Creation of a read-only role.

Hello.

When we start using of any replication system (e.g. Slony) we need to create
a "read-only" role for access the database. This role must be able to read
anything, but should NOT be able to INSERT, UPDATE or DELETE for all
database objects.

Overall, we need 3 roles:

1. Administrator: can do anything with a database (by default this user is
already exists - "postgres").
2. Read-only: can only read. Runs on all slave nodes.
3. Read-write: can write, but cannot change the database schema. Runs on
master node only.

Is any way to easily create and maintain these standard roles?

Now I have written a stored procedure which iterates over the pg_catalog and
runs a lot of REVOKE & GRANT commands, but it seems to be not an universal
solution, because:

1. I have to re-run this procedure after I change the database schema. (Very
bad item! Can we avoid it?)
2. It looks like a "broot-force" method, and nothing said about it in the
Slony documentation (strange).
3. In MySQL (e.g.) there is a one-command way to create these three roles.

Again, these 3 roles seems to be a de-facto standard for replication
systems, but I found nothing about this question in the Google.