You can run vacuum verbose on just the postgres database and get the global numbers at the end. gotta be a superuser as well.
# \c postgres postgres postgres=# vacuum verbose; .... lots deleted. DETAIL: A total of 7664 page slots are in use (including overhead). 7664 page slots are required to track all free space. Current limits are: 1004800 page slots, 5000 relations, using 6426 kB. -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
