官方文档

https://dev.mysql.com/doc/

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如果英文不好的话，可以参考 searchdoc 翻译的中文版本

http://www.searchdoc.cn/rdbms/mysql/dev.mysql.com/doc/refman/5.7/en/index.com.coder114.cn.html
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使用索引扫描来优化排序

存储引擎： Innodb

重点：优化排序手段：利用索引

两个思路： 1 通过排序操作、 2 按照索引顺序扫描数据

索引的列顺序和Order By子句的顺序完全一致

举几个例子

mysql> show create table rental \G;
*************************** 1. row ***************************
       Table: rental
Create Table: CREATE TABLE `rental` (
  `rental_id` int(11) NOT NULL AUTO_INCREMENT,
  `rental_date` datetime NOT NULL,
  `inventory_id` mediumint(8) unsigned NOT NULL,
  `customer_id` smallint(5) unsigned NOT NULL,
  `return_date` datetime DEFAULT NULL,
  `staff_id` tinyint(3) unsigned NOT NULL,
  `last_update` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
  PRIMARY KEY (`rental_id`),
  UNIQUE KEY `rental_date` (`rental_date`,`inventory_id`,`customer_id`),
  KEY `idx_fk_inventory_id` (`inventory_id`),
  KEY `idx_fk_customer_id` (`customer_id`),
  KEY `idx_fk_staff_id` (`staff_id`),
  CONSTRAINT `fk_rental_customer` FOREIGN KEY (`customer_id`) REFERENCES `customer` (`customer_id`) ON UPDATE CASCADE,
  CONSTRAINT `fk_rental_inventory` FOREIGN KEY (`inventory_id`) REFERENCES `inventory` (`inventory_id`) ON UPDATE CASCADE,
  CONSTRAINT `fk_rental_staff` FOREIGN KEY (`staff_id`) REFERENCES `staff` (`staff_id`) ON UPDATE CASCADE
) ENGINE=InnoDB AUTO_INCREMENT=16050 DEFAULT CHARSET=utf8mb4
1 row in set (0.00 sec)

ERROR: 
No query specified

mysql> explain select * from rental where rental_date > '2005-01-01' order by rental_id \G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: rental
   partitions: NULL
         type: index              ---------------> 索引
possible_keys: rental_date
          key: PRIMARY
      key_len: 4
          ref: NULL
         rows: 16008
     filtered: 50.00
        Extra: Using where
1 row in set, 1 warning (0.00 sec)

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