Postgres 13

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CREATE TABLE foo (foo_id int PRIMARY KEY);
ALTER TABLE foo DROP CONSTRAINT foo_pkey;

CREATE TABLE

ALTER TABLE

SELECT attname, attnotnull
FROM   pg_attribute
WHERE  attrelid = 'foo'::regclass
AND    attnum > 0;

attname
attnotnull

foo_id
t

SELECT 1

CREATE TABLE bar (foo_id int, bar_id int, PRIMARY KEY(foo_id, bar_id));
ALTER TABLE bar DROP CONSTRAINT bar_pkey;

CREATE TABLE

ALTER TABLE

SELECT attname, attnotnull
FROM   pg_attribute
WHERE  attrelid = 'bar'::regclass
AND    attnum > 0;

attname
attnotnull

foo_id
t

bar_id
t

SELECT 2