autovacuum vacuums entire database...is this right?

Hi,

I'm just wondering if autovacuum is ever supposed to vacuum the entire
database during one of its runs.  As far as I remember, it's supposed
to vacuum one table at a time, based on the
autovacuum_vacuum_threshold, autovacuum_analyze_threshold, etc.
settings.

For some reason, autovacuum decided to run a vacuum on my entire
database (29GB large), and it's taking forever:

select now(), query_start, current_query, backend_start, procpid,
usename from pg_stat_activity where current_query <> '<IDLE>';
              now              |          query_start          |
current_query |         backend_start         | procpid | usename
-------------------------------+-------------------------------
+---------------+-------------------------------+---------+----------
 2007-04-15 06:34:27.925042-07 | 2007-04-14 22:23:31.283894-07 |
VACUUM        | 2007-04-14 22:23:31.274121-07 |    9406 | postgres

Is this expected behavior?

--Richard