Re: Mutable foreign key constraints

"David G. Johnston" <david(dot)g(dot)johnston(at)gmail(dot)com> writes:
> On Thursday, September 12, 2024, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> A possible objection is that if anybody has such a setup and
>> hasn't noticed a problem because they never change their
>> timezone setting, they might not appreciate us breaking it.
>> So I certainly wouldn't propose back-patching this. But
>> maybe we should add it as a foot-gun defense going forward.

> I’m disinclined to begin enforcing this. If they got a volatile data type
> in a key column and don’t attempt to index the key, which would fail on the
> volatile side, I’d be mighty surprised.

Um, neither type is "volatile" and each can be indexed just fine.
It's the cross-type comparison required by the FK that brings the
hazard.

> I suggest adding the commentary and queries used to check for just such a
> situation to the “don’t do this page” of the wiki and there just explain
> while allowed for backward compatibility it is definitely not a recommended
> setup.

Yeah, that's a possible approach.

regards, tom lane