From: | Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us> |
---|---|
To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
Cc: | Michael Meskes <meskes(at)postgresql(dot)org>, pgsql-general(at)postgresql(dot)org |
Subject: | Re: Re: Does PostgreSQL support EXISTS? |
Date: | 2001-06-13 15:38:09 |
Message-ID: | 200106131538.f5DFc9v09998@candle.pha.pa.us |
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Lists: | pgsql-general |
> Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us> writes:
> > Got it. How does an IN subquery returning NULL behave differently from
> > one returning FALSE? I can't think of a test that would be affected.
>
> After we fix IS TRUE and friends to respond to nulls correctly (Conway's
> promised to do that, IIRC) it'll be possible to write
>
> (foo IN (SELECT ...)) IS NOT FALSE
>
> and get the "intuitive" behavior. But right now that doesn't work.
OK, so I wasn't missing anything in our current code. I can see how
this capability would change things.
> Hm. Maybe we could recognize that construct as a whole, and translate
> it to an optimizable join? It'd become the usual locution, I imagine.
Are we anywhere with optimizing IN to EXISTS? I didn't think there was
any work being done in that area.
--
Bruce Momjian | http://candle.pha.pa.us
pgman(at)candle(dot)pha(dot)pa(dot)us | (610) 853-3000
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