关于mysql索引explain时候extra的问题

Question

CREATE TABLE `teacher_card` (
 `tcid` int(3) NOT NULL,
 `tcdesc` varchar(20) NOT NULL,
 PRIMARY KEY (`tcid`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;

CREATE TABLE `teacher` (
 `tid` int(3) NOT NULL,
 `tname` varchar(20) NOT NULL,
 `tcid` int(3) NOT NULL,
 PRIMARY KEY (`tid`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;

tname索引:

alter table teacher add index a (tname)

执行 explain SELECT * FROM teacher WHERE tname = "张三" extra为空
执行 explain SELECT * FROM teacher WHERE tname = "张三" order by tcid
extra 为 Using index condition; Using filesort
我的疑问是：为什么第一条执行的时候不是Using index condition？

Answer 1

Using index condition是索引下推，你的查询并没有索引下推这一种操作，只是简单利用索引定位到tname="张三"，然后就回表了。