Horton

Re: cataloguing NOT NULL constraints

Alvaro Herrera <alvherre@alvh.no-ip.org>

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From: Alvaro Herrera <alvherre@alvh.no-ip.org>
To: Dean Rasheed <dean.a.rasheed@gmail.com>
Cc: Peter Eisentraut <peter@eisentraut.org>, pgsql-hackers@lists.postgresql.org
Date: 2023-07-24T10:55:26Z
Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Revert structural changes to not-null constraints

  2. Fix inconsistencies in error messages

  3. Disallow direct change of NO INHERIT of not-null constraints

  4. Disallow NO INHERIT not-null constraints on partitioned tables

  5. Better handle indirect constraint drops

  6. Don't try to assign smart names to constraints

  7. Fix restore of not-null constraints with inheritance

  8. ATTACH PARTITION: Don't match a PK with a UNIQUE constraint

  9. Fix propagating attnotnull in multiple inheritance

  10. Check stack depth in new recursive functions

  11. Move privilege check to the right place

  12. Update information_schema definition for not-null constraints

  13. Fix not-null constraint test

  14. Disallow changing NO INHERIT status of a not-null constraint

  15. Catalog not-null constraints

  16. parallel_schedule: add comment on event_trigger test dependency

  17. Revert "Catalog NOT NULL constraints" and fallout

  18. Adjust contrib/sepgsql regression test expected outputs.

  19. Fix table name clash in recently introduced test

  20. Catalog NOT NULL constraints

  21. Change the rules for inherited CHECK constraints to be essentially the same

On 2023-Jul-24, Dean Rasheed wrote:

> Hmm, I'm not so sure. I think perhaps multiple NOT NULL constraints on
> the same column should just be allowed, otherwise things might get
> confusing. For example:
> 
 create table p1 (a int not null check (a > 0));
 create table p2 (a int not null check (a > 0));
 create table foo () inherits (p1, p2);

Have a look at the conislocal / coninhcount values.  These should
reflect the fact that the constraint has multiple sources; and the
constraint does disappear if you drop it from both sources.

> If I then drop the p1 constraints:
> 
> alter table p1 drop constraint p1_a_check;
> alter table p1 drop constraint p1_a_not_null;
> 
> I end up with column "a" still being not null, and the "p1_a_not_null"
> constraint still being there on foo, which seems even more
> counter-intuitive, because I just dropped that constraint, and it
> really should now be the "p2_a_not_null" constraint that makes "a" not
> null:

I can see that it might make sense to not inherit the constraint name in
some cases.  Perhaps:

1. never inherit a name.  Each table has its own constraint name always
2. only inherit if there's a single parent
3. always inherit the name from the first parent (current implementation)

> So I'd say that ALTER TABLE ... ADD NOT NULL should always add a
> constraint, even if there already is one. For example ALTER TABLE ...
> ADD UNIQUE does nothing to prevent multiple unique constraints on the
> same column(s). It seems pretty dumb, but maybe there is a reason to
> allow it, and it doesn't feel like we should be second-guessing what
> the user wants.

That was my initial implementation but I changed it to allowing a single
constraint because of the way the standard describes SET NOT NULL;
specifically, 11.15 <set column not null clause> says that "If the
column descriptor of C does not contain an indication that C is defined
as NOT NULL, then:" a constraint is added; otherwise (i.e., such an
indication does exist), nothing happens.

-- 
Álvaro Herrera         PostgreSQL Developer  —  https://www.EnterpriseDB.com/
"La virtud es el justo medio entre dos defectos" (Aristóteles)