mysql性能的检查和调优方法（2）

但是，如果我用别的userid查询，结果又会有所不同：

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 | userid | 51 | const | 2944 | Using where; Using filesort |

+----+-------------+-------+------+---------------+--------+---------+-------+------+-----------------------------+

1 row in set (0.00 sec)

这个结果和userid="7mini"的结果基本相同，但是mysql用userid索引一次搜索后结果集的大小达到2944条，这2944条记录都会加入内存进行filesort，效率比起7mini那次来说就差很多了。这时可以有两种办法可以解决，第一种办法是再加一个索引和判断条件，因为我只需要根据点击量取最大的10条数据，所以有很多数据我根本不需要加进来排序，比如点击量小于10的，这些数据可能占了很大部分。

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)