From: Tom Lane <tgl@sss.pgh.pa.us>
To: Markus Schaber <schabi@logix-tt.com>
Cc: PostgreSQL SQL List <pgsql-sql@postgresql.org>
Subject: Re: Set generating functions and subqueries
Date: Fri, 10 Mar 2006 09:48:48 -0500
Message-ID: <5142.1142002128@sss.pgh.pa.us> (raw)
In-Reply-To: <44116ED2.9060901@logix-tt.com>
References: <44116ED2.9060901@logix-tt.com>
Markus Schaber <schabi@logix-tt.com> writes:
> postgres=# select (select generate_series(1,2)),'othercol';
> ERROR: more than one row returned by a subquery used as an expression
> So it seems that set-returning functions "blow up" the resultset by
> duplicating rows - so why is this not allowed for subqueries?
The behavior of the subquery expression is dictated by the SQL spec:
1) If the cardinality of a <scalar subquery> or a <row subquery> is
greater than 1, then an exception condition is raised: cardinal-
ity violation.
The fact that the other form is even allowed is more of a holdover from
PostQUEL than something we have consciously decided is a good idea.
(IMHO it's actually a fairly *bad* idea, because it does not work nicely
when there's more than one SRF in the same targetlist.) It'll probably
get phased out someday, if we can find a way to replace the
functionality. I seem to recall speculating that SQL2003's LATERAL
tables might do the job.
> Btw, having several set-returning functions with equal or different set
> lengths produce interesting results:
No kidding.
regards, tom lane
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To: pgsql-sql@postgresql.org
Cc: tgl@sss.pgh.pa.us, schabi@logix-tt.com
Subject: Re: Set generating functions and subqueries
In-Reply-To: <5142.1142002128@sss.pgh.pa.us>
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