Possibly slow query

Folks,

I'm using PostgreSQL 7.4.1 on Linux, and I'm trying to figure out weather a
query I have is going to be slow when I have more information in my tables.
both tables involved will likely have ~500K rows within a year or so.

Specifically I can't tell if I'm causing myself future problems with the
subquery, and should maybe re-write the query to use a join. The reason I
went with the subquery is that I don't know weather a row in Assignments
will have a corresponding row in Assignment_Settings

The query is:
SELECT User_ID
FROM Assignments A
WHERE A.User_ID IS NOT NULL
AND (SELECT Value FROM Assignment_Settings WHERE Setting='Status' AND
Assignment_ID=A.Assignment_ID) IS NULL
GROUP BY User_ID;

The tables and an explain analyze of the query are as follows:

neo=# \d assignments;
Table "shopper.assignments"
Column | Type |
Modifiers
---------------+--------------------------------+---------------------------
----------------------------------------------
assignment_id | integer | not null default
nextval('shopper.assignments_assignment_id_seq'::text)
sample_id | integer | not null
user_id | integer |
time | timestamp(0) without time zone | not null default now()
address_id | integer |
Indexes:
"assignments_pkey" primary key, btree (assignment_id)
"assignments_sample_id" unique, btree (sample_id)
"assignments_address_id" btree (address_id)
"assignments_user_id" btree (user_id)
Triggers:
assignments_check_assignment BEFORE INSERT ON assignments FOR EACH ROW
EXECUTE PROCEDURE check_assignment()

neo=# \d assignment_settings
Table
"shopper.assignment_settings"
Column | Type |
Modifiers
-----------------------+------------------------+---------------------------
--------------------------------------------------------------
assignment_setting_id | integer | not null default
nextval('shopper.assignment_settings_assignment_setting_id_seq'::text)
assignment_id | integer | not null
setting | character varying(250) | not null
value | text |
Indexes:
"assignment_settings_pkey" primary key, btree (assignment_setting_id)
"assignment_settings_assignment_id_setting" unique, btree
(assignment_id, setting)

neo=# explain analyze SELECT User_ID FROM Assignments A WHERE A.User_ID IS
NOT NULL AND (SELECT Value FROM Assignment_Settings WHERE Setti
ng='Status' AND Assignment_ID=A.Assignment_ID) IS NULL GROUP BY User_ID;
QUERY PLAN
----------------------------------------------------------------------------
------------------------------------------------
HashAggregate (cost=1.01..1.01 rows=1 width=4) (actual time=0.057..0.058
rows=1 loops=1)
-> Seq Scan on assignments a (cost=0.00..1.01 rows=1 width=4) (actual
time=0.033..0.040 rows=2 loops=1)
Filter: ((user_id IS NOT NULL) AND ((subplan) IS NULL))
SubPlan
-> Seq Scan on assignment_settings (cost=0.00..0.00 rows=1
width=13) (actual time=0.001..0.001 rows=0 loops=2)
Filter: (((setting)::text = 'Status'::text) AND
(assignment_id = $0))
Total runtime: 0.159 ms
(7 rows)

Thanks in advance for any help!

Thanks,
Peter Darley