Re: last_autovacuum & last_autoanalyze showing NULL

> Truncate should not affect those values.
OK.

> Has your Postgres instance crashed or been restarted with immediate mode
since the last vacuum? This will cause the autovacuum information to be
null.
No, it's been up and running for almost a week.  I even checked with "select
pg_postmaster_start_time()".

> Also, are you sure that autovacuum has processed these tables at least
once?
Yes, am very sure about this.  As I told earlier, I keep tab on this
particular table and even noticed last autovacuum and last autoanalyze every
15 minutes (using cron job I write it to a file) with timestamp before.