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Explain工具介绍

使用EXPLAIN关键字可以模拟优化器执行SQL语句，分析你的查询语句或是结构的性能瓶颈
在 select 语句之前增加 explain 关键字，MySQL 会在查询上设置一个标记，执行查询会返回执行计划的信息，而不是执行这条SQL
注意：如果 from 中包含子查询，仍会执行该子查询，将结果放入临时表中

Explain分析示例

示例表：
DROP TABLE IF EXISTS `actor`; 
CREATE TABLE `actor` (`id` int(11) NOT NULL,`name` varchar(45) DEFAULT NULL,`update_time` datetime DEFAULT NULL,PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;INSERT INTO `actor` (`id`, `name`, `update_time`) VALUES (1,'a','2017-12-22 15:27:18'), (2,'b','2017-12-22 15:27:18'), (3,'c','2017-12-22 15:27:18');DROP TABLE IF EXISTS `film`;
CREATE TABLE `film` (`id` int(11) NOT NULL AUTO_INCREMENT,`name` varchar(10) DEFAULT NULL,PRIMARY KEY (`id`),KEY `idx_name` (`name`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;INSERT INTO `film` (`id`, `name`) VALUES (3,'film0'),(1,'film1'),(2,'film2');DROP TABLE IF EXISTS `film_actor`;
CREATE TABLE `film_actor` (`id` int(11) NOT NULL,`film_id` int(11) NOT NULL,`actor_id` int(11) NOT NULL,`remark` varchar(255) DEFAULT NULL,PRIMARY KEY (`id`),KEY `idx_film_actor_id` (`film_id`,`actor_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;INSERT INTO `film_actor` (`id`, `film_id`, `actor_id`) VALUES (1,1,1),(2,1,2),(3,2,1);

索引最佳实践

示例表：
CREATE TABLE `employees` (`id` int(11) NOT NULL AUTO_INCREMENT,`name` varchar(24) NOT NULL DEFAULT '' COMMENT '姓名',`age` int(11) NOT NULL DEFAULT '0' COMMENT '年龄',`position` varchar(20) NOT NULL DEFAULT '' COMMENT '职位',`hire_time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP COMMENT '入职时间',PRIMARY KEY (`id`),KEY `idx_name_age_position` (`name`,`age`,`position`) USING BTREE
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8 COMMENT='员工记录表';INSERT INTO employees(name,age,position,hire_time) VALUES('LiLei',22,'manager',NOW());
INSERT INTO employees(name,age,position,hire_time) VALUES('HanMeimei', 23,'dev',NOW());
INSERT INTO employees(name,age,position,hire_time) VALUES('Lucy',23,'dev',NOW());