pg_buffercache view

Dear all,

About the pg_buffercache view:
I couldn't find the description for this view in the manual at
http://www.postgresql.org/docs/8.2/interactive/catalogs.html
However I found the readme file provided in the /contrib./pg_buffercache of
the source code for version 8.2.3
Here it's written the following description:

Column | references | Description

----------------+----------------------+------------------------------------
bufferid | | Id, 1..shared_buffers.
relfilenode | pg_class.relfilenode | Refilenode of the relation.
reltablespace | pg_tablespace.oid | Tablespace oid of the relation.
reldatabase | pg_database.oid | Database for the relation.
relblocknumber | | Offset of the page in the
relation.
isdirty | | Is the page dirty?

I've 2 questions:
1)
I was not able to find the field "oid" from pg_database view. Could you
please tell me what is the actual name of the column for which reldatabase
is reffering to?
2)
In readme file is also written:
"Unused buffers are shown with all fields null except buffered".
A "used" buffer means that is used 100% or could it be filled only
partially?
Is there any way to know at a certain moment with precision how much shared
memory expressed in Mb is used?

With best regards,
Sorin