pg_database_size(oid)

Hi All,

I have just started with postgres and after going through manual nearly for
2 hours, I need help.

I have created a database, which I plan to load with several tables. I am
trying to find the size of the databases and came
across pg_database_size(oid) function. Since it requires databse oid, I
thought there must be a view where you get it - then came across
"pg_database" table and still all in vain. The "pg_database" table lists all
databases in the server but won't provide me the oid
for pg_database_size().

I noticed this being true for almost all reference views. For example,
pg_tablespace lists the tablespaces but no tablespace oid.
Function pg_tablespace_databases (tablespace_oid) , needs tablespace_oid,
but where I could get it from?

Please help!

-Thanks,
DP.