Re: Functional dependencies and GROUP BY

Peter Eisentraut <peter_e(at)gmx(dot)net> writes:
> On tis, 2010-06-08 at 09:59 +0900, Hitoshi Harada wrote:
>> In addition, what if y is implicitly a constant? For example,
>>
>> SELECT x, y FROM tab2 WHERE y = a AND a = 5 GROUP BY x;

> Yes, as I said, my implementation is incomplete in the sense that it
> only recognizes some functional dependencies. To recognize the sort of
> thing you show, you would need some kind of complex deduction or proof
> engine, and that doesn't seem worthwhile, at least for me, at this
> point.

The question is why bother to recognize *any* cases of this form.
I find it really semantically ugly to have the parser effectively
doing one deduction of this form when the main engine for that type
of deduction is elsewhere; so unless there is a really good argument
why we have to do this case (and NOT "it was pretty easy"), I don't
want to do it.

As far as I recall, at least 99% of the user requests for this type
of behavior, maybe 100%, would be satisfied by recognizing the
group-by-primary-key case. So I think we should do that and be happy.

regards, tom lane