Mysql进阶优化篇02——索引失效的10种情况及原理（一）

#班级表
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;
#学员表
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 ;
#假如要删除
#drop PROCEDURE insert_stu;

#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 ;
#假如要删除
#drop PROCEDURE insert_class;

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

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 ;

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';

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);

EXPLAIN SELECT SQL_NO_CACHE * FROM student WHERE age=30 AND classId=4;