我对clicks加一个索引，然后加入一个where条件再查询：
create index clicks on imgs(clicks);
mysql> desc select * from imgs where userid="admin" order by clicks desc limit 10;
+----+-------------+-------+------+---------------+--------+---------+-------+------+-----------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+--------+---------+-------+------+-----------------------------+
| 1 | SIMPLE | imgs | ref | userid,clicks | userid | 51 | const | 2944 | Using where; Using filesort |
+----+-------------+-------+------+---------------+--------+---------+-------+------+-----------------------------+
1 row in set (0.00 sec)
这时可以看到possible_keys变成了userid,clicks，possible_keys是可以匹配的所有索引，mysql会从possible_keys中自己判断并取用其中一个索引来执行语句，值得注意的是，mysql取用的这个索引未必是最优化的。这次查询mysql还是使用userid这个索引来查询的，并没有按照我的意愿，所以结果还是没有什么变化。改一下sql加上use index强制mysql使用clicks索引：
mysql> desc select * from imgs use index (clicks) where userid='admin' and clicks>10 order by clicks desc limit 10
+----+-------------+-------+-------+---------------+--------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+---------------+--------+---------+------+------+-------------+
| 1 | SIMPLE | imgs | range | clicks | clicks | 4 | NULL | 5455 | Using where |
+----+-------------+-------+-------+---------------+--------+---------+------+------+-------------+
1 row in set (0.00 sec)
这时mysql用到了clicks索引进行查询，但是结果集比userid还要大!看来还要再进行限制：
mysql> desc select * from imgs use index (clicks) where userid='admin' and clicks>1000 order by clicks desc limit 10
+----+-------------+-------+-------+---------------+--------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+---------------+--------+---------+------+------+-------------+
| 1 | SIMPLE | imgs | range | clicks | clicks | 4 | NULL | 312 | Using where |
+----+-------------+-------+-------+---------------+--------+---------+------+------+-------------+
1 row in set (0.00 sec)
加到1000的时候结果集变成了312条，排序效率应该是可以接受。
不过，采用换索引这种优化方式需要取一个采样点，比如这个例子中的1000这个数字，这样，对userid的每个数值，都要去找一个采样点，这样对程序来说是很难办的。如果按1000取样的话，那么userid='7mini'这个例子中，取到的结果将不会是8条，而是2条，给用户造成了困惑。
