Martijn van Oosterhout <kleptog@svana.org> writes:
> On Tue, Jun 04, 2002 at 01:15:38AM -0400, Tom Lane wrote:
>> I don't doubt that this transformation is valid in some cases ... but
>> I do doubt that it is valid in all cases. If someone can supply a
>> rigorous proof about when it is valid, I'd be willing to look into
>> doing the necessary programming.
> IIRC, union does an implicit DISTINCT (there's UNION ALL, right). So if what
> is being selected is anything other than a simple statement, it'll be very
> hard to prove equivalence (i guess this is what the iscachable is
> for).
Yeah, the UNION vs. UNION ALL difference is one of the things that would
need to be thought about. I think it's more likely that the
transformation would work for UNION ALL than for UNION ... but I have
not had the time to try to work it out.
regards, tom lane
