PSQL 8.4 - partittions - join tables - not optimal plan

Hello,

could you help me with joined query from partitioned table, please? I
have a table "data" with partitions by period_id

CREATE TABLE data
(
period_id smallint NOT NULL DEFAULT 0,
store_id smallint NOT NULL DEFAULT 0,
product_id integer NOT NULL DEFAULT 0,
s_pcs real NOT NULL DEFAULT 0,
s_val real NOT NULL DEFAULT 0
)

CONSTRAINT data_561_period_id_check CHECK (period_id = 561)
CONSTRAINT data_562_period_id_check CHECK (period_id = 562)
...

When I run a simple query with a condition period_id = something I get
best query plan:

explain select sum(s_pcs),sum(s_val)
from data d inner join periods p on d.period_id=p.period_id
where p.period_id=694;
QUERY PLAN
----------------------------------------------------------------------------------------
Aggregate (cost=214028.71..214028.72 rows=1 width=8)
-> Nested Loop (cost=0.00..181511.71 rows=6503400 width=8)
-> Index Scan using pk_periods on periods p (cost=0.00..8.27
rows=1 width=2)
Index Cond: (period_id = 694)
-> Append (cost=0.00..116469.44 rows=6503400 width=10)
-> Seq Scan on data_694 d (cost=0.00..116446.44
rows=6503395 width=10)
Filter: (d.period_id = 694)
(8 rows)

but when I try make a condition by join table, the query plan is not
optimal:

select period_id from periods where y=2009 and w=14;
period_id
-----------
704
(1 row)

explain select sum(s_pcs),sum(s_val)
from data d inner join periods p on d.period_id=p.period_id
where p.y=2009 and p.w=14;
QUERY PLAN
----------------------------------------------------------------------------------------
Aggregate (cost=15313300.27..15313300.28 rows=1 width=8)
-> Hash Join (cost=8.92..15293392.89 rows=3981476 width=8)
Hash Cond: (d.period_id = p.period_id)
-> Append (cost=0.00..12267462.15 rows=796295215 width=10)
-> Seq Scan on data d (cost=0.00..20.40 rows=1040 width=10)
-> Seq Scan on data_561 d (cost=0.00..66903.25
rows=4342825 width=10)
-> Seq Scan on data_562 d (cost=0.00..73481.02
rows=4769802 width=10)
-> Seq Scan on data_563 d (cost=0.00..73710.95
rows=4784695 width=10)
-> Seq Scan on data_564 d (cost=0.00..71869.75
rows=4665175 width=10)
-> Seq Scan on data_565 d (cost=0.00..72850.37
rows=4728837 width=10)
...

I get same result with constraint_exclusion = partition and
constraint_exclusion = on.
Do you have any idea where can be a problem?

For simple query the partitions works perfect on this table (about
2*10^9 records) but the joined query is an problem.

Thank you very much, Vrata