Nested loops are killing throughput

Postgresql 8.1

I've tried turning off nested loops to see what the query planner would choose instead of nested loops. It chose a hash
join,and it still had a nested loop in the mix! How can I entice the query planner to use a simpler join scheme? What
criteriais used to determine whether or not a merge join will be used?
 

Here's an explain analyze...

Subquery Scan foo  (cost=762.21..762.50 rows=1 width=12) (actual time=10784.849..10786.992 rows=9 loops=1) ->
GroupAggregate (cost=762.21..762.24 rows=1 width=30) (actual time=10767.702..10768.539 rows=9 loops=1)       ->  Sort
(cost=762.21..762.21rows=1 width=30) (actual time=10767.525..10767.989 rows=360 loops=1)             Sort Key:
(dpdl.field_value)::timestampwith time zone             ->  Nested Loop  (cost=2.01..762.20 rows=1 width=30) (actual
time=283.824..10749.007rows=360 loops=1)                   ->  Nested Loop  (cost=0.00..744.28 rows=1 width=16) (actual
time=31.227..6980.765rows=5436 loops=1)                         ->  Nested Loop  (cost=0.00..135.29 rows=101 width=16)
(actualtime=25.514..273.660 rows=5436 loops=1)                               ->  Index Scan using ut_company_name_idx
onuser_table ut  (cost=0.00..21.96 rows=5 width=11) (actual time=6.596..6.649 rows=9 loops=1)
         Index Cond: ((company_name)::text = Acme, Inc.'::text)                               ->  Index Scan using
packet_user_idxon packet dp  (cost=0.00..19.89 rows=222 width=27) (actual time=16.939..28.025 rows=604 loops=9)
                           Index Cond: ((dp.username)::text = ("outer".username)::text)
   Filter: (trans_date > (date_trunc('month'::text, (now() - '1 mon'::interval)) - '1 year 6 mons'::interval))
              ->  Index Scan using packet_status_puuid_pkey on packet_status dps  (cost=0.00..6.02 rows=1 width=16)
(actualtime=1.226..1.228 rows=1 loops=5436)                               Index Cond: (dps.packet_uuid =
"outer".packet_uuid)                              Filter: ((status & 2) = 0)                   ->  Bitmap Heap Scan on
packet_datalinkdpdl  (cost=2.01..17.87 rows=4 width=30) (actual time=0.685..0.688 rows=0 loops=5436)
    Recheck Cond: (dpdl.packet_uuid = "outer".packet_uuid)                         Filter: (((field_name)::text =
'event_date'::text)AND ((field_value)::date >= ('now'::text)::date))                         ->  Bitmap Index Scan on
packet_dl_puuid_idx (cost=0.00..2.01 rows=4 width=0) (actual time=0.402..0.402 rows=4 loops=5436)
       Index Cond: (dpdl.packet_uuid = "outer".packet_uuid)
 
Total runtime: 10787.198 ms

Also, no one here can figure out why the row count prediction is off on the packet table either. It's vacuumed and
analyzed.

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