last_autovacuum & last_autoanalyze showing NULL

Hi,

Our production server is running PostgreSQL v8.2.3 on RHEL5.  Autovacuum
daemon is also running in the server.

A brief background on my use case:  We heavily use a table for storing of
data temporarily inside PostgreSQL functions.  Eventually, there will not be
any record in the table.  Because this table is used heavily, I usually keep
tab of this particular table in "pg_stat_user_tables", so that disk space is
within control.

Recently, I noticed that "last_autovacuum" and "last_autoanalyze" column for
this table showing a <NULL> value.  My question here is, what
condition/scenario would make this column to update to <NULL> value by
autovacuum?  I can add one more point here, which I remember, that is I
performed a TRUNCATE on this table just before.  I believe that this is not
because of TRUNCATE  TABLE?

Regards,
Gnanam