Re: cataloguing NOT NULL constraints

On 2024-Apr-22, Alvaro Herrera wrote:

> > On d9f686a72~1 this script results in:
> > ERROR:  cannot change NO INHERIT status of inherited NOT NULL constraint "t_a_not_null" on relation "t"
> 
> Right.  Now I'm beginning to wonder if allowing ADD CONSTRAINT to mutate
> a pre-existing NO INHERIT constraint into a inheritable constraint
> (while accepting a constraint name in the command that we don't heed) is
> really what we want.  Maybe we should throw some error when the affected
> constraint is the topmost one, and only accept the inheritance status
> change when we're recursing.

So I added a restriction that we only accept such a change when
recursively adding a constraint, or during binary upgrade.  This should
limit the damage: you're no longer able to change an existing constraint
from NO INHERIT to YES INHERIT merely by doing another ALTER TABLE ADD
CONSTRAINT.

One thing that has me a little nervous about this whole business is
whether we're set up to error out where some child table down the
hierarchy has nulls, and we add a not-null constraint to it but fail to
do a verification scan.  I tried a couple of cases and AFAICS it works
correctly, but maybe there are other cases I haven't thought about where
it doesn't.

-- 
Álvaro Herrera               48°01'N 7°57'E  —  https://www.EnterpriseDB.com/
"You're _really_ hosed if the person doing the hiring doesn't understand
relational systems: you end up with a whole raft of programmers, none of
whom has had a Date with the clue stick."              (Andrew Sullivan)
https://postgr.es/m/20050809113420.GD2768@phlogiston.dyndns.org