Horton

Re: cataloguing NOT NULL constraints

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

View thread
From: Alvaro Herrera <alvherre@alvh.no-ip.org>
To: Laurenz Albe <laurenz.albe@cybertec.at>
Cc: Pg Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2022-08-18T09:04:25Z
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 2022-Aug-18, Laurenz Albe wrote:

> On Wed, 2022-08-17 at 20:12 +0200, Alvaro Herrera wrote:

> > 1. In my implementation, you can have more than one NOT NULL
> >    pg_constraint row for a column.  What should happen if the user does
> >    ALTER TABLE .. ALTER COLUMN .. DROP NOT NULL;
> >    ?  Currently it throws an error about the ambiguity (ie. which
> >    constraint to drop).
> 
> I'd say that is a good solution, particularly if there is a hint to drop
> the constraint instead, similar to when you try to drop an index that
> implements a constraint.

Ah, I didn't think about the hint.  I'll add that, thanks.

> >    Using ALTER TABLE DROP CONSTRAINT works fine, and the 'attnotnull'
> >    bit is lost when the last one such constraint goes away.
> 
> Wouldn't it be the correct solution to set "attnotnumm" to FALSE only
> when the last NOT NULL constraint is dropped?

... when the last NOT NULL or PRIMARY KEY constraint is dropped.  We
have to keep attnotnull set when a PK exists even if there's no specific
NOT NULL constraint.

> > 2. If a table has a primary key, and a table is created that inherits
> >    from it, then the child has its column(s) marked attnotnull but there
> >    is no pg_constraint row for that.  This is not okay.  But what should
> >    happen?
> > 
> >    1. a CHECK(col IS NOT NULL) constraint is created for each column
> >    2. a PRIMARY KEY () constraint is created
> 
> I think it would be best to create a primary key constraint on the
> partition.

Sorry, I wasn't specific enough.  This applies to legacy inheritance
only; partitioning has its own solution (as you say: the PK constraint
exists), but legacy inheritance works differently.  Creating a PK in
children tables is not feasible (because unicity cannot be maintained),
but creating a CHECK (NOT NULL) constraint is possible.

I think a PRIMARY KEY should not be allowed to exist in an inheritance
parent, precisely because of this problem, but it seems too late to add
that restriction now.  This behavior is absurd, but longstanding:

55432 16devel 1787364=# create table parent (a int primary key);
CREATE TABLE
55432 16devel 1787364=# create table child () inherits (parent);
CREATE TABLE
55432 16devel 1787364=# insert into parent values (1);
INSERT 0 1
55432 16devel 1787364=# insert into child values (1);
INSERT 0 1
55432 16devel 1787364=# select * from parent;
 a 
───
 1
 1
(2 filas)

-- 
Álvaro Herrera        Breisgau, Deutschland  —  https://www.EnterpriseDB.com/
"But static content is just dynamic content that isn't moving!"
                http://smylers.hates-software.com/2007/08/15/fe244d0c.html