| From: | Bruno Wolff III <bruno(at)wolff(dot)to> |
|---|---|
| To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
| Cc: | 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 02:51:06 |
| Message-ID: | 20031215025106.GA15847@wolff.to |
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| Thread: | |
| Lists: | pgsql-hackers |
On Sun, Dec 14, 2003 at 18:09:33 -0500,
Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>
> This was discussed before --- see the archives. I believe the
> conclusion was that the results would actually be nondeterministic
> if we used two sort steps (that's what the code comment means by
> "rather unpredictable"). This is not unrelated to the reasons why
> people consider DISTINCT ON to be a messy feature ... ideally it
> should be orthogonal to ORDER BY, but it simply isn't.
If the sort used to select the records sorts on both the distinct
expressions and the order by expressions you will get a sensible
deterministic result. That result can then be sorted using just
the order by expressions.
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