Re: Set generating functions and subqueries

Markus Schaber <schabi(at)logix-tt(dot)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