spark sql中的to_json函数

Question

我正在尝试使用Spark 2.1中的新 to_json 函数将Map类型列转换为JSON。以下代码段：

import org.apache.spark.sql.functions.to_json

    val df = spark.sql("SELECT *, to_json(Properties) AS Prop, to_json(Measures) AS Meas FROM tempTable")

但是，我收到以下错误。什么想法可能是错的？

    org.apache.spark.sql.AnalysisException: Undefined function: 'to_json'. This function is neither a registered temporary function nor a permanent function registered in the database 'default'.; line 1 pos 10