Bad n_distinct estimation; hacks suggested?

Folks,

Params:  PostgreSQL 8.0.1 on Solaris 10
Statistics = 500
(tablenames have been changed to protect NDA)

e1=# select tablename, null_frac, correlation, n_distinct from pg_stats where
tablename = 'clickstream1' andattname = 'session_id';
      tablename       | null_frac | correlation | n_distinct
----------------------+-----------+-------------+------------
 clickstream1         |         0 |    0.412034 |     378174
(2 rows)

e1=# select count(distinct session_id) from clickstream1;
  count
---------
 3174813

As you can see, n_distinct estimation is off by a factor of 10x and it's
causing query planning problems.   Any suggested hacks to improve the
histogram on this?

(BTW, increasing the stats to 1000 only doubles n_distinct, and doesn't solve
the problem)

--
--Josh

Josh Berkus
Aglio Database Solutions
San Francisco