Re: Prevent ALTER TABLE DROP NOT NULL on child tables if parent column has it

On Wed, Jun 15, 2016 at 10:34 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Michael Paquier <michael.paquier@gmail.com> writes:
> This doesn't sound like the right approach; in particular, it won't really
> help for deciding whether to propagate a DROP NOT NULL on a parent rel to
> its children.  What we've discussed in the past is to store NOT NULL
> constraints in pg_constraint, much like CHECK constraints are already, and
> use use-count logic identical to the CHECK case to keep track of whether
> NOT NULL constraints are inherited or not.  My feeling is that we'd keep
> the pg_attribute.attnotnull field and continue to drive actual enforcement
> off that, but it would just reflect a summary of the pg_constraint state.

OK, I see. Hm, by storing this information I would actually think that
we want to drop this attnotnull so as we don't need to bother about
updating pg_attribute through the whole tree when dropping a NOT NULL
constraint on the parent, and we do not actually need to store this
information in two different places..

I would also rather do nothing for the DDL interface regarding for
example the possibility to change the constraint names for domains and
tables to keep things simple. A patch of this caliber would be
complicated enough if a catalog switch is done.

> IIRC, Alvaro posted a WIP patch for that awhile back.  Not sure what the
> current state is.

Are you talking about that?
https://www.postgresql.org/message-id/20110707213401.GA27098%40alvh.no-ip.org
This is not a small patch :)

Alvaro, others, any opinions?
-- 
Michael