RelFileNode to Relation

Hi

I need to get a Relation instance but I have only a RelFileNode!
I see the relcache.h, only the following function seems helpful

extern Relation RelationIdGetRelation(Oid relationId);

However, there is another problem, In the RelFileNode, only relNode exist
and as comment said this is equivalent to pg_class.relfilenode and in some
situation relation oid (pg_class.oid) and relation file node are not the
same.

So i cant use above function!

Any idea?

Regards,
Soroosh Sardari
Sharif University of Tech.