MySQL Boolean类型的坑

MySQL中，Boolean只是 tinyint(1) 的别名，也就是说，MySQL中并没有真正的bool类型。

而SQLAlchemy生成SQL的时候并没有检测到 这一点，这就导致一个问题，当使用 bool 类型作为查询条件时，用不上索引，从而导致扫表的行为：

> SELECT COUNT(*) FROM message WHERE message.is_national = 1
 AND message.updated_at > '2020-01-01 00:00:00' AND message.deleted_at IS NULL;
+----------+
| COUNT(*) |
+----------+
| 0        |
+----------+
1 row in set
Time: 0.018s
> SELECT COUNT(*) FROM message WHERE message.is_national is true 
AND message.updated_at > '2020-01-01 00:00:00' AND message.deleted_at IS NULL;
+----------+
| COUNT(*) |
+----------+
| 0        |
+----------+
1 row in set
Time: 2.162s

注意观察第一行和第二行的时间，很明显第二行没有用上索引，我们来看看 EXPLAIN 的结果便知道了：

> EXPLAIN SELECT COUNT(*) FROM message WHERE message.is_national = 1 AND message.updated_at > '2020-01-01 00:00:00' AND message.de
        leted_at IS NULL;
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
| 1  | SIMPLE | message | ref  | ix_message_updated_at,idx_updated_at_is_national,ix_message_is_national | ix_message_is_national | 1 | const | 1 | Using where |
> EXPLAIN SELECT COUNT(*) FROM message WHERE message.is_national is true AND message.updated_at > '2020-01-01 00:00:00' AND messag

e.deleted_at IS NULL;

| id | select_type | table   | type | possible_keys | key    | key_len | ref    | rows    | Extra |

| 1  | SIMPLE | message | ALL  | ix_message_updated_at,idx_updated_at_is_national | <null> | <null>  | <null> | 一个很大的数字 | Using whe

re |

 

 

mysql文档给出的解释

java.lang.Boolean if the configuration property tinyInt1isBit is set to true (the default) and the storage size is 1, or java.lang.Integer if not.

要注意下面这个提示

The ResultSet.getObject() method uses the type conversions between MySQL and Java types, following the JDBC specification where appropriate. The values returned by ResultSetMetaData.GetColumnTypeName()and ResultSetMetaData.GetColumnClassName() are shown in the table below. For more information on the JDBC types, see the reference on the java.sql.Types class.

文档地址：https://dev.mysql.com/doc/connector-j/8.0/en/connector-j-reference-type-conversions.html

 

解决方案:

1.使用ifnull(column, 0)处理该字段,个人测试过可以;
2.在JDBC的URL增加 tinyInt1isBit=false参数，注意参数名区分大小写，否则不生效（默认为true）
即：jdbc:mysql://${ucmha.proxy1_2.host}/${db.mysql.db}?tinyInt1isBit=false
3.避免使用长度为1的tinyint类型字段存储数字格式的数据;

参考资料：

• https://jiajunhuang.com/articles/2020_03_06-mysql_boolean_tinyint_index.md.html
• https://dev.mysql.com/doc/refman/8.0/en/numeric-type-syntax.html
• https://www.jianshu.com/p/6885cad1cb14/
• https://dev.mysql.com/doc/connector-j/8.0/en/connector-j-reference-type-conversions.html