我有一个具有以下架构的数据框：
root
|-- _id: long (nullable = true)
|-- student_info: struct (nullable = true)
| |-- firstname: string (nullable = true)
| |-- lastname: string (nullable = true)
| |-- major: string (nullable = true)
| |-- hounour_roll: boolean (nullable = true)
|-- school_name: string (nullable = true)
如何仅获取“student_info”下的列列表？IE["firstname","lastname","major","honour_roll"]