Re: Reject ADD CONSTRAINT NOT NULL if name mismatches existing domain not-null constraint

> On Mar 1, 2026, at 09:53, jian he <jian(dot)universality(at)gmail(dot)com> wrote:
>
> Hi.
>
> Similar to https://git.postgresql.org/cgit/postgresql.git/commit/?id=96e2af605043974137d84edf5c0a24561956919e
> We apply this logic to the domain not-null constraint too.
> It would error out if ALTER DOMAIN ADD CONSTRAINT NOT NULL, the new
> constraint name does not
> matches the existing domain's not-null constraint
>
> create domain d1 as int constraint nn not null;
> src4=# alter domain d1 add constraint nn1 not null;
> ERROR: cannot create not-null constraint "nn1" for domain "d1"
> DETAIL: A not-null constraint named "nn" already exists for domain "d1".
>
> However, repeated ALTER DOMAIN SET NOT NULL or ALTER DOMAIN ADD NOT
> NULL statements are allowed,
> This aligns with the NOT NULL constraints on tables.
>
> No need to worry about CREATE DOMAIN.
> We already disallow multiple NOT NULL constraints in CREATE DOMAIN.
> Like this would fail:
> create domain d2 as text collate "C" constraint nn not null constraint
> nn2 not null;
>
>
>
>
> --
> jian
> https://www.enterprisedb.com/
> <v1-0001-Reject-ADD-CONSTRAINT-NOT-NULL-if-name-mismatches-existing-domain.patch>

The code change looks good to me. This patch uses the same error code and similar error message as 96e2af605043974137d84edf5c0a24561956919e, so they should be fine. I saw conTup returned from findDomainNotNullConstraint() is not free-ed, but that seems as as we can rely on memory context to free the memory.

But when I played with the patch, I saw a problem. In the test script, we can see:
```
alter domain connotnull add constraint constr1 not null;
alter domain connotnull add constraint constr1 not null; — redundant
```

If we first create a named constraint “constr1” then create an unnamed one, that’s fine, the unnamed is considered as redundant. However, if I do the reverse order, add a unnamed first, then “constr1”, it failed:
```
evantest=# create domain connotnull integer;
CREATE DOMAIN
evantest=# alter domain connotnull add not null;
ALTER DOMAIN
evantest=# alter domain connotnull add constraint constr1 not null;
ERROR: cannot create not-null constraint "constr1" for domain "connotnull"
DETAIL: A not-null constraint named "connotnull_not_null" already exists for domain "connotnull".
```

Is that an expected behavior?

Best regards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/