MySQL：EXPLAIN中EXTRA字段显示结果与预期不符

Question

MySQL版本：8.0.16

创建学生表student，包含编号、姓名、年龄三个字段

CREATE TABLE student (
    id INT NOT NULL AUTO_INCREMENT, 
    name VARCHAR(10) NOT NULL, 
    age INT NOT NULL,
    PRIMARY KEY (id),
    INDEX (name) 
);

插入测试数据

INSERT INTO student(name, age) VALUES('Tim', 18);
INSERT INTO student(name, age) VALUES('Jack', 25);

通过姓名查找学生

mysql> explain SELECT id, name, age FROM student WHERE name = 'Tim';

id	select_type	table	partitions	type	possible_keys	key	key_len	ref	rows	filtered	Extra
1	SIMPLE	student	NULL	ref	name	name	32	const	1	100.00	NULL

预期结果：Extra显示using index condition
实际结果：Extra显示NULL

先通过二级索引查找name，再通过聚簇索引查找age，此时应该发生了回表，但为何Extra字段显示NULL呢？

Answer 1

Explain output columns

Column	Meaning
id	The SELECT identifier
select_type	The SELECT type
table	The table for the output row
partitions	The matching partitions
type	The join type
possible_keys	The possible indexes to choose
key	The index actually chosen
key_len	The length of the chosen key
ref	The columns compared to the index
rows	Estimate of rows to be examined
filtered	Percentage of rows filtered by table condition
Extra	Additional information

这么多列有索引有关，你为啥只看 Extra 呢 ...