我正在尝试使用 MySQL 命令show indexes from table_name来找出哪些索引是唯一的。但它错误地报告所有索引都是非唯一的：
mysql> desc books;
+------------------+--------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+------------------+--------------+------+-----+---------+-------+
| id | int(11) | NO | PRI | NULL | |
| name | varchar(255) | YES | | NULL | |
| author_id | int(11) | YES | | NULL | |
| coauthor_id | int(11) | YES | | NULL | |
| publisher_id | int(11) | YES | | NULL | |
| isbn | varchar(255) | YES | | NULL | |
| publication_year | int(11) | YES | | NULL | |
| shelf_id | int(11) | YES | | NULL | |
+------------------+--------------+------+-----+---------+-------+
8 rows in set (0.00 sec)
mysql> show indexes from books;
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| books | 0 | PRIMARY | 1 | id | A | 0 | NULL | NULL | | BTREE | | |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
1 row in set (0.00 sec)
mysql> create unique index books_isbn on books (isbn);
Query OK, 0 rows affected (0.23 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> show indexes from books;
+-------+------------+------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| books | 0 | PRIMARY | 1 | id | A | 0 | NULL | NULL | | BTREE | | |
| books | 0 | books_isbn | 1 | isbn | A | 0 | NULL | NULL | YES | BTREE | | |
+-------+------------+------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
2 rows in set (0.00 sec)
mysql> insert into books (id, name, isbn) values (0, 'foo', 'bar');
Query OK, 1 row affected, 1 warning (0.04 sec)
mysql> insert into books (id, name, isbn) values (1, 'foo2', 'bar');
ERROR 1062 (23000): Duplicate entry 'bar' for key 'books_isbn'
mysql>
为什么会show indexes在列中报告错误的内容non_unique，以及如何获得关于哪些索引是唯一的真实答案？
（这是 MySQL 5.5.24，表是 InnoDB。）