What does MySQL "USE INDEX()" without any arguments do?
I've just did a test, and apparently it forces the query not to use any index:

mysql&gt; explain select email from B2 where length(email)&gt;30;
+----+-------------+-------+-------+---------------+-------+---------+------+--------+--------------------------+
| id | select_type | table | type  | possible_keys | key   | key_len | ref  | rows   | Extra                    |
+----+-------------+-------+-------+---------------+-------+---------+------+--------+--------------------------+
|  1 | SIMPLE      | B2    | index | NULL          | email | 67      | NULL | 118097 | Using where; Using index |
+----+-------------+-------+-------+---------------+-------+---------+------+--------+--------------------------+
1 row in set (0.00 sec)

mysql&gt; explain select email from B2 use index() where length(email)&gt;30;
+----+-------------+-------+------+---------------+------+---------+------+--------+-------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows   | Extra       |
+----+-------------+-------+------+---------------+------+---------+------+--------+-------------+
|  1 | SIMPLE      | B2    | ALL  | NULL          | NULL | NULL    | NULL | 118097 | Using where |
+----+-------------+-------+------+---------------+------+---------+------+--------+-------------+
1 row in set (0.00 sec)


So, you SHOULD remove this clause if it is empty.

On another note, I think you should not force an index, with the exception of few rare cases. If the you notice that a query is using an index which is not optimal, you'd better analyze the situation, and know why is this happening, and fix it. (Some times, optimize would do the job)