Re: cataloguing NOT NULL constraints

On Thu, Aug 18, 2022 at 6:04 PM Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org> wrote:
> On 2022-Aug-18, Laurenz Albe wrote:
> > On Wed, 2022-08-17 at 20:12 +0200, Alvaro Herrera wrote:
> > > 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.

Yeah, I think it makes sense to think of the NOT NULL constraints on
their own in this case, without worrying about the PK constraint that
created them in the first place.

BTW, maybe you are aware, but the legacy inheritance implementation is
not very consistent about wanting to maintain the same NULLness for a
given column in all members of the inheritance tree. For example, it
allows one to alter the NULLness of an inherited column:

create table p (a int not null);
create table c (a int) inherits (p);
\d c
Table "public.c"
Column | Type | Collation | Nullable | Default
--------+---------+-----------+----------+---------
a | integer | | not null |
Inherits: p

alter table c alter a drop not null ;
ALTER TABLE
\d c
Table "public.c"
Column | Type | Collation | Nullable | Default
--------+---------+-----------+----------+---------
a | integer | | |
Inherits: p

Contrast that with the partitioning implementation:

create table pp (a int not null) partition by list (a);
create table cc partition of pp default;
\d cc
Table "public.cc"
Column | Type | Collation | Nullable | Default
--------+---------+-----------+----------+---------
a | integer | | not null |
Partition of: pp DEFAULT

alter table cc alter a drop not null ;
ERROR: column "a" is marked NOT NULL in parent table

IIRC, I had tried to propose implementing the same behavior for legacy
inheritance back in the day, but maybe we left it alone for not
breaking compatibility.

--
Thanks, Amit Langote
EDB: http://www.enterprisedb.com