利用联合索引优化filesoert

  业务场景：查找某个人的历史记录，如果前端传来userid，我们直接在userid建立索引的话，那么按照时间排序就会有排序的情况，即extra会有filesort标志，我们可以利用联合索引建立（userid，time）的联合索引，保证他们两个就是有序的。注意，这里如果数据量过小，mysql的优化器就会将他们全表扫描

INSERT INTO history_record VALUES ('1','1','baidu','tencent',NOW(),12,13);
INSERT INTO history_record VALUES ('2','1','baidu','tencent',NOW(),12,13);
INSERT INTO history_record VALUES ('3','1','baidu','tencent',NOW(),12,13);
INSERT INTO history_record VALUES ('4','1','baidu','tencent',NOW(),12,13);
INSERT INTO history_record VALUES ('5','1','baidu','tencent',NOW(),12,13);
INSERT INTO history_record VALUES ('6','1','baidu','tencent',NOW(),12,13);
INSERT INTO history_record VALUES ('7','1','baidu','tencent',NOW(),12,13);
SELECT * FROM history_record 
EXPLAIN SELECT user_id, TIME FROM history_record WHERE user_id = 1 ORDER BY TIME

CREATE TABLE `history_record` (
  `id` INT(32) PRIMARY KEY AUTO_INCREMENT,
  `user_id` INT(32) NOT NULL,
  `url` VARCHAR(255) NOT NULL,
  `cover` VARCHAR(255) NOT NULL,
  `time` DATETIME NOT NULL,
  `width` INT(6) NOT NULL,
  `height` INT(6) NOT NULL,
  KEY `user_id_time` (`user_id`,`time`)
) ENGINE=INNODB DEFAULT CHARSET=utf8mb4

SELECT COUNT(*) FROM history_record

EXPLAIN SELECT * FROM history_record WHERE user_id = 1 ORDER BY TIME 

TRUNCATE TABLE history_record; #清空表数据
DROP PROCEDURE IF EXISTS proc_init_data; #如果存在此存储过程则删掉
DELIMITER $ -- 使用delimiter后，将不把分号当做语句结束，会将该段整个提交
CREATE PROCEDURE proc_init_data()
BEGIN
    
    DECLARE j INT DEFAULT 1;
    DECLARE i INT DEFAULT 1;
    WHILE j<=200 DO
    SET i = 1;
	    WHILE i<=50 DO
		INSERT INTO history_record(user_id, url, cover, TIME, width,height) VALUES (i,'baidu','tencent',NOW(),12,13);
		SET i = i+1;
	    END WHILE;
	    SET j = j + 1;
    END WHILE;	    
END $
CALL proc_init_data();