Re: NOT ENFORCED constraint feature

On Tue, 11 Feb 2025 at 08:36, Álvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>
wrote:

> On 2025-Feb-10, Isaac Morland wrote:
>
> > I'm having a lot of trouble understanding the operational distinction
> > between your 'u' and 'U'. If it's not enforced, it cannot be assumed to
> be
> > valid, regardless of whether it was valid in the past. I'm not sure what
> I
> > think of a single character vs. 2 booleans, but there are only 3 sensible
> > states either way: valid enforced, invalid enforced, and invalid
> unenforced.
>
> I kinda agree with you and would prefer that things were that way as
> well. But look at the discussion starting at
>
> https://postgr.es/m/CAExHW5tV23Sw+Nznv0KpdNg_t7LrXY1WM9atiC=eKKSsKHSnuQ@mail.gmail.com
> whereby it was apparently established that if you have a
> NOT VALID NOT ENFORCED
> constraint, and you make it enforced, then you should somehow end up
> with a NOT VALID ENFORCED constraint, which says to me that we need to
> store the fact that the constraint was NOT VALID to start with; and
> correspondingly if it's VALID NOT ENFORCED and you enforce it, then it
> ends up VALID ENFORCED. If we take this view of the world (with which,
> I repeat, I disagree) then we must keep track of whether the constraint
> was valid or not valid to start with. And this means that we need to
> keep convalidated=true _regardless_ of whether conenforced is false.
> So in this view of the world there aren't three states but four.
>
> I would prefer there to be three states as well, but apparently I'm
> outvoted on this.
>

Sounds like we agree. I think the problem is with the statement in the
linked discussion that “If the constraint is VALID and later marked as NOT
ENFORCED, changing it to ENFORCED should also keep it VALID.” This ignores
that if it is changed to NOT ENFORCED that should immediately change it to
NOT VALID if it is not already so.

Has anybody argued for how it makes any sense at all to have a constraint
that is VALID (and therefore will be assumed to be true by the planner),
yet NOT ENFORCED (and therefore may well not be true)? What next, a patch
to the planner so that it only treats as true constraints that are both
VALID and ENFORCED?

Re: the 3 or 4 values for the single character status, there is a similar
issue with relkind, where one can imagine writing "relkind IN ('r')" when
one meant "relkind IN ('r', 'v')" or something else; but on the other hand,
one can easily imagine actually wanting the first one of those. But here,
it's not at all clear to me when you would ever want to distinguish between
'u' and 'U', but it is clear to me that it would be natural to write "… =
'U'" when one actually needs to write "… IN ('u', 'U')", or perhaps "…
ILIKE 'u'" (not what I would want to see).