ill-planned queries inside a stored procedure

Hi all,
do you know any clean workaround at ill-planned queries inside a stored procedure?
Let me explain with an example:

empdb=# select count(*) from user_logs;
count
---------
5223837
(1 row)

empdb=# select count(*) from user_logs where id_user = 5024;
count
--------
239453
(1 row)

empdb=# explain analyze select login_time from user_logs where id_user = 5024 order by id_user_log desc limit 1;
QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------------------------------
Limit (cost=0.00..22.62 rows=1 width=12) (actual time=3.921..3.922 rows=1 loops=1)
-> Index Scan Backward using user_logs_pkey on user_logs (cost=0.00..5355619.65 rows=236790 width=12) (actual time=3.918..3.918 rows=1 loops=1)
Filter: (id_user = 5024)
Total runtime: 3.963 ms
(4 rows)

same select in a prepared query ( I guess the stored procedure use same plan ):

empdb=# explain analyze execute test(5024);
QUERY PLAN
--------------------------------------------------------------------------------------------------------------------------------------------------------
Limit (cost=759.60..759.61 rows=1 width=12) (actual time=45065.755..45065.756 rows=1 loops=1)
-> Sort (cost=759.60..760.78 rows=470 width=12) (actual time=45065.748..45065.748 rows=1 loops=1)
Sort Key: id_user_log
-> Index Scan using idx_user_user_logs on user_logs (cost=0.00..738.75 rows=470 width=12) (actual time=8.936..44268.087 rows=239453 loops=1)
Index Cond: (id_user = $1)
Total runtime: 45127.256 ms
(6 rows)

There is a way to say: replan this query at execution time ?

Regards
Gaetano Mendola