From: | Neil Conway <neilc(at)samurai(dot)com> |
---|---|
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 |
Subject: | Re: ORDER BY and DISTINCT ON |
Date: | 2003-12-15 02:44:55 |
Message-ID: | 87fzfmg5p4.fsf@mailbox.samurai.com |
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Lists: | pgsql-hackers |
Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> writes:
> 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").
Does the non-determinism you're referring to result from an ORDER BY
on a non-deterministic expression, or the non-determinism that results
from picking an effectively random row because the ORDER BY isn't
sufficient?
I searched the archives and found Stephen Szabo's comment[1] that:
The query you've written is potential non-deterministic if you have
a people_id that has multiple rows with different last names that
meet the where clause.
Which seems like an unconvincing justification for rejecting the
query: we accept DISTINCT ON with no ORDER BY clause at all, for
example.
-Neil
[1] http://archives.postgresql.org/pgsql-hackers/2001-07/msg00588.php
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