发生这种情况是因为您没有为DataFrameReader提供架构.结果，Spark必须急切地扫描数据集以推断输出模式.

It happens because you don't provide schema for DataFrameReader. As a result Spark has to eagerly scan data set to infer output schema.

由于未缓存mappedRdd，因此将被评估两次:

Since mappedRdd is not cached it will be evaluated twice:

• 一次进行模式推断
• 一次致电data.show

如果要阻止，则应为阅读器提供架构(Scala语法):

If you want to prevent you should provide schema for reader (Scala syntax):

val schema: org.apache.spark.sql.types.StructType = ???
spark.read.schema(schema).json(mappedRdd)