Table scan instead of index scan

Hi,

I am sure PostgreSQL is a very powerful product!

I use PostgreSQL 7.1.2.4 in two servers under RedHat 7.0.
Our database has a table like that:
create table tb_client
(
id_client varchar(32),
client_name varchar(40),
bdate date,
...
);

create index xiftb_client on tb_client(id_client);

When I execute the following query:

SELECT * FROM tb_cliente WHERE id_client LIKE '1234%';

One server executes index search and other full table scan.

I don't know the reason.

If you could help me I would appreciate a lot.

Thanks,

Mário Sérgio