Adding a field to each tuple

Ok,

I decided to learn more about postgresql.

My challenge:
Given an oid, efficiently determine the most derived class the instance
belongs to.

My intended solution is to add an extra attribute to each tuple
which contains the oid of the most derived class it belongs to.

On inserting a tuple I set this field to the value of the table/class used
for the
insert.

Using with a negative index I can allways retreive the oid of
the most derived class.

How to proceed?
Step: 1
Add the new field to the HeapTupleData struct in htup.h.

Step: 2
Modify inserting function to add the oid of the class into which
the insertion takes place.

Step 3
???

Step 4

Can anyone give clues as whatelse I will have to do to get this to work?

Thanks,
Maurice