flink1.7.1 dataStream table API 的 distinc问题

  tenv.registerDataStream("data", dataStream, 'a, 'b, 'c, 'time.rowtime)
  import org.apache.flink.table.api.scala._
  import org.apache.flink.streaming.api.scala._
  order
      .groupBy('a)
      .select('a, 'b.sum.distinct  as 'd )
      .toRetractStream[Row]
    .print()

参照官网文档写的代码,但是distinct 却不能用,求解.

Error:(119, 26) method distinct in class DistinctAgg cannot be accessed in org.apache.flink.table.expressions.DistinctAgg

  .select('a, 'b.sum.distinct  as 'd )