bug: error occurs while the mysql external dictionary table contains null

[Dragonliu2018]

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• I had searched in the issues and found no similar issues.

Version

main(7f3ec69)

What's Wrong?

When querying an external dictionary using MySQL as the data source, if there are NULL values in the MySQL table and they are queried, the following error will occur.

error: APIError: ResponseError with 3115: Dictionary Sqlx Error, cause: error occurred while decoding column 1: unexpected null; try decoding as an `Option`

How to Reproduce?

1. Create a table in the data source (MySQL) and insert data containing NULL values.

   CREATE TABLE user(id int, name varchar(100));
   
   insert into user values(1, 'Amy'), (2, null);

2. Create the corresponding external dictionary in Databend and query.

   CREATE OR REPLACE DICTIONARY mysql_dic(id int, name string) PRIMARY KEY id SOURCE(mysql(host='localhost' port='3306' username='root' password='123456' db='test' table='user'));
   
   select dict_get(mysql_dic, 'name', 2);

Are you willing to submit PR?

• Yes I am willing to submit a PR!