Evaluate usage of idnumber fileld in Moodle database tables

While looking at MDL-15187 (assigning roles via CSV upload), I noticed that we are making an assumption that the idnumber field can be used as a unique record identifier for the user and course tables. I also noticed that we do not have an idnumber field for the course_category or groups tables which I would advocate for adding to Moodle 2.0 for increased interaction between Moodle and other student information systems (SISs) which may have unique internal identifiers for course categories and groups. I do not see a need for an idnumber field for the course_modules and block_instance tables. If the idnumber fields are assumed to be unique identifiers and are used as such for importing records, I wonder if we ought to enforce that either via an index or some checking similar to how we handle checking for duplicate usernames. I figured now is a good time to take a look at these issues and see if we wanted to add the idnumber field to course_categories and groups tables. Peace - Anthony