Datetime类型字段上的索引和记录条数有关吗？

report 表索引字段 calldate(datetime)

查询1:
explain SELECT * FROM report WHERE calldate >= '2009-04-09 00:00:00';

+----+-------------+----------+------+---------------+------+---------+------+--------+-------------+
| id | select_type | table    | type | possible_keys | key  | key_len | ref  | rows   | Extra       |
+----+-------------+----------+------+---------------+------+---------+------+--------+-------------+
|  1 | SIMPLE      | report  | ALL  | calldate      | NULL | NULL    | NULL | 128305 | Using where |
+----+-------------+----------+------+---------------+------+---------+------+--------+-------------+
1 row in set (0.00 sec)

查询2:
explain SELECT * FROM report  WHERE calldate >= '2009-04-10 00:00:00';

+----+-------------+----------+-------+---------------+----------+---------+------+-------+-------------+
| id | select_type | table    | type  | possible_keys | key      | key_len | ref  | rows  | Extra       |
+----+-------------+----------+-------+---------------+----------+---------+------+-------+-------------+
|  1 | SIMPLE      | report  | range | calldate      | calldate | 8       | NULL | 23029 | Using where |
+----+-------------+----------+-------+---------------+----------+---------+------+-------+-------------+
1 row in set (0.00 sec)


为什么两个时间字段的范围不一样就导致了索引的使用