long-running query - needs tuning

Hi List;

I've just started working with a new client and they have amoung other issues
with their databases a particular update that basically locks out users.

The below query was running for over 6 hours this morning and the CPU load had
climbed to a point where new connections simply hung waiting to connect. We
had to kill the query to allow business users to connect to the applications
that connect to the database, thus I could not post an explain analyze.

In any case the query looks like this:

update dat_customer_mailbox_counts
set total_contacts = contacts.ct,
total_contact_users = contacts.dct
from
( select customer_id, count(*) as ct,
count( distinct con.user_id ) as dct
from dat_user_contacts con
group by customer_id )
contacts where contacts.customer_id = dat_customer_mailbox_counts.customer_id

Here's the latest counts from the system catalogs:

dat_customer_mailbox_counts: 423
dat_user_contacts 59,469,476

And here's an explain plan:

QUERY PLAN
----------------------------------------------------------------------------------------------------
Merge Join (cost=17118858.51..17727442.30 rows=155 width=90)
Merge Cond: ("outer".customer_id = "inner".customer_id)
-> GroupAggregate (cost=17118772.93..17727347.34 rows=155 width=8)
-> Sort (cost=17118772.93..17270915.95 rows=60857208 width=8)
Sort Key: con.customer_id
-> Seq Scan on dat_user_contacts con (cost=0.00..7332483.08
rows=60857208 width=8)
-> Sort (cost=85.57..88.14 rows=1026 width=74)
Sort Key: dat_customer_mailbox_counts.customer_id
-> Seq Scan on dat_customer_mailbox_counts (cost=0.00..34.26
rows=1026 width=74)
(9 rows)

Any thoughts, comments, Ideas for debugging, etc would be way helpful...

Thanks in advance.

/Kevin