| From: | Bruce Momjian <maillist(at)candle(dot)pha(dot)pa(dot)us> |
|---|---|
| To: | tgl(at)sss(dot)pgh(dot)pa(dot)us (Tom Lane) |
| Cc: | clark(dot)evans(at)manhattanproject(dot)com, pgsql-hackers(at)postgreSQL(dot)org |
| Subject: | Re: [HACKERS] SELECT DISTINCT i FROM dtest ORDER BY j |
| Date: | 1998-12-20 19:03:45 |
| Message-ID: | 199812201903.OAA01434@candle.pha.pa.us |
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| Lists: | pgsql-hackers |
> The trouble is that if you have several tuples with the same i and
> different j, doing the DISTINCT first implies throwing away all but
> one of those tuples. Which one do you keep? It matters because
> some of those tuples might sort differently than others.
>
> As far as I can see, this combination of features is not well-defined on
> its surface. You have to make some additional assumptions (about which
> of the possible j values is kept for sorting) in order to define a
> unique result.
That is an interesting argument. It suggests it should not be a TODO
item. I can see cases where you would want to do the DISTINCT before
the ORDER BY, so the current behavior seems correct.
Removing it from TODO list.
--
Bruce Momjian | http://www.op.net/~candle
maillist(at)candle(dot)pha(dot)pa(dot)us | (610) 853-3000
+ If your life is a hard drive, | 830 Blythe Avenue
+ Christ can be your backup. | Drexel Hill, Pennsylvania 19026
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