Fields of composite types are always nullable

[plcplc]

What

We used to mark fields of composite type as not-nullable in the NDC schema.

This is wrong. Nullability is a property of columns of tables, not of fields of record types.

This is also demonstrated by the following transcript:

postgres=# create table a_table (nullable_text text, not_nullable_text text not null);
CREATE TABLE
postgres=# create table derived_table (a_table a_table, other text);
CREATE TABLE

postgres=# insert into derived_table values (('nullable', 'not nullable'), 'other text');
INSERT 0 1
postgres=# insert into derived_table values ((null, 'not nullable'), 'other text');
INSERT 0 1

postgres=# select (a_table).* from derived_table;
 nullable_text | not_nullable_text
---------------+-------------------
 nullable      | not nullable
               | not nullable
(2 rows)

postgres=# insert into a_table select (a_table).* from derived_table;
INSERT 0 2
postgres=# select * from a_table;
 nullable_text | not_nullable_text
---------------+-------------------
 nullable      | not nullable
               | not nullable
(2 rows)

-- We can easily construct a record with (not_nullable_text=null) when on **the composite type** a_table:
postgres=# insert into derived_table values (('nullable', null), 'other text');
INSERT 0 1
postgres=# select * from derived_table;
          a_table          |   other
---------------------------+------------
 (nullable,"not nullable") | other text
 (,"not nullable")         | other text
 (nullable,)               | other text
(3 rows)

postgres=# select (a_table).* from derived_table;
 nullable_text | not_nullable_text
---------------+-------------------
 nullable      | not nullable
               | not nullable
 nullable      |
(3 rows)

-- ... But we cannot insert this into **the table** a_table.
postgres=# insert into a_table select (a_table).* from derived_table;
ERROR:  null value in column "not_nullable_text" of relation "a_table" violates not-null constraint
DETAIL:  Failing row contains (nullable, null).

This also means that fields of table type on a composite type cannot respect the column nullabilities specified on the table, because those are not part of the table's type! This should be documented.

How

We simply make the schema endpoint always return nullable fields of composite types.

[plcplc]

Strictly speaking, fixing this will require a configuration version bump, since it changes the schema :-/

[soupi]

I'm fine with approving this, I would just like to see ndc-test pass with the changes here: https://github.com/hasura/ndc-postgres/pull/564/files#r1709631720