Mysql的索引优化原则详解

create database idx_optimize character set 'utf8';
CREATE TABLE users(
  id INT PRIMARY KEY AUTO_INCREMENT,
  user_name VARCHAR(20) NOT NULL COMMENT '姓名',
  user_age INT NOT NULL DEFAULT 0 COMMENT '年龄',
  user_level VARCHAR(20) NOT NULL COMMENT '用户等级',
  reg_time TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP COMMENT '注册时间'
);
INSERT INTO users(user_name,user_age,user_level,reg_time)
VALUES('tom',17,'A',NOW()),('jack',18,'B',NOW()),('lucy',18,'C',NOW());

ALTER TABLE users ADD INDEX idx_nal (user_name,user_age,user_level) USING BTREE;

EXPLAIN SELECT * FROM users WHERE user_name = 'tom' 
AND user_age = 17 AND user_level = 'A';

EXPLAIN SELECT * FROM users WHERE user_age = 17 AND user_level = 'A';

EXPLAIN SELECT * FROM users WHERE 
user_age = 17 AND user_name = 'tom' AND user_level = 'A';

INSERT INTO users(user_name,user_age,user_level,reg_time) VALUES('11223344',22,'D',NOW());

EXPLAIN SELECT * FROM users WHERE LEFT(user_name, 6) = '112233';

EXPLAIN SELECT * FROM users WHERE 

-- 条件只有一个 user_name
EXPLAIN SELECT * FROM users WHERE user_name = 'tom';

EXPLAIN SELECT * FROM users WHERE user_name = 'tom' AND user_age = 17;

EXPLAIN SELECT * FROM users WHERE user_name = 'tom' 
AND user_age = 17 AND user_level = 'A';

-----使用范围条件  user_age>17  ,user_level索引就失效了
EXPLAIN SELECT * FROM users WHERE user_name = 'tom' 
AND user_age > 17 AND user_level = 'A';

EXPLAIN SELECT * FROM users WHERE user_name IS NULL;
---Impossible where： 表示where条件不成立，不能返回任何的行

EXPLAIN SELECT * FROM users WHERE user_name IS NOT NULL;  ---全表扫描

EXPLAIN SELECT * FROM users WHERE user_name != 'tom';
EXPLAIN SELECT * FROM users WHERE user_name = 'tom' or user_name = 'jack';

EXPLAIN SELECT * FROM users WHERE user_name LIKE '%tom%';
EXPLAIN SELECT * FROM users WHERE user_name LIKE '%tom';

EXPLAIN SELECT * FROM users WHERE user_name LIKE 'tom%';

EXPLAIN SELECT user_name FROM users WHERE user_name LIKE '%jack%';
EXPLAIN SELECT user_name,user_age,user_level FROM users WHERE user_name LIKE '%jack%';