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！

sql如下：sql强制用了into_time索引

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

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

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

SET timestamp=1487043301;

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：

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

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

Name: customers

Engine: InnoDB

Version: 10

Row_format: Dynamic

Rows: 2504609

Avg_row_length: 710

Data_length: 1780383744

Max_data_length: 0

Index_length: 1253048320

Data_free: 6291456

Auto_increment: 2546101

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

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

Check_time: NULL

Collation: utf8_general_ci

Checksum: NULL

Create_options:

Comment:

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

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

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

| count(*) |

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

| 2439147 |

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

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

Create Table: CREATE TABLE `customers` (

`id` int(11) NOT NULL AUTO_INCREMENT,

PRIMARY KEY (`id`),

KEY `newdata` (`newdata`),

KEY `cusname` (`cusname`),

KEY `type` (`type`,`ownerid`),

KEY `operator` (`operator`),

KEY `into_time` (`into_time`),

KEY `isarea` (`isarea`),

KEY `linkcase` (`linkcase`),

KEY `score` (`score`),

FULLTEXT KEY `fdx_cusname` (`cusname_idx`)

) ENGINE=InnoDB AUTO_INCREMENT=2546101 DEFAULT CHARSET=utf8

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

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;

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

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

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

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

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

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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