Postgres: foreign key constraint for components of a composite type?

Question

In Postgres, is it possible to set up a foreign key contraint involving components of a composite type, in a situation like the following one?

Suppose I have

CREATE TABLE main (
    -- a and b are separate fields
    a INTEGER,
    b INTEGER,
    PRIMARY KEY (a,b)
);
CREATE TYPE abtype AS (
    a INTEGER,
    b INTEGER
);
CREATE TABLE sub (
    -- a and b are components of a composite type
    ab abtype,
    PRIMARY KEY (ab)
);

How do I set up a foreign key constraint specifying that sub.ab references main.(a,b)?

The following attempts are all syntactically wrong.

ALTER TABLE sub ADD CONSTRAINT sub_ab_fkey FOREIGN KEY
... (ab) REFERENCES main(a,b);
... (ab) REFERENCES main((a,b));   
... (ab) REFERENCES main((a,b)::abtype);
... (ab) REFERENCES main(a,b)::abtype;
... ((ab).a,(ab).b) REFERENCES main(a,b);
... (ab.a,ab.b) REFERENCES main(a,b);

Answer 1

As I pointed out in comments, you could add a new composite column in main table:

CREATE TYPE abtype AS (
    a INTEGER,
    b INTEGER
);

CREATE TABLE main (
    -- a and b are separate fields
    a INTEGER,
    b INTEGER,
    ab abtype generated always as ((a,b)) stored,
    PRIMARY KEY (a,b)
);

CREATE TABLE sub (
    -- a and b are components of a composite type
    ab abtype,
    PRIMARY KEY (ab)
);

And set a unique index:

create unique index on main (ab);

Now you can reference your FK to this column:

ALTER TABLE sub ADD CONSTRAINT sub_ab_fkey FOREIGN KEY (ab) references main(ab);

db<>fiddle here