Which file belongs to which database?

Hello all,

just out of curiosity: how can I find out which files in the PG_DATA directory
belong to which database/table?

I have looked through the documentation of the system catalogs, but couldn't
find any reference to that.

The field datpath in pg_database is empty in my system (7.2 on Windows and 7.3.1
on Linux)

Cheers
Thomas