disjoint union types

Hi,

I was after opinions as to the best way to lay tables out to get the
effect of a "disjoint union" type (also known as a "tagged union").
When I have to do this at the moment, I'm creating a structure like:

  CREATE TABLE circle ( id SERIAL PRIMARY KEY, radius  REAL NOT NULL );
  CREATE TABLE square ( id SERIAL PRIMARY KEY, sidelen REAL NOT NULL );

  CREATE TABLE shapes (
    id SERIAL PRIMARY KEY,

    tag INTEGER NOT NULL,

    circleid INTEGER REFERENCES circle
      CHECK ((tag = 1) = (circleid IS NOT NULL)),
    squareid INTEGER REFERENCES square
      CHECK ((tag = 2) = (squareid IS NOT NULL))
  );

I can then put data into this by doing:

  BEGIN;
  INSERT INTO circle (radius) VALUES (1);
  INSERT INTO shapes (tag,circleid) VALUES (1,currval('circle_id_seq'));
  COMMIT;

This works, but it's sometimes a bit of a headache turning things around
so they fit this structure.  Are there standard solutions to this that
work better?


Thanks,
  Sam