mysql explain

mysql> explain select app_id,uid from client_month_visit_user_201206 where (login_days & (b'11' << 9)) !=0 limit 100;
+----+-------------+--------------------------------+------+---------------+------+---------+------+----------+-------------+
| id | select_type | table                          | type | possible_keys | key  | key_len | ref  | rows     | Extra       |
+----+-------------+--------------------------------+------+---------------+------+---------+------+----------+-------------+
|  1 | SIMPLE      | client_month_visit_user_201206 | ALL  | NULL          | NULL | NULL    | NULL | 17098195 | Using where |
+----+-------------+--------------------------------+------+---------------+------+---------+------+----------+-------------+
1 row in set (0.00 sec)

mysql> explain select app_id,uid from client_month_visit_user_201206 where (login_days & (b'11' << 9)) !=0 order by app_id limit 100;
+----+-------------+--------------------------------+-------+---------------+---------+---------+------+----------+-------------+
| id | select_type | table                          | type  | possible_keys | key     | key_len | ref  | rows     | Extra       |
+----+-------------+--------------------------------+-------+---------------+---------+---------+------+----------+-------------+
|  1 | SIMPLE      | client_month_visit_user_201206 | index | NULL          | PRIMARY | 8       | NULL | 17098195 | Using where |
+----+-------------+--------------------------------+-------+---------------+---------+---------+------+----------+-------------+
1 row in set (0.00 sec)

mysql> desc client_month_visit_user_201206;
+------------+---------+------+-----+---------+-------+
| Field      | Type    | Null | Key | Default | Extra |
+------------+---------+------+-----+---------+-------+
| uid        | int(11) | NO   | PRI | NULL    |       |
| app_id     | int(11) | NO   | PRI | 0       |       |
| login_days | bit(31) | YES  |     |         |       |
+------------+---------+------+-----+---------+-------+
3 rows in set (0.00 sec)

mysql> show create table client_month_visit_user_201206;
+--------------------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table                          | Create Table                                                                                                                                                                                                                       |
+--------------------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| client_month_visit_user_201206 | CREATE TABLE `client_month_visit_user_201206` (
  `uid` int(11) NOT NULL,
  `app_id` int(11) NOT NULL default '0',
  `login_days` bit(31) default '\0\0\0\0',
  PRIMARY KEY  (`app_id`,`uid`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8 |
+--------------------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)