From: | Greg Stark <gsstark(at)mit(dot)edu> |
---|---|
To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
Cc: | Marko Tiikkaja <marko(dot)tiikkaja(at)cs(dot)helsinki(dot)fi>, pgsql-hackers(at)postgresql(dot)org |
Subject: | Re: wCTE: why not finish sub-updates at the end, not the beginning? |
Date: | 2011-02-25 17:31:05 |
Message-ID: | AANLkTinDwQVezdd9rV_3XtvdXnYrd0k12nTpqWqectjY@mail.gmail.com |
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On Fri, Feb 25, 2011 at 2:58 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> However, the real reason for doing it isn't any of those, but rather
> to establish the principle that the executions of the modifying
> sub-queries are interleaved not sequential. We're never going to be
> able to do any significant optimization of such queries if we have to
> preserve the behavior that the sub-queries execute sequentially.
> And I think it's inevitable that users will manage to build such an
> assumption into their queries if the first release with the feature
> behaves that way.
Does the interleaved execution have sane semantics?
With a query like:
WITH
a as update x set x.i=x.i+1 returning x.i,
b as update x set x.i=x.i+1 returning x.i
select * from a natural join b;
Is there any way to tell what it will return or what state it will
leave the table in?
--
greg
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