limit 的奇怪问题

语句 select * from T order by id desc 加了limit子句后，因limit offset差异而效率既然不同。
T的id字段为主键，以下是offset 为47和48时的explain结果，48时主键居然没有用上，这是为何？

SQL code

mysql> explain select * from T order by id desc limit 47, 30;  
+----+-------------+--------+-------+---------------+---------+---------+------+------+-------+
| id | select_type | table  | type  | possible_keys | key     | key_len | ref  | rows | Extra |
+----+-------------+--------+-------+---------------+---------+---------+------+------+-------+
|  1 | SIMPLE      | T      | index | NULL          | PRIMARY | 4       | NULL |   77 |       | 
+----+-------------+--------+-------+---------------+---------+---------+------+------+-------+
1 row in set (0.00 sec)

mysql> explain select * from T order by id desc limit 48, 30; 
+----+-------------+--------+------+---------------+------+---------+------+----------+----------------+
| id | select_type | table  | type | possible_keys | key  | key_len | ref  | rows     | Extra          |
+----+-------------+--------+------+---------------+------+---------+------+----------+----------------+
|  1 | SIMPLE      | T      | ALL  | NULL          | NULL | NULL    | NULL | 44224755 | Using filesort | 
+----+-------------+--------+------+---------------+------+---------+------+----------+----------------+
1 row in set (0.00 sec)

表结构怎样的？另外表的数据有多少呢（虽然这个不影响 explain）？