Postgresql, Restrict update on specific columns (Read Only)

Question

Is it possible to have postresql restrict/prevent an update on a specific record if the update includes changes to specific columns?

How would this be implemented. A

Answer 1

No, but it should be pretty trivial to write. Just set up a BEFORE UPDATE trigger that compares old field against new field and does a RAISE ERROR if they're different. The pgSQL docs have a few examples of how to write a trigger function.