any need for it to correlate with the age of the
> tuple.  My use of step.ctid<s.ctid was not to get the earliest or
> latest row, but just to *choose* one.  Perhaps there's some other
> query that would modify only one of each pair of equal-keyed rows?

How do you know there is only 1 duplicate?

Anyway, if (x,y) are the same but (z) is not then you can compare 
against max(z) or min(z). Something like:

SELECT t1.x AS update_me_x, t1.y AS update_me_y, t1.z AS update_me_z
FROM
   test_tbl AS t1,
   (
     SELECT x,y,max(z) AS max_z
     FROM test_tbl
     GROUP BY x,y
   ) AS t2
WHERE
   t1.x = t2.x AND t1.y = t2.y AND t1.z = t2.max_z
-- 
   Richard Huxton
   Archonet Ltd