scala - 用于火花案例类的 scala 通用编码器

Question

我怎样才能让这个方法编译。奇怪的是，已经导入了隐含的火花。

def loadDsFromHive[T <: Product](tableName: String, spark: SparkSession): Dataset[T] = {
    import spark.implicits._
    spark.sql(s"SELECT * FROM $tableName").as[T]
  }

这是错误：

Unable to find encoder for type stored 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.
[error]     spark.sql(s"SELECT * FROM $tableName").as[T]

Answer 1

根据源代码org.apache.spark.sql.SQLImplicits，您需要类型的类型类TypeTag，以便隐式Encoder存在：

import scala.reflect.runtime.universe.TypeTag
def loadDsFromHive[T <: Product: TypeTag](tableName: String, spark: SparkSession): Dataset[T] = ...