order by排序使用联合索引

表结构：

CREATE TABLE `UserInfo` ( `Id` BIGINT(20) UNSIGNED NOT NULL COMMENT '唯一id', `UserId` INT(10) UNSIGNED NOT NULL, `Name` INT(10) UNSIGNED NOT NULL, `Age` TINYINT(4) NOT NULL, `CreateTime` DATETIME NOT NULL, PRIMARY KEY (`Id`), INDEX `Index 2` (`Age`, `CreateTime`) ) COLLATE='utf8_general_ci' ENGINE=InnoDB ; sql语句，差别咋就这么大呢，待查资料分析原理...：

1）：

SELECT Age,CreateTime FROM `UserInfo` where Age=18 ORDER BY CreateTime desc;

2）：

SELECT Age,CreateTime,UserId FROM `UserInfo` where Age=18 ORDER BY CreateTime desc;

插入11条记录，类似这种：

INSERT INTO `UserInfo` (`Id`, `UserId`, `Name`, `Age`, `CreateTime`) VALUES (1, 1, '0gfd', 18, '2017-03-10 16:22:58'); INSERT INTO `UserInfo` (`Id`, `UserId`, `Name`, `Age`, `CreateTime`) VALUES (3, 1, 'gfds', 18, '2017-03-10 16:22:58'); INSERT INTO `UserInfo` (`Id`, `UserId`, `Name`, `Age`, `CreateTime`) VALUES (2, 2, '0fd', 19, '2017-03-10 16:22:58');

3）： explain SELECT Age,CreateTime FROM `UserInfo` where Age=18 ORDER BY CreateTime desc; 4）： explain SELECT Age,CreateTime,UserId FROM `UserInfo` where Age=18 ORDER BY CreateTime desc;