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