我有这个查询（如下），当它应该使用created索引时它会一直命中表。我在这里遗漏了一些简单的东西吗？我怎样才能让它使用索引而不是表？
mysql> select version();
+------------+
| version() |
+------------+
| 5.5.27-log |
+------------+
mysql> explain SELECT `Lead`.`subid` AS `subid` FROM `leads` AS `Lead` WHERE `Lead`.`is_live` = '1' AND `Lead`.`created` >= '2012-12-13 00:00:00' AND `Lead`.`created` <= '2012-12-13 23:59:59' AND NOT (`Lead`.`subid` IS NULL) GROUP BY `Lead`.`subid`;
+----+-------------+-------+-------+-----------------------+---------+---------+------+-------+----------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+-----------------------+---------+---------+------+-------+----------------------------------------------+
| 1 | SIMPLE | Lead | range | is_live,subid,created …