Re: d25ea01275 and partitionwise join

Amit Langote <amitlangote09(at)gmail(dot)com> writes:
> On Tue, Apr 7, 2020 at 2:41 AM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> I *think* that it might be all right, because although what we
>> are matching to is a user-written COALESCE() not an actual
>> FULL JOIN USING column, it has to behave in somewhat the same
>> way. In particular, by construction it must be a coalesce of
>> some representation of the matching partition columns of the
>> full join's inputs. So, even though it might go to null in
>> different cases than an actual USING variable would do, it
>> does not break the ability to partition the join.

> Seems fine to me too. Maybe users should avoid writing it by hand if
> possible anyway, because even slight variation in the way it's written
> will affect this:

I'm not particularly concerned about users intentionally trying to trigger
this behavior. I just want to be sure that if someone accidentally does
so, we don't produce a wrong plan.

I waited till after the "advanced partitionwise join" patch went
in because that seemed more important (plus I wondered a bit if
that would subsume this). But this patch seems to still work,
and the other thing doesn't fix the problem, so pushed.

regards, tom lane