Horton

Re: cataloguing NOT NULL constraints

Laurenz Albe <laurenz.albe@cybertec.at>

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From: Laurenz Albe <laurenz.albe@cybertec.at>
To: Alvaro Herrera <alvherre@alvh.no-ip.org>
Cc: Pg Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2022-08-18T15:00:52Z
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 Thu, 2022-08-18 at 11:04 +0200, Alvaro Herrera wrote:
> On 2022-Aug-18, Laurenz Albe wrote:
> > On Wed, 2022-08-17 at 20:12 +0200, Alvaro Herrera wrote:
> > >    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.

Of course, I forgot that.
I hope that is not too hard to implement.

> > > 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:

My mistake; you clearly said "inherits".

Since such an inheritance child currently does not have a primary key, you
can insert duplicates.  So automatically adding a NUT NULL constraint on the
inheritance child seems the only solution that does not break backwards
compatibility.  pg_upgrade would have to be able to cope with that.

Forcing a primary key constraint on the inheritance child could present an
upgrade problem.  Even if that is probably a rare and strange case, I don't
think we should risk that.  Moreover, if we force a primary key on the
inheritance child, using ALTER TABLE ... INHERIT might have to create a
unique index on the table, which can be cumbersome if the table is large.

So I think a NOT NULL constraint is the least evil.

Yours,
Laurenz Albe