pgsql-bugs@postgresql.org writes:
> Subselects don't allow one to use tuple set operators like UNION,
> INTERSECT, EXCEPT.
Yeah, we know. The existing implementation of UNION etc. is a horrid
kluge that only works at the top level of a SELECT (and not even very
well there). Fixing this will require a redesign of querytrees, which
is currently planned for 7.2.
> It would be gr8 if single column subselects would allow themselves to
> be treated the same way as lists do as right-side arguments of IN
> operator.
As Lamar pointed out, this works now.
> Subselects cannot be used as arguments to aggregation functions
You have that backwards. You don't do the select as an argument to
the aggregate, you use it as a context for the aggregate, eg
... (SELECT max(foo) FROM bar WHERE baz) ...
regards, tom lane
