From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
---|---|
To: | Martijn van Oosterhout <kleptog(at)svana(dot)org> |
Cc: | Nils Zonneveld <nils(at)mbit(dot)nl>, pgsql-general(at)postgresql(dot)org |
Subject: | Re: Re: Does PostgreSQL support EXISTS? |
Date: | 2001-06-13 13:51:46 |
Message-ID: | 26043.992440306@sss.pgh.pa.us |
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Lists: | pgsql-general |
Martijn van Oosterhout <kleptog(at)svana(dot)org> writes:
> On postgres at least, exists is faster than in.
> They are equivalent though.
Not really.
For one thing, IN can return a NULL (don't know) result; EXISTS cannot.
regression=# select * from foo;
f1
----
1
(2 rows)
regression=# select 2 in (select f1 from foo);
?column?
----------
(1 row)
regression=# select exists (select 1 from foo where f1 = 2);
?column?
----------
f
(1 row)
Yes, this behavior is spec-compliant. Think: we don't know what the NULL
represents, therefore we don't know whether 2 is in the column or not,
therefore IN should return NULL.
regards, tom lane
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