Implementing DB2's "distinct" types

Hi,

I recently stumbled upon a really cool feature in DB2: distinct types.

DB2 lets you define your own types (just as Postgres) but with the added benefit that you can mark them such that they are _not_ comparable, e.g. to avoid comparing "apples to oranges".

Take the following example:

create type sno as varchar(50)
with comparisons;

create type pno as varchar(50)
with comparisons;

create table s
(
sno sno not null primary key,
.... other columns
);

create table p
(
pno pno not null primary key,
.... other columns
);

The following query will be rejected because sno and pno are not comparable (even though both are varchar columns):

select *
from p
join s on s.sno = p.pno;

I wonder if a similar behaviour can be achieved with Postgres' types as well.

As a type definition in Postgres can also include comparison functions, I have the feeling that this should be possible, but I don't have an idea on how to start to be honest.

Any ideas?

Regards
Thomas