Re: Teaching planner to short-circuit empty UNION/EXCEPT/INTERSECT inputs

David Rowley <dgrowleyml(at)gmail(dot)com> writes:
> In [1] there was a report that set operations didn't correctly detect
> when inputs were provably empty sets. While this is not the bug that
> the report claimed it to be, as it's just a missing optimisation, I
> did decide to look at it to check if there was much performance to
> gain from doing this.

I'm kind of resistant to the amount of code this patch adds in
comparison to the likely benefit. Sure, a badly written query can
profit, but is it worth debugging and maintaining a couple hundred
lines of code for that?

The first few hunks of changes seem fine by this light, but I think
you're expending too much effort on the EXCEPT-with-dummy-inputs
cases. I'm wondering if it could be shortened a great deal by
handling left-input-dummy and EXCEPT-ALL-with-right-input-dummy
but leaving the EXCEPT-with-right-input-dummy case unimproved.

regards, tom lane