Wrong plan or what ?

Hi all,
I'm running Postgres7.3.3 and I'm performing this simple select:

select *
from user_logs ul,
user_data ud,
class_default cd
where
ul.id_user = ud.id_user and
ud.id_class = cd.id_class and
cd.id_provider = 39;

these are the number of rows for each table:

user_logs: 1258955
class_default: 31 ( only one with id_provider = 39 )
user_data: 10274;

this is the explain analyze for that query:

QUERY PLAN
Hash Join (cost=265.64..32000.76 rows=40612 width=263) (actual
time=11074.21..11134.28 rows=10 loops=1)
Hash Cond: ("outer".id_user = "inner".id_user)
-> Seq Scan on user_logs ul (cost=0.00..24932.65 rows=1258965 width=48)
(actual time=0.02..8530.21 rows=1258966 loops=1)
-> Hash (cost=264.81..264.81 rows=331 width=215) (actual
time=30.22..30.22 rows=0 loops=1)
-> Nested Loop (cost=0.00..264.81 rows=331 width=215) (actual
time=29.95..30.20 rows=6 loops=1)
-> Seq Scan on class_default cd (cost=0.00..1.39 rows=1
width=55) (actual time=0.08..0.10 rows=1 loops=1)
Filter: (id_provider = 39)
-> Index Scan using idx_user_data_class on user_data ud
(cost=0.00..258.49 rows=395 width=160) (actual time=29.82..29.96 rows=6
loops=1)
Index Cond: (ud.id_class = "outer".id_class)
Total runtime: 11135.65 msec
(10 rows)

I'm able to performe that select with these 3 steps:

SELECT id_class from class_default where id_provider = 39;
id_class
----------
48
(1 row)

SELECT id_user from user_data where id_class in ( 48 );
id_user
---------
10943
10942
10934
10927
10910
10909
(6 rows)

SELECT * from user_logs where id_user in (
10943, 10942, 10934, 10927, 10910, 10909
);
[SNIPPED]

and the time ammount is a couple of milliseconds.

Why the planner or the executor ( I don't know ) do not follow
the same strategy ?

Thank you
Gaetano Mendola