请教一个 spark dataframe 问题

代码：

val dfArr = df.map(row => {
  ...
  ...
  val DF = spark.createDataFrame(rdd, schema)
  DF // 返回 dataframe
})

报错：

error: Unable to find encoder for type org.apache.spark.sql.DataFrame. An implicit Encoder[org.apache.spark.sql.DataFrame] is needed to store org.apache.spark.sql.DataFrame instances in a Dataset. Primitive types (Int, String, etc) and Product types (case classes) are supported by importing spark.implicits._  Support for serializing other types will be added in future releases.

将 df.take(n)到 driver 不会报错，不 take 的话报错，原因应该是序列化的时候没找合适的 encoder ，奇怪的是我在创建 df 的时候已经给了 schema.