Re: Allowing join removals for more join types

David Rowley <dgrowleyml(at)gmail(dot)com> writes:
> On Mon, Jul 7, 2014 at 4:15 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> I poked around to see if we didn't have some code already for that, and
>> soon found that not only do we have such code (equality_ops_are_compatible)
>> but actually almost this entire patch duplicates logic that already exists
>> in optimizer/util/pathnode.c, to wit create_unique_path's subroutines
>> query_is_distinct_for et al. So I'm thinking what this needs to turn into
>> is an exercise in refactoring to allow that logic to be used for both
>> purposes.

> Well, it seems that might just reduce the patch size a little!
> I currently have this half hacked up to use query_is_distinct_for, but I
> see there's no code that allows Const's to exist in the join condition. I
> had allowed for this in groupinglist_is_unique_on_restrictinfo() and I
> tested it worked in a regression test (which now fails). I think to fix
> this, all it would take would be to modify query_is_distinct_for to take a
> list of Node's rather than a list of column numbers then just add some
> logic that skips if it's a Const and checks it as it does now if it's a Var
> Would you see a change of this kind a valid refactor for this patch?

I'm a bit skeptical as to whether testing for that case is actually worth
any extra complexity. Do you have a compelling use-case? But anyway,
if we do want to allow it, why does it take any more than adding a check
for Consts to the loops in query_is_distinct_for? It's the targetlist
entries where we'd want to allow Consts, not the join conditions.

regards, tom lane