Re: Removing Functionally Dependent GROUP BY Columns

David Rowley <david(dot)rowley(at)2ndquadrant(dot)com> writes:
> On 23 January 2016 at 12:44, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> * What you did to join.sql/join.out seems a bit bizarre. The existing
>> test case that you modified was meant to test something else, and
>> conflating this behavior with the pre-existing one (and not documenting
>> it) is confusing as can be. A bit more work on regression test cases
>> seems indicated.

> The history behind that is that at one point during developing the
> patch that test had started failing due to the group by item being
> removed therefore allowing the join removal conditions to be met. On
> testing again with the old test query I see this no longer happens, so
> I've removed the change, although the expected output still differs
> due to the group by item being removed.

Hmm ... but ... it seems to me that the test as it stands *should* fail
after this patch, because once the non-pkey grouping column is removed
the join removal optimization should apply. I think we should look a bit
more closely at what's happening there.

(IOW, I wasn't so much unhappy with the change to that test case as
that it was being used as the only test case for this new behavior.
I see you added some new, separate test cases, so that's good; but
there's something fishy if the existing case doesn't change behavior.)

regards, tom lane