在调查迁移失败的问题时，我发现了以下奇怪的行为。在我的客户会话上使用SET NAMES utf8会更改REPLACE(uuid(),'','')调用的行为。
mysql> select replace(uuid(),'','') from mysql.user;
+--------------------------------------+
| replace(uuid(),'','') |
+--------------------------------------+
| 4b483d57-ecdc-11e8-844f-0242ac120002 |
| 4b483d57-ecdc-11e8-844f-0242ac120002 |
| 4b483d57-ecdc-11e8-844f-0242ac120002 |
+--------------------------------------+
3 rows in set (0.00 sec)
mysql> set names utf8;
Query OK, 0 rows affected (0.01 sec)
mysql> select replace(uuid(),'','') from mysql.user;
+--------------------------------------+
| replace(uuid(),'','') |
+--------------------------------------+
| 539c0b5c-ecdc-11e8-844f-0242ac120002 |
| 539c0b79-ecdc-11e8-844f-0242ac120002 |
| 539c0b7f-ecdc-11e8-844f-0242ac120002 |
+--------------------------------------+
3 rows in set (0.01 sec)
如您所见，生成的UUID仅在将NAMES设置为utf8的之后是唯一的。我发现的关于SET NAMES utf8的方式是通过MySQL Workbench传递查询。
如果有人可以解释字符集（NAMES）如何影响REPLACE(UUID(), ...)调用的输出，我将不胜感激。预先感谢。
更新：添加摘要以证明问题1）并非UUID()产生非唯一值，而2）与utf8mb4字符集有关
mysql> SHOW VARIABLES LIKE 'char%';
+--------------------------+----------------------------+
| Variable_name | Value |
+--------------------------+----------------------------+
| character_set_client | utf8mb4 |
| character_set_connection | utf8mb4 |
| character_set_database | utf8mb4 |
| character_set_filesystem | binary |
| character_set_results | utf8mb4 |
| character_set_server | utf8mb4 |
| character_set_system | utf8 |
| character_sets_dir | /usr/share/mysql/charsets/ |
+--------------------------+----------------------------+
8 rows in set (0.01 sec)
mysql> select uuid, replace(uuid,'','') from ( select uuid() as uuid from mysql.user) tmp;
+--------------------------------------+--------------------------------------+
| uuid | replace(uuid,'','') |
+--------------------------------------+--------------------------------------+
| e41d2fe4-ed70-11e8-844f-0242ac120002 | e41d2dc2-ed70-11e8-844f-0242ac120002 |
| e41d3042-ed70-11e8-844f-0242ac120002 | e41d2dc2-ed70-11e8-844f-0242ac120002 |
| e41d309c-ed70-11e8-844f-0242ac120002 | e41d2dc2-ed70-11e8-844f-0242ac120002 |
+--------------------------------------+--------------------------------------+
3 rows in set (0.00 sec)
mysql> set names utf8;
Query OK, 0 rows affected (0.00 sec)
mysql> select uuid, replace(uuid,'','') from ( select uuid() as uuid from mysql.user) tmp;
+--------------------------------------+--------------------------------------+
| uuid | replace(uuid,'','') |
+--------------------------------------+--------------------------------------+
| e9059092-ed70-11e8-844f-0242ac120002 | e9059117-ed70-11e8-844f-0242ac120002 |
| e90591a1-ed70-11e8-844f-0242ac120002 | e905923e-ed70-11e8-844f-0242ac120002 |
| e9059380-ed70-11e8-844f-0242ac120002 | e90593e1-ed70-11e8-844f-0242ac120002 |
+--------------------------------------+--------------------------------------+
3 rows in set (0.00 sec)
mysql> set names utf8mb4;
Query OK, 0 rows affected (0.00 sec)
mysql> select uuid, replace(uuid,'','') from ( select uuid() as uuid from mysql.user) tmp;
+--------------------------------------+--------------------------------------+
| uuid | replace(uuid,'','') |
+--------------------------------------+--------------------------------------+
| ef564f32-ed70-11e8-844f-0242ac120002 | ef564d0c-ed70-11e8-844f-0242ac120002 |
| ef564fa4-ed70-11e8-844f-0242ac120002 | ef564d0c-ed70-11e8-844f-0242ac120002 |
| ef565019-ed70-11e8-844f-0242ac120002 | ef564d0c-ed70-11e8-844f-0242ac120002 |
+--------------------------------------+--------------------------------------+
3 rows in set (0.00 sec)
更新2 ：下面添加了EXPLAIN个查询以跟踪@qbolec建议的实际SQL代码。这表明使用CONVERT(... using ...)是非唯一UUID的罪魁祸首。我仍然不太清楚为什么会这样，因为这不是我期望的CONVERT()函数的行为。
mysql> set names utf8mb4;
Query OK, 0 rows affected (0.00 sec)
mysql> explain select replace(uuid(),'','') from mysql.user;\W
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| 1 | SIMPLE | user | NULL | index | NULL | PRIMARY | 276 | NULL | 7 | 100.00 | Using index |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
Note (Code 1003): /* select#1 */ select replace(convert(uuid() using utf8mb4),'','') AS `replace(uuid(),'','')` from `mysql`.`user`
Show warnings enabled.
mysql> set names utf8;
Query OK, 0 rows affected (0.00 sec)
mysql> explain select replace(uuid(),'','') from mysql.user;\W
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| 1 | SIMPLE | user | NULL | index | NULL | PRIMARY | 276 | NULL | 7 | 100.00 | Using index |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
Note (Code 1003): /* select#1 */ select replace(uuid(),'','') AS `replace(uuid(),'','')` from `mysql`.`user`
Show warnings enabled.