weird EXPLAIN

hi ,
I have just migrated to PostgreSQL...and found the following wierd :

suryadb=# explain select * from dbwin where id in (select id from wdwin
where word='bacd');
NOTICE: QUERY PLAN:
Seq Scan on dbwin (cost=0.00..8158.20 rows=1000 width=76)
^^^^^^^^^^^^^^^^^^
SubPlan
-> Materialize (cost=8.14..8.14 rows=10 width=4)
-> Index Scan using wdkwin on wdwin (cost=0.00..8.14 rows=10 width=4)

i wanted to know why a sequential scan is done on the field id
while it is a primary key in the table dbwin.this can be clearly
confirmed from the query below :

suryadb=# explain select * from dbwin where id=1234;
NOTICE: QUERY PLAN:
Index Scan using dbwin_pkey on dbwin (cost=0.00..8.14 rows=10 width=76)

why does it do a sequential scan for the query above??
Also i would be happy if someone could explain me the numbers in the
cost thing.. i read the docs....but it is kinda confusing and incomplete.

--
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Varun
Intel Inside - Idiot outside