为什么分区不使用索引键

Question

CREATE TABLE `tmp` (
  `CallID` bigint(8) unsigned NOT NULL,
  `InfoID` bigint(8) unsigned NOT NULL,
  `CallStartTime` datetime NOT NULL,
  `PartitionID` int(4) unsigned NOT NULL,
  KEY `CallStartTime`(`CallStartTime`)
  ) ENGINE=InnoDB DEFAULT CHARSET=latin1
    PARTITION BY RANGE (`PartitionID`)(
    PARTITION p0 VALUES LESS THAN ( 50 ),
    PARTITION p1 VALUES LESS THAN ( 100 ),
    PARTITION p2 VALUES LESS THAN ( 150 ),
    PARTITION p3 VALUES LESS THAN ( 200 ),
    PARTITION p4 VALUES LESS THAN ( 250 ),
    PARTITION p5 VALUES LESS THAN ( 300 ),
    PARTITION p6 VALUES LESS THAN ( 350 ),
    PARTITION p7 VALUES LESS THAN MAXVALUE );

我正在执行此查询：

EXPLAIN PARTITIONS 
 SELECT * 
   FROM tmp 
  WHERE (`PartitionID` BETWEEN dayofyear('2013-06-19 01:00:00') 
                           AND dayofyear('2013-08-13 12:10:06') )
    AND (CallStartTime  BETWEEN '2013-06-19 01:00:00' AND '2013-08-13 12:10:06')

结果：

id  select_type table   partitions  type    possible_keys   key key_len ref rows    Extra
1   SIMPLE  tmp           p3,p4     ALL CallStartTime               11  Using where