From: | Bruno Wolff III <bruno(at)wolff(dot)to> |
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To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Greg Stark <gsstark(at)mit(dot)edu>, pgsql-hackers(at)postgresql(dot)org, Neil Conway <neilc(at)samurai(dot)com> |
Subject: | Re: ORDER BY and DISTINCT ON |
Date: | 2003-12-15 14:03:34 |
Message-ID: | 20031215140334.GA20111@wolff.to |
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Lists: | pgsql-hackers |
On Mon, Dec 15, 2003 at 06:14:59 -0600,
Bruno Wolff III <bruno(at)wolff(dot)to> wrote:
>
> Doing things as above is pretty much the same as normal distinct on
> for purposes of which rows get selected. Of the possible rows that
> might get returned for a specific set of values from the distinct on
> expressions you will get the row that is first as ordered by the
> expressions in the order by clause. If the order by clause isn't selective
> enough there may be several rows that could be selected, but that is true
> for how distinct on works now.
Specifically the interpretation I think makes sense is that
SELECT DISTINCT ON (a, b, c) * FROM tablename ORDER BY d, e, f
should be treated as equvialent to
SELECT * FROM
(SELECT DISTINCT ON (a, b, c) FROM tablename ORDER BY a, b, c, d, e, f) AS t
ORDER BY d, e, f
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