Very slow planning performance on partition table

Hi,

I have a table partitioned with about 60 children tables. Now I found
the planning time of simple query with partition key are very slow.
# explain analyze select count(*) as cnt from article where pid=88 and
hash_code='2ca3ff8b17b163f0212c2ba01b80a064';
QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------------------------------
Aggregate (cost=16.55..16.56 rows=1 width=0) (actual
time=0.259..0.259 rows=1 loops=1)
-> Append (cost=0.00..16.55 rows=2 width=0) (actual
time=0.248..0.250 rows=1 loops=1)
-> Seq Scan on article (cost=0.00..0.00 rows=1 width=0)
(actual time=0.002..0.002 rows=0 loops=1)
Filter: ((pid = 88) AND (hash_code =
'2ca3ff8b17b163f0212c2ba01b80a064'::bpchar))
-> Index Scan using article_88_hash_idx on article_88
article (cost=0.00..16.55 rows=1 width=0) (actual time=0.246..0.248
rows=1 loops=1)
Index Cond: (hash_code =
'2ca3ff8b17b163f0212c2ba01b80a064'::bpchar)
Filter: (pid = 88)
Total runtime: 3.816 ms
(8 rows)

Time: 30999.986 ms

You can see the timing output that the actual run time of the 'explain
analyze' is 30 seconds while the select sql itself takes only 3 ms. My
partition key is on article.pid and the constraint is simple like this:
CONSTRAINT article_88_pid_check CHECK (pid = 88). What's wrong and how
can I improve the planning performance?