CREATE OR REPLACE FUNCTION delete_relation() RETURNS trigger AS
$$
BEGIN
    DELETE FROM table_C
        USING table_B
        WHERE table_C.id = table_B.id_C;
    DELETE FROM table_C
        USING table_B
        WHERE table_D.id = table_B.id_D;
    DELETE FROM table_B WHERE table_B.id_A = OLD.id;
END;
$$ LANGUAGE plpgsql;

CREATE TRIGGER
    delete_table_A
AFTER DELETE ON
    table_A
FOR EACH ROW EXECUTE PROCEDURE delete_relation();

Function executed no return statement: 7
ERROR: control reached end of trigger procedure without RETURN
CONTEXT: PL/pgSQL function delete_relation()