Typical cases of MySQL index failure

CREATE TABLE `student_info` (
  `id` int(11) NOT NULL,
  `name` varchar(10) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `idx_name` (`name`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4

CREATE TABLE `student_score` (
  `id` int(11) NOT NULL,
  `name` varchar(10) DEFAULT NULL,
  `score` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `idx_name` (`name`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8

mysql> insert into student_info values ​​(1,'zhangsan'),(2,'lisi'),(3,'wangwu'),(4,'zhaoliu');
Query OK, 4 rows affected (0.01 sec)
Records: 4 Duplicates: 0 Warnings: 0

mysql> insert into student_score values ​​(1,'zhangsan',60),(2,'lisi',70),(3,'wangwu',80),(4,'zhaoliu',90);
Query OK, 4 rows affected (0.01 sec)
Records: 4 Duplicates: 0 Warnings: 0

mysql> select * from student_info;
+----+----------+
| id | name |
+----+----------+
| 2 | lisi |
| 3 | wangwu |
| 1 | zhangsan |
| 4 | zhaoliu |
+----+----------+
4 rows in set (0.00 sec)

mysql> select * from student_score;
+----+----------+-------+
| id | name | score |
+----+----------+-------+
| 1 | zhangsan | 60 |
| 2 | lisi | 70 |
| 3 | wangwu | 80 |
| 4 | zhaoliu | 90 |
+----+----------+-------+
4 rows in set (0.00 sec)

mysql> explain select B.*
        from
        student_info A,student_score B
        where A.name=B.name and A.id=1;
+----+-------------+-------+------------+-------+------------------+---------+---------+-------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+------------------+---------+---------+-------+------+----------+-------------+
| 1 | SIMPLE | A | NULL | const | PRIMARY,idx_name | PRIMARY | 4 | const | 1 | 100.00 | NULL |
| 1 | SIMPLE | B | NULL | ALL | NULL | NULL | NULL | NULL | 4 | 100.00 | Using where |
+----+-------------+-------+------------+-------+------------------+---------+---------+-------+------+----------+-------------+
2 rows in set, 1 warning (0.00 sec)

select * from student_score where CONVERT(name USING utf8mb4)=$LA.name

mysql> alter table student_score modify name varchar(10) character set utf8mb4;
Query OK, 4 rows affected (0.03 sec)
Records: 4 Duplicates: 0 Warnings: 0

mysql> explain select B.* from student_info A,student_score B where A.name=B.name and A.id=1;
+----+-------------+-------+------------+-------+------------------+----------+---------+-------+------+------+------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+------------------+----------+---------+-------+------+------+------+
| 1 | SIMPLE | A | NULL | const | PRIMARY,idx_name | PRIMARY | 4 | const | 1 | 100.00 | NULL |
| 1 | SIMPLE | B | NULL | ref | idx_name | idx_name | 43 | const | 1 | 100.00 | NULL |
+----+-------------+-------+------------+-------+------------------+----------+---------+-------+------+------+------+
2 rows in set, 1 warning (0.01 sec)