[GitHub] [spark] viirya commented on a change in pull request #31468: [SPARK-34353][SQL] CollectLimitExec avoid shuffle if input rdd has single partition

[GitBox <gi...@apache.org>]

viirya commented on a change in pull request #31468:
URL: https://github.com/apache/spark/pull/31468#discussion_r573989674



##########
File path: sql/core/src/main/scala/org/apache/spark/sql/execution/limit.scala
##########
@@ -52,16 +53,25 @@ case class CollectLimitExec(limit: Int, child: SparkPlan) extends LimitExec {
     SQLShuffleReadMetricsReporter.createShuffleReadMetrics(sparkContext)
   override lazy val metrics = readMetrics ++ writeMetrics
   protected override def doExecute(): RDD[InternalRow] = {
-    val locallyLimited = child.execute().mapPartitionsInternal(_.take(limit))
-    val shuffled = new ShuffledRowRDD(
-      ShuffleExchangeExec.prepareShuffleDependency(
-        locallyLimited,
-        child.output,
-        SinglePartition,
-        serializer,
-        writeMetrics),
-      readMetrics)
-    shuffled.mapPartitionsInternal(_.take(limit))
+    val childRDD = child.execute()
+    if (childRDD.getNumPartitions == 0) {
+      new ParallelCollectionRDD(sparkContext, Seq.empty[InternalRow], 1, Map.empty)

Review comment:
       EmptyRDD?




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