Re: [PATCH] Keeps tracking the uniqueness with UniqueKey

Heikki Linnakangas <hlinnaka(at)iki(dot)fi> writes:
>> I can understand why we need EquivalenceClass for UniqueKey, but I can't
>> understand why we need opfamily here.

> Thinking a bit harder, I guess we don't. Because EquivalenceClass
> includes the operator family already, in the ec_opfamilies field.

No. EquivalenceClasses only care about equality, which is why they
might potentially mention several opfamilies that share an equality
operator. If you care about sort order, you *cannot* rely on an
EquivalenceClass to depict that. Now, abstract uniqueness also only
cares about equality, but if you are going to implement it via sort-
and-unique then you need to settle on a sort order.

I agree we are overspecifying DISTINCT by settling on a sort operator at
parse time, rather than considering all the possibilities at plan time.
But given that opfamilies sharing equality are mostly a hypothetical
use-case, I'm not in a big hurry to fix it. Before we had ASC/DESC
indexes, there was a real use-case for making a "reverse sort" opclass,
with the same equality as the type's regular opclass but the opposite sort
order. But that's ancient history now, and I've seen few other plausible
use-cases.

I have not been following this thread closely enough to understand
why we need a new "UniqueKeys" data structure at all. But if the
motivation is only to remove this overspecification, I humbly suggest
that it ain't worth the trouble.

regards, tom lane