Why is seq search preferred here by planner?

Hi Folks,

I want to update the country feild in table email_bank by looking
up another table personal_account_details by matching userid.
any anyone tellme why an seq scan on personal_account_details
for getting the coutry. the query is:

UPDATE email_bank set country=personal_account_details.country where userid > 0 and
userid=personal_account_details.userid ;

both tables have been vacuum analyzed before running the queries.
my question is is the the best plan ?

becoz i most get similar plan when i update in
this manner.

Its quite a decent IBM xseries server with
2GB ram and 4 Intel(R) XEON(TM) CPU 2.00GHz.

and explain analyze is below:

tradein_clients=# begin work;explain analyze UPDATE email_bank set
country=personal_account_details.country where userid > 0 and
userid=personal_account_details.userid ;BEGIN
Time: 669.57 ms

QUERY PLAN
---------------------------------------------------------------------------------------------------------------------------------------------- Hash Join (cost=14496.79..36722.98 rows=216658 width=137) (actual time=3505.31..49707.67
rows=150756 loops=1) Hash Cond: ("outer".userid = "inner".userid)
-> Seq Scan on email_bank (cost=0.00..16268.10 rows=216658 width=123) (actual
time=0.06..43033.91 rows=155714 loops=1) Filter: (userid > 0)
-> Hash (cost=14113.23..14113.23 rows=153423 width=14) (actual time=3505.08..3505.08 rows=0
loops=1) -> Seq Scan on personal_account_details (cost=0.00..14113.23 rows=153423 width=14)
(actual time=0.04..3196.12 rows=153431 loops=1) Total runtime: 209472.14 msec
(7 rows)

Regds
Mallah.

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