Referencing pg_user

Hi

Just subscribed to the list.

How do I put a column constraint that references
the pg_user/pg_shadow system table?

I'm trying to create a table with a field attribute like this:

create table feedback (
...
usesysid integer references pg_shadow,
...
);

But I get "ERROR: PRIMARY KEY for referenced table "pg_shadow"
not found." I tried this as postgres user.

I'm putting the constraint so that only records associated with
PostgreSQL users can be inserted into the table.

--
Benjamin Oris Jr.
bastabob(at)flash(dot)net(dot)ph