Mysql中索引长度key_len探索

mysql> desc emp;
+--------------+---------------+------+-----+-------------------+-----------------------------+
| Field        | Type          | Null | Key | Default           | Extra                       |
+--------------+---------------+------+-----+-------------------+-----------------------------+
| empno        | int(10)       | NO   | PRI | NULL              | auto_increment              |
| ename        | varchar(50)   | YES  | MUL | NULL              |                             |
| job          | varchar(100)  | YES  |     | NULL              |                             |
| mgr          | int(10)       | YES  |     | NULL              |                             |
| hiredate     | datetime      | YES  |     | NULL              |                             |
| sal          | decimal(10,2) | YES  |     | NULL              |                             |
| comm         | decimal(10,2) | YES  |     | NULL              |                             |
| deptno       | int(11)       | YES  |     | NULL              |                             |
| created_time | datetime      | YES  |     | CURRENT_TIMESTAMP |                             |
| updated_time | datetime      | YES  |     | NULL              | on update CURRENT_TIMESTAMP |
| is_deleted   | tinyint(1)    | YES  |     | 0                 |                             |
| version      | int(1)        | YES  |     | 1                 |                             |
+--------------+---------------+------+-----+-------------------+-----------------------------+
12 rows in set (0.07 sec)

mysql> show index from emp;
+-------+------------+----------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name       | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+----------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| emp   |          0 | PRIMARY        |            1 | empno       | A         |          48 | NULL     | NULL   |      | BTREE      |         |               |
| emp   |          1 | idx_ename      |            1 | ename       | A         |          24 | NULL     | NULL   | YES  | BTREE      |         |               |
| emp   |          1 | idx_mgr        |            1 | mgr         | A         |           7 | NULL     | NULL   | YES  | BTREE      |         |               |
| emp   |          1 | idx_hiredate   |            1 | hiredate    | A         |          20 | NULL     | NULL   | YES  | BTREE      |         |               |
| emp   |          1 | idx_sal        |            1 | sal         | A         |           7 | NULL     | NULL   | YES  | BTREE      |         |               |
| emp   |          1 | idx_is_deleted |            1 | is_deleted  | A         |           2 | NULL     | NULL   | YES  | BTREE      |         |               |
+-------+------------+----------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
6 rows in set (0.08 sec)

mysql> explain select * from emp where ename = '张三';
+----+-------------+-------+------------+------+---------------+-----------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key       | key_len | ref   | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+-----------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | emp   | NULL       | ref  | idx_ename     | idx_ename | 153     | const |   20 |   100.00 | NULL  |
+----+-------------+-------+------------+------+---------------+-----------+---------+-------+------+----------+-------+
1 row in set (0.10 sec)

mysql>