Query much faster with enable_seqscan=0

Hello,

I have received some help from the IRC channel, however, the problem still exists. When running the following query with enable_seqscan set to 0, it takes less than a second, whereas with it set to 1, the query returns in 14 seconds. The machine itself has 8GB Ram and is running PostgreSQL 9.0 on Debian Lenny. The database size is about 7GB.

Query:
SELECT tr.id, tr.sid
FROM
test_registration tr,
INNER JOIN test_registration_result r on (tr.id = r.test_registration_id)
WHERE.
tr.test_administration_id='32a22b12-aa21-11df-a606-96551e8f4e4c'::uuid
GROUP BY tr.id, tr.sid

demo=# \d test_registration
Table "public.test_registration"
Column | Type | Modifiers
------------------------+-----------------------------+------------------------
id | uuid | not null
sid | character varying(36) | not null
created_date | timestamp without time zone | not null default now()
modified_date | timestamp without time zone | not null
test_administration_id | uuid | not null
teacher_number | character varying(15) |
test_version_id | uuid |
Indexes:
"test_registration_pkey" PRIMARY KEY, btree (id)
"test_registration_sid_key" UNIQUE, btree (sid, test_administration_id)
"test_registration_teacher" btree (teacher_number)
"test_registration_test_id" btree (test_administration_id)

demo=# \d test_registration_result
Table "public.test_registration_result"
Column | Type | Modifiers
----------------------+-----------------------+-----------
answer | character varying(15) |
question_id | uuid | not null
score | double precision |
test_registration_id | uuid | not null
Indexes:
"test_registration_result_pkey" PRIMARY KEY, btree (question_id, test_registration_id)
"test_registration_result_answer" btree (test_registration_id, answer, score)
"test_registration_result_test" btree (test_registration_id)

Explain Analyze:

--------------------------------------------------------------------------------------------------------------------------------------------------------------------------
HashAggregate (cost=951169.97..951198.37 rows=2840 width=25) (actual time=14669.039..14669.843 rows=2972 loops=1)
-> Hash Join (cost=2988.07..939924.85 rows=2249024 width=25) (actual time=551.464..14400.061 rows=638980 loops=1)
Hash Cond: (r.test_registration_id = tr.id)
-> Seq Scan on test_registration_result r (cost=0.00..681946.72 rows=37199972 width=16) (actual time=0.015..6073.101 rows=37198734 loops=1)
-> Hash (cost=2952.57..2952.57 rows=2840 width=25) (actual time=2.516..2.516 rows=2972 loops=1)
Buckets: 1024 Batches: 1 Memory Usage: 160kB
-> Bitmap Heap Scan on test_registration tr (cost=44.29..2952.57 rows=2840 width=25) (actual time=0.528..1.458 rows=2972 loops=1)
Recheck Cond: (test_administration_id = 'e26a165a-c19f-11df-be2f-778af560e5a2'::uuid)
-> Bitmap Index Scan on test_registration_test_administration_id (cost=0.00..43.58 rows=2840 width=0) (actual time=0.507..0.507 rows=2972 loops=1)
Index Cond: (test_administration_id = 'e26a165a-c19f-11df-be2f-778af560e5a2'::uuid)
Total runtime: 14670.337 ms
(11 rows)

real 0m14.698s
user 0m0.000s
sys 0m0.008s

With "set enable_seqscan=0;"

SET
QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
HashAggregate (cost=1225400.19..1225428.59 rows=2840 width=25) (actual time=748.397..749.160 rows=2972 loops=1)
-> Nested Loop (cost=0.00..1214155.07 rows=2249024 width=25) (actual time=0.107..465.165 rows=638980 loops=1)
-> Index Scan using test_registration_test_administration_id on test_registration tr (cost=0.00..4413.96 rows=2840 width=25) (actual time=0.050..1.610 rows=2972 loops=1)
Index Cond: (test_administration_id = 'e26a165a-c19f-11df-be2f-778af560e5a2'::uuid)
-> Index Scan using test_registration_result_answer on test_registration_result r (cost=0.00..416.07 rows=792 width=16) (actual time=0.019..0.106 rows=215 loops=2972)
Index Cond: (r.test_registration_id = tr.id)
Total runtime: 749.745 ms
(7 rows)

real 0m0.759s
user 0m0.008s
sys 0m0.000s

The following parameters are changed in postgresql.conf and I have routinely vacuum analyzed the tables and database:

shared_buffers = 2048MB
work_mem = 8MB
maintenance_work_mem = 256MB
wal_buffers = 640kB
random_page_cost = 4.0
effective_cache_size = 7000MB
default_statistics_target = 200

free -m:
total used free shared buffers cached
Mem: 8003 7849 153 0 25 7555
-/+ buffers/cache: 268 7735
Swap: 7640 0 7639

Any help would be appreciated. Thank you very much.

Ogden