Github user scwf commented on a diff in the pull request: https://github.com/apache/spark/pull/16677#discussion_r97700723 --- Diff: sql/core/src/main/scala/org/apache/spark/sql/execution/limit.scala --- @@ -90,25 +95,100 @@ trait BaseLimitExec extends UnaryExecNode with CodegenSupport { } /** - * Take the first `limit` elements of each child partition, but do not collect or shuffle them. + * Take the `limit` elements of the child output. */ -case class LocalLimitExec(limit: Int, child: SparkPlan) extends BaseLimitExec { +case class GlobalLimitExec(limit: Int, child: SparkPlan) extends UnaryExecNode { - override def outputOrdering: Seq[SortOrder] = child.outputOrdering + override def output: Seq[Attribute] = child.output override def outputPartitioning: Partitioning = child.outputPartitioning -} -/** - * Take the first `limit` elements of the child's single output partition. - */ -case class GlobalLimitExec(limit: Int, child: SparkPlan) extends BaseLimitExec { + override def outputOrdering: Seq[SortOrder] = child.outputOrdering - override def requiredChildDistribution: List[Distribution] = AllTuples :: Nil + private val serializer: Serializer = new UnsafeRowSerializer(child.output.size) - override def outputPartitioning: Partitioning = child.outputPartitioning + protected override def doExecute(): RDD[InternalRow] = { + val childRDD = child.execute() + val partitioner = FakePartitioning(child.outputPartitioning, + childRDD.getNumPartitions) + val shuffleDependency = ShuffleExchange.prepareShuffleDependency( + childRDD, child.output, partitioner, serializer) + val numberOfOutput: Seq[Int] = if (shuffleDependency.rdd.getNumPartitions != 0) { + // submitMapStage does not accept RDD with 0 partition. + // So, we will not submit this dependency. + val submittedStageFuture = sparkContext.submitMapStage(shuffleDependency) + submittedStageFuture.get().numberOfOutput.toSeq + } else { + Nil + } - override def outputOrdering: Seq[SortOrder] = child.outputOrdering + // Try to keep child plan's original data parallelism or not. It is enabled by default. + val respectChildParallelism = sqlContext.conf.enableParallelGlobalLimit + + val sumOfOutput = numberOfOutput.sum + if (sumOfOutput <= limit) { + childRDD + } else if (!respectChildParallelism) { + // This is mainly for tests. + // We take the rows of each partition until we reach the required limit number. + var countForRows = 0 + val takeAmounts = new mutable.HashMap[Int, Int]() + numberOfOutput.zipWithIndex.foreach { case (num, index) => + if (countForRows + num < limit) { + countForRows += num + takeAmounts += ((index, num)) + } else { + val toTake = limit - countForRows + countForRows += toTake + takeAmounts += ((index, toTake)) + } + } + val shuffled = new ShuffledRowRDD(shuffleDependency) + shuffled.mapPartitionsWithIndexInternal { case (index, iter) => + takeAmounts.get(index).map { size => + iter.take(size) + }.getOrElse(iter) --- End diff -- getOrElse(empty iter)?
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