SQL索引失效的11种情况详析

CREATE DATABASE atguigudb2;
USE atguigudb2;

#############    class 表    #################
CREATE TABLE `class` (
`id` INT(11) NOT NULL AUTO_INCREMENT,
`className` VARCHAR(30) DEFAULT NULL,
`address` VARCHAR(40) DEFAULT NULL,
`monitor` INT NULL ,
PRIMARY KEY (`id`)
) ENGINE=INNODB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;

#############    student 表    #################
CREATE TABLE `student` (
`id` INT(11) NOT NULL AUTO_INCREMENT,
`stuno` INT NOT NULL ,
`name` VARCHAR(20) DEFAULT NULL,
`age` INT(3) DEFAULT NULL,
`classId` INT(11) DEFAULT NULL,
PRIMARY KEY (`id`)
#CONSTRAINT `fk_class_id` FOREIGN KEY (`classId`) REFERENCES `t_class` (`id`)
) ENGINE=INNODB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;

#################################

SET GLOBAL log_bin_trust_function_creators=1; # 不加global只是当前窗口有效。

#随机产生字符串
DELIMITER //
CREATE FUNCTION rand_string(n INT) RETURNS VARCHAR(255)
BEGIN
DECLARE chars_str VARCHAR(100) DEFAULT
'abcdefghijklmnopqrstuvwxyzABCDEFJHIJKLMNOPQRSTUVWXYZ';
DECLARE return_str VARCHAR(255) DEFAULT '';
DECLARE i INT DEFAULT 0;
WHILE i < n DO
SET return_str =CONCAT(return_str,SUBSTRING(chars_str,FLOOR(1+RAND()*52),1));
SET i = i + 1;
END WHILE;
RETURN return_str;
END //
DELIMITER ;
#假如要删除
#drop function rand_string;

#用于随机产生多少到多少的编号
DELIMITER //
CREATE FUNCTION rand_num (from_num INT ,to_num INT) RETURNS INT(11)
BEGIN
DECLARE i INT DEFAULT 0;
SET i = FLOOR(from_num +RAND()*(to_num - from_num+1)) ;
RETURN i;
END //
DELIMITER ;
#假如要删除
#drop function rand_num;

#创建往stu表中插入数据的存储过程
DELIMITER //
CREATE PROCEDURE insert_stu( START INT , max_num INT )
BEGIN
DECLARE i INT DEFAULT 0;
SET autocommit = 0; #设置手动提交事务
REPEAT #循环
SET i = i + 1; #赋值
INSERT INTO student (stuno, NAME ,age ,classId ) VALUES
((START+i),rand_string(6),rand_num(1,50),rand_num(1,1000));
UNTIL i = max_num
END REPEAT;
COMMIT; #提交事务
END //
DELIMITER ;

#执行存储过程，往class表添加随机数据
DELIMITER //
CREATE PROCEDURE `insert_class`( max_num INT )
BEGIN
DECLARE i INT DEFAULT 0;
SET autocommit = 0;
REPEAT
SET i = i + 1;
INSERT INTO class ( classname,address,monitor ) VALUES
(rand_string(8),rand_string(10),rand_num(1,100000));
UNTIL i = max_num
END REPEAT;
COMMIT;
END //
DELIMITER ;

#执行存储过程，往class表添加1万条数据
CALL insert_class(10000);

#执行存储过程，往stu表添加50万条数据
CALL insert_stu(100000,500000);

SELECT COUNT(*) FROM class;
SELECT COUNT(*) FROM student;

############################### 删除索引的存储过程 ########################
DELIMITER //
CREATE PROCEDURE `proc_drop_index`(dbname VARCHAR(200),tablename VARCHAR(200))
BEGIN
DECLARE done INT DEFAULT 0;
DECLARE ct INT DEFAULT 0;
DECLARE _index VARCHAR(200) DEFAULT '';
DECLARE _cur CURSOR FOR SELECT index_name FROM
information_schema.STATISTICS WHERE table_schema=dbname AND table_name=tablename AND
seq_in_index=1 AND index_name <>'PRIMARY' ;
#每个游标必须使用不同的declare continue handler for not found set done=1来控制游标的结束
DECLARE CONTINUE HANDLER FOR NOT FOUND SET done=2 ;
#若没有数据返回,程序继续,并将变量done设为2
OPEN _cur;
FETCH _cur INTO _index;
WHILE _index<>'' DO
SET @str = CONCAT("drop index " , _index , " on " , tablename );
PREPARE sql_str FROM @str ;
EXECUTE sql_str;
DEALLOCATE PREPARE sql_str;
SET _index='';
FETCH _cur INTO _index;
END WHILE;
CLOSE _cur;
END //
DELIMITER ;
# 执行存储过程
CALL proc_drop_index("dbname","tablename");

# 【1】. 全值匹配
# student表,主键id,此时无索引,耗时大
EXPLAIN SELECT SQL_NO_CACHE * FROM student WHERE age = 30;
EXPLAIN SELECT SQL_NO_CACHE * FROM student WHERE age = 30 AND classId = 4;
EXPLAIN SELECT SQL_NO_CACHE * FROM student WHERE age = 30 AND classId = 4 AND NAME = 'abcd';

# 注：SQL_NO_CACHE 不使用查询缓存

# 建立索引
CREATE INDEX idx_age ON student(age);
CREATE INDEX idx_age_classid ON student(age,classId);
CREATE INDEX idx_age_classid_name ON student(age,classId,NAME);	
# 此时第三条查询语句默认使用最后一条索引,而不是前两个

# 【2】. 最佳左前缀法则
EXPLAIN SELECT SQL_NO_CACHE * FROM student WHERE student.age = 30 AND student.name = 'abcd';	
# 查age&name，用age的索引

EXPLAIN SELECT SQL_NO_CACHE * FROM student WHERE student.classid = 1 AND student.name = 'abcd';	
# 查classid&name，classid在前，有索引的话先找classid相同的，再找name，
#但现在没有这样的索引，idx_age_classid_name的字段顺序是先找age，所以不符合，所以此时不能用索引

EXPLAIN SELECT SQL_NO_CACHE * FROM student 
WHERE classid = 4 AND student.age = 30 AND student.name = 'abcd';	
#idx_age_classid_name 联合索引中所有字段均出现，可以使用该索引

EXPLAIN SELECT SQL_NO_CACHE * FROM student
WHERE student.age = 30 AND student.name = 'abcd';
# 现在，删除idx_age和idx_age_classid，发现用到idx_age_classid_name，而key_len=5,即只用到age字段，int(4)+null(1)
#因为索引完age后没有classid了，不能再查找到name

# 【4】. 计算、函数、类型转换(自动或手动)导致索引失效
##### 例1：
EXPLAIN SELECT SQL_NO_CACHE * FROM student WHERE student.name LIKE 'abc%';	#更好，能够使用上索引
# type=range 使用了索引中的排序

EXPLAIN SELECT SQL_NO_CACHE * FROM student WHERE LEFT(student.name,3) = 'abc';	# left(text,num_chars):截取左侧n个字符
# type = all 全表的访问
# 该语句的执行过程：针对每一条数据，一个一个取出，先作用一遍函数，再拿函数结果与abc对比，用不上b+树

CREATE INDEX idx_name ON student(NAME);

##### 例2：
CREATE INDEX idx_sno ON student(stuno);
EXPLAIN SELECT SQL_NO_CACHE id,stuno,NAME FROM student WHERE stuno+1 = 900001; 	# type = all 需要做运算，无法直接用索引找值

EXPLAIN SELECT SQL_NO_CACHE id,stuno,NAME FROM student WHERE stuno = 900000; 	# type = ref

# 【5】. 类型转换导致索引失效
# 未使用到索引
EXPLAIN SELECT SQL_NO_CACHE * FROM student WHERE NAME=123;	# 这里使用了隐式转换
# 使用到索引
EXPLAIN SELECT SQL_NO_CACHE * FROM student WHERE NAME='123'; 	# name本身就是字符串类型

# 【6】. 范围条件右边的列索引失效 （ > < >= <= between 等）
SHOW INDEX FROM student;
CALL proc_drop_index('atguigudb2','student');

CREATE INDEX idx_age_classid_name ON student(age,classId,NAME);

EXPLAIN SELECT SQL_NO_CACHE * FROM student
WHERE student.age = 30 AND student.classId > 20 AND student.name = 'abc';	# 这三个and先写谁无所谓，优化器会调优
# key_len = 10, age=5,classId=5,name用不上。classId 是范围，索引右侧的name用不上

# 改写索引：
CREATE INDEX idx_age_name_cid ON student(age,NAME,classId); 	#把需要排序的classid放到最后
# 此时在执行上面的语句，就使用了这个索引，key_len=73

# 【7】. 不等于(!= 或者<>)索引失效
CREATE INDEX idx_name ON student(NAME);

EXPLAIN SELECT SQL_NO_CACHE * FROM student WHERE student.name <> 'abc';	# 索引失效 索引查的是等于

# 【8】. is null可以使用索引，is not null无法使用索引
EXPLAIN SELECT SQL_NO_CACHE * FROM student WHERE age IS NULL;	# type=ref 相当于等于某个值
EXPLAIN SELECT SQL_NO_CACHE * FROM student WHERE age IS NOT NULL;	# 索引失效 相当于不等于

# 【9】. like以通配符%开头索引失效
EXPLAIN SELECT SQL_NO_CACHE * FROM student WHERE NAME LIKE 'ab%';	# 可用索引

EXPLAIN SELECT SQL_NO_CACHE * FROM student WHERE NAME LIKE '%ab';	# type = all 索引失效

# 【10】. OR 前后存在非索引的列，索引失效 

CALL proc_drop_index('atguigudb2','student');
SHOW INDEX FROM student;
# 创建一个age的索引
CREATE INDEX idx_age ON student(age);

EXPLAIN SELECT SQL_NO_CACHE * FROM student WHERE age = 10 OR classid = 100;	# 未使用索引，索引+全表扫描->全表扫描
# 再加一个字段的单独索引
CREATE INDEX idx_cid ON student(classid);
# 再执行上条语句，此时 type = index_merge ，key = idx_age,idx_cid。