Referencing the foreign key of another schema
To create a foreign key that references an object in a different schema, you just need to qualify the object name
ALTER TABLE B.table_b ADD CONSTRAINT fk_b_a FOREIGN KEY (apk) REFERENCES a.table_a( apk )This also requires that the user B has sufficient privileges on A.table_a. The user would need to have the REFERENCES privilege and would, presumably, need to have the SELECT privilege on the table as well.