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
# 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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