MySQL 8.0.26版本升级32版本查询数据为空的跟踪
某业务系统将MySQL 8.0.26升级为 GreatSQL 8.0.32-24 后，某些特定的SQL语句不能查询到数据。经测试 MySQL 8.0.32也存在相同的问题
此BUG已在 GreatSQL 8.0.32-25 版本中解决
接到客户反馈的问题后，对问题进行了复现和分析。
版本信息
greatsql> select version();
+-----------+
| version() |
+-----------+
| 8.0.32-24 |
+-----------+
1 row in set (0.00 sec)
建表语句
greatsql> show create table t_student;
CREATE TABLE `t_student1` (
`id` int NOT NULL AUTO_INCREMENT,
`name` varchar(255) NOT NULL,
`age` int NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci;
表数据
greatsql> select * from t_student;
+----+--------+-----+
| id | name | age |
+----+--------+-----+
| 1 | abc | 10 |
| 2 | 汤姆 | 20 |
+----+--------+-----+
2 rows in set (0.08 sec)
查询数据
greatsql> select * from (select * from t_student union select * from t_student) temp where name='汤姆';
Empty set, 2 warnings (0.00 sec)
greatsql> show warnings;
+---------+------+-------------------------------------------------------------------------+
| Level | Code | Message |
+---------+------+-------------------------------------------------------------------------+
| Warning | 1300 | Cannot convert string '\xE6\xB1\xA4\xE5\xA7\x86' from utf8mb4 to binary |
| Warning | 1300 | Cannot convert string '\xE6\xB1\xA4\xE5\xA7\x86' from utf8mb4 to binary |
+---------+------+-------------------------------------------------------------------------+
2 rows in set (0.00 sec)
greatsql> select * from (select * from t_student union select * from t_student) temp;
+----+--------+-----+
| id | name | age |
+----+--------+-----+
| 1 | abc | 10 |
| 2 | 汤姆 | 20 |
+----+--------+-----+
2 rows in set (0.00 sec)
greatsql> select * from (select * from t_student union select * from t_student) temp where name='abc';
+----+------+-----+
| id | name | age |
+----+------+-----+
| 1 | abc | 10 |
+----+------+-----+
1 row in set (0.00 sec)
可以看到 直接查询temp这个view或者 在视图上添加英文过滤条件均能查询出数据，在view上添加中文过滤条件sql语句返回结果为空，有warnings提醒，warnings具体内容为Cannot convert string
在官方网站进行搜索，以下连接有相似内容：
https://bugs.mysql.com/bug.php?id=110228
https://bugs.mysql.com/bug.php?id=110955
MySQL 8.0.32版本中对于UNION/UNION ALL 后的结果中的字符串字段过滤筛选不生效，客户端中提示：
Cannot convert string '%\x...' from utf8mb4 to binary
官方已确认是8.0.32中的一个bug，已在8.0.33版本中修复，对于8.0.32版本，官方给出的解决方式如下：
set optimizer_switch="derived_condition_pushdown=off";
让所有Session都生效，需要在配置文件中将optimizer_switch设置为off
此BUG已在 GreatSQL 8.0.32-25 版本中解决
解决方法
方法一：
设置set optimizer_switch="derived_condition_pushdown=off";后SQL语句能正常执行
greatsql> set optimizer_switch='derived_condition_pushdown=off';
Query OK, 0 rows affected (0.00 sec)
greatsql> select * from (select * from t_student union select * from t_student) temp where name='汤姆';
+----+--------+-----+
| id | name | age |
+----+--------+-----+
| 2 | 汤姆 | 20 |
+----+--------+-----+
1 row in set (0.00 sec)
方法二：
对单个SQL语句设置NO_DERIVED_CONDITION_PUSHDOWN hint
greatsql> set optimizer_switch='derived_condition_pushdown=on';
Query OK, 0 rows affected (0.00 sec)
greatsql> select * from (select * from t_student union select * from t_student) temp where name='汤姆';
Empty set, 2 warnings (0.00 sec)
greatsql> select /*+ NO_DERIVED_CONDITION_PUSHDOWN(temp) */ * from (select * from t_student union select * from t_student) temp where name='汤姆';
+----+--------+-----+
| id | name | age |
+----+--------+-----+
| 2 | 汤姆 | 20 |
+----+--------+-----+
1 row in set (0.00 sec)
《深入浅出MGR》视频课程
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https://www.bilibili.com/medialist/play/1363850082?business=space_collection&business_id=343928&desc=0
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