这篇文章主要介绍MySQL中SQL语句优化的示例分析，文中介绍的非常详细，具有一定的参考价值，感兴趣的小伙伴们一定要看完！

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sql如下：sql强制用了into_time索引

1. # Time: 2017-02-14T11:35:01.594499+08:00

2. # User@Host: oms_readonly[oms_readonly] @ [10.44.xxx.xxx] Id: 41636892

3. # Query_time: 9.299612 Lock_time: 0.000124 Rows_sent: 20 Rows_examined: 2439330

4. SET timestamp=1487043301;

5. select * from customers force index(`into_time`) where `type` = 1 AND `status` < 7 AND `isarea` = 8 AND `into_time`<='2017-01-31 23:59:59' order by score desc limit 0,20;

查看表的相关状态：

1. mysql> show table status like 'customers' \G;

2. *************************** 1. row ***************************

3.            Name: customers

4.          Engine: InnoDB

5.         Version: 10

6.      Row_format: Dynamic

7.            Rows: 2504609

8.  Avg_row_length: 710

9.     Data_length: 1780383744

10. Max_data_length: 0

11.    Index_length: 1253048320

12.       Data_free: 6291456

13.  Auto_increment: 2546101

14.     Create_time: 2017-01-07 01:59:34

15.     Update_time: 2017-02-14 13:58:17

16.      Check_time: NULL

17.       Collation: utf8_general_ci

18.        Checksum: NULL

19.  Create_options:

20.         Comment:

21. 1 row in set (0.00 sec)

表一共大约有250万行记录，查看下满足into_time<='2017-01-31 23:59:59'这个条件的有多少行

1. mysql> select count(*) from customers where `into_time`<='2017-01-31 23:59:59';

2. +----------+

3. | count(*) |

4. +----------+

5. | 2439147 |

6. +----------+

7. 1 row in set (0.95 sec)

显然into_time这个列的索引已经不合适了，查看下表上都有那些索引

1. Create Table: CREATE TABLE `customers` (

2.   `id` int(11) NOT NULL AUTO_INCREMENT,

3.   PRIMARY KEY (`id`),

4.   KEY `newdata` (`newdata`),

5.   KEY `cusname` (`cusname`),

6.   KEY `type` (`type`,`ownerid`),

7.   KEY `operator` (`operator`),

8.   KEY `into_time` (`into_time`),

9.   KEY `isarea` (`isarea`),

10.   KEY `linkcase` (`linkcase`),

11.   KEY `score` (`score`),

12.   FULLTEXT KEY `fdx_cusname` (`cusname_idx`)

13. ) ENGINE=InnoDB AUTO_INCREMENT=2546101 DEFAULT CHARSET=utf8

14. 1 row in set (0.00 sec)

可以看到score列有索引，如果能采用这个列的索引是个比较好的选择，去掉强制索引看下执行计划

1. 
   
   

1. mysql> explain select * from customers where `type` = 1 AND `status` < 7 AND `isarea` = 8 AND `into_time`<='2017-01-31 23:59:59' order by score desc limit 0,20;

2. +----+-------------+-----------+------------+-------+-----------------------------------------------------------------+-------+---------+------+------+----------+-------------+

3. | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |

4. +----+-------------+-----------+------------+-------+-----------------------------------------------------------------+-------+---------+------+------+----------+-------------+

5. | 1 | SIMPLE | customers | NULL | index | type,into_time,isarea,status,idx_isarea_renew_owner,type_status | score | 2 | NULL | 270 | 0.92 | Using where |

6. +----+-------------+-----------+------------+-------+-----------------------------------------------------------------+-------+---------+------+------+----------+-------------+

7. 1 row in set, 1 warning (0.00 sec)

可以看到用了score索引，执行时间从最9秒多，到优化后的0.0几秒。

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