----------------------+
| pad |
+----------------------------------------------------+
| qqqqqqqqqqwwwwwwwwwweeeeeeeeeerrrrrrrrrrtttttttttt |
| qqqqqqqqqqwwwwwwwwwweeeeeeeeeerrrrrrrrrrtttttttttt |
| qqqqqqqqqqwwwwwwwwwweeeeeeeeeerrrrrrrrrrtttttttttt |
| qqqqqqqqqqwwwwwwwwwweeeeeeeeeerrrrrrrrrrtttttttttt |
| qqqqqqqqqqwwwwwwwwwweeeeeeeeeerrrrrrrrrrtttttttttt |
| qqqqqqqqqqwwwwwwwwwweeeeeeeeeerrrrrrrrrrtttttttttt |
| qqqqqqqqqqwwwwwwwwwweeeeeeeeeerrrrrrrrrrtttttttttt |
| qqqqqqqqqqwwwwwwwwwweeeeeeeeeerrrrrrrrrrtttttttttt |
| qqqqqqqqqqwwwwwwwwwweeeeeeeeeerrrrrrrrrrtttttttttt |
| qqqqqqqqqqwwwwwwwwwweeeeeeeeeerrrrrrrrrrtttttttttt |
| qqqqqqqqqqwwwwwwwwwweeeeeeeeeerrrrrrrrrrtttttttttt |
| qqqqqqqqqqwwwwwwwwwweeeeeeeeeerrrrrrrrrrtttttttttt |
| qqqqqqqqqqwwwwwwwwwweeeeeeeeeerrrrrrrrrrtttttttttt |
| qqqqqqqqqqwwwwwwwwwweeeeeeeeeerrrrrrrrrrtttttttttt |
| qqqqqqqqqqwwwwwwwwwweeeeeeeeeerrrrrrrrrrtttttttttt |
| qqqqqqqqqqwwwwwwwwwweeeeeeeeeerrrrrrrrrrtttttttttt |
| qqqqqqqqqqwwwwwwwwwweeeeeeeeeerrrrrrrrrrtttttttttt |
mysql> desc sbtest;
+-------+------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------+------------------+------+-----+---------+----------------+
| id | int(10) unsigned | NO | PRI | NULL | auto_increment |
| k | int(10) unsigned | NO | MUL | 0 | |
| c | char(120) | NO | | | |
| pad | varchar(60) | YES | | NULL | |
+-------+------------------+------+-----+---------+----------------+
＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝无索引的情况下：＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝
varchar(60)的情况下：
mysql> desc select * from sbtest order by pad;
+----+-------------+--------+------+---------------+------+---------+------+---------+----------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+------+---------------+------+---------+------+---------+----------------+
| 1 | SIMPLE | sbtest | ALL | NULL | NULL | NULL | NULL | 1000117 | Using filesort |
+----+-------------+--------+------+---------------+------+---------+------+---------+----------------+
mysql> select count(*) from sbtest order by pad;
+----------+
| count(*) |
+----------+
| 1000000 |
+----------+
1 row in set (0.51 sec)
mysql> show profile cpu,block io for query 11;
+----------------------+----------+----------+------------+--------------+---------------+
| Status | Duration | CPU_user | CPU_system | Block_ops_in | Block_ops_out |
+----------------------+----------+----------+------------+--------------+---------------+
| starting | 0.000081 | 0.000000 | 0.000000 | 0 | 0 |
| checking permissions | 0.000014 | 0.000000 | 0.000000 | 0 | 0 |
| Opening tables | 0.000033 | 0.000000 | 0.000000 | 0 | 0 |
| System lock | 0.000025 | 0.000000 | 0.000000 | 0 | 0 |
| init | 0.000028 | 0.000000 | 0.000000 | 0 | 0 |
| optimizing | 0.000009 | 0.000000 | 0.000000 | 0 | 0 |
| statistics | 0.000019 | 0.000000 | 0.000000 | 0 | 0 |
| preparing | 0.000013 | 0.000000 | 0.000000 | 0 | 0 |
| executing | 0.000011 | 0.000000 | 0.000000 | 0 | 0 |
| Sending data | 0.506327 | 0.489925 | 0.018997 | 0 | 0 |
| end | 0.000026 | 0.001000 | 0.000000 | 0 | 0 |
| query end | 0.000010 | 0.000000 | 0.000000 | 0 | 0 |
| closing tables | 0.000022 | 0.000000 | 0.000000 | 0 | 0 |
| freeing items | 0.000034 | 0.000000 | 0.000000 | 0 | 0 |
| logging slow query | 0.000003 | 0.000000 | 0.000000 | 0 | 0 |
| logging slow query | 0.000114 | 0.000000 | 0.000000 | 0 | 8 |
| cleaning up | 0