PG_STAT_DATABASE

I am looking into the PostgreSQL dictionary. The view pg_stat_activity is the PgSQL version of v$session. There are interesting views named pg_stat_database and pg_stat_user_tables. What is the starting point for those stat views? Is that since the database and table, in case of the latter view, was created? Is it since the last restart of the instance? I was unable to find that in the documentation.
Here is the description of the view in question:

View "pg_catalog.pg_stat_database"     Column     |  Type   | Modifiers ---------------+---------+----------- datid         | oid     | datname       | name    | numbackends   | integer | xact_commit   | bigint  | xact_rollback | bigint  | blks_read     | bigint  | blks_hit      | bigint  | tup_returned  | bigint  | tup_fetched   | bigint  | tup_inserted  | bigint  | tup_updated   | bigint  | tup_deleted   | bigint  | Mladen Gogala Sr. Oracle DBA 1500 Broadway New York, NY 10036 (212) 329-5251 www.vmsinfo.com The Leader in Integrated Media Intelligence Solutions