Expected frequency of auto_vacuum activity

Hi there,

I'm wondering if I'm seeing the appropriate amount of vacuuming in my
8.4 database.

I have a database with about twenty tables ranging from small, mostly
static, tables to tables with tens or hundreds of thousands of rows and
a fair number of inserts and updates (but very few deletes).

I have auto_vacuum with the default values (except
"log_autovacuum_min_duration=0") but when I check the logs I see lots of
vacuuming of a few pg_catalog tables - every 30 mins or so - but very
few vacuums of my public tables. Just two occurences in the last week.

According to pg_stat_all_tables only two tables in the public schema
have ever been auto-vacuumed.

Is this expected?

Thanks,

Dave.