mysql 执行计划走分区

mysql> EXPLAIN PARTITIONS   select count(*) from  history where clock<1438876800;
+----+-------------+---------+------------+-------+---------------+-----------+---------+------+------+--------------------------+
| id | select_type | table   | partitions | type  | possible_keys | key       | key_len | ref  | rows | Extra                    |
+----+-------------+---------+------------+-------+---------------+-----------+---------+------+------+--------------------------+
|  1 | SIMPLE      | history | p20150806  | index | NULL          | history_1 | 12      | NULL | 5250 | Using where; Using index |
+----+-------------+---------+------------+-------+---------------+-----------+---------+------+------+--------------------------+
1 row in set (0.00 sec)

mysql>  EXPLAIN PARTITIONS   select count(*) from  history where clock<143899999;
+----+-------------+---------+------------+-------+---------------+-----------+---------+------+------+--------------------------+
| id | select_type | table   | partitions | type  | possible_keys | key       | key_len | ref  | rows | Extra                    |
+----+-------------+---------+------------+-------+---------------+-----------+---------+------+------+--------------------------+
|  1 | SIMPLE      | history | p20150806  | index | NULL          | history_1 | 12      | NULL | 5250 | Using where; Using index |
+----+-------------+---------+------------+-------+---------------+-----------+---------+------+------+--------------------------+
1 row in set (0.00 sec)

mysql>  EXPLAIN PARTITIONS   select count(*) from  history where clock<1439049600;
+----+-------------+---------+-------------------------------+-------+---------------+-----------+---------+------+-------+--------------------------+
| id | select_type | table   | partitions                    | type  | possible_keys | key       | key_len | ref  | rows  | Extra                    |
+----+-------------+---------+-------------------------------+-------+---------------+-----------+---------+------+-------+--------------------------+
|  1 | SIMPLE      | history | p20150806,p20150807,p20150808 | index | NULL          | history_1 | 12      | NULL | 71310 | Using where; Using index |
+----+-------------+---------+-------------------------------+-------+---------------+-----------+---------+------+-------+--------------------------+
1 row in set (0.00 sec)