Recently, you have encountered such a problem in JMeter connection database:

Response message:java.sql.SQLException: Cannot create PoolableConnectionFactory (Unknown initial character set index '255' received from server. Initial client character set can be forced via the 'characterEncoding' property.)

The reason for this error is the coding problem, as shown in the following figure:

Modify the Database URL in JDBC Connection Configuration to

jdbc:mysql://127.0.0.1:{port_number}/{db_name}?useUnicode=true&characterEncoding=utf8

For example: jdbc:mysql://127.0.0.1:3306/stu?useUnicode=true&characterEncoding=utf8

The problem is solved~