InnoDB 是数据和索引存储在一块的

辅助索引树/二级索引树
key是辅助索引的值，data是所在记录行的主键值。要搜索的如果在辅助树上没有，将回表。

假如现在要执行这句话 select * from student where age=20 ORDER BY name;。可以采用多列索引，需要注意的是索引顺序要和筛选顺序对得上。

mysql> CREATE INDEX name_age_idx on student(age,name);
Query OK, 0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> explain select * from student where age=20 ORDER BY name;
+----+-------------+---------+------------+------+---------------+--------------+---------+-------+------+----------+-----------------------+
| id | select_type | table   | partitions | type | possible_keys | key          | key_len | ref   | rows | filtered | Extra                 |
+----+-------------+---------+------------+------+---------------+--------------+---------+-------+------+----------+-----------------------+
|  1 | SIMPLE      | student | NULL       | ref  | name_age_idx  | name_age_idx | 1       | const |    2 |   100.00 | Using index condition |
+----+-------------+---------+------------+------+---------------+--------------+---------+-------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)