zhengruifeng commented on code in PR #46101:
URL: https://github.com/apache/spark/pull/46101#discussion_r1568725499
##########
sql/core/src/main/scala/org/apache/spark/sql/execution/limit.scala:
##########
@@ -118,18 +118,52 @@ case class CollectLimitExec(limit: Int = -1, child:
SparkPlan, offset: Int = 0)
* logical plan, which happens when the user is collecting results back to the
driver.
*/
case class CollectTailExec(limit: Int, child: SparkPlan) extends LimitExec {
+ assert(limit >= 0)
+
override def output: Seq[Attribute] = child.output
override def outputPartitioning: Partitioning = SinglePartition
override def executeCollect(): Array[InternalRow] = child.executeTail(limit)
+ private val serializer: Serializer = new
UnsafeRowSerializer(child.output.size)
+ private lazy val writeMetrics =
+ SQLShuffleWriteMetricsReporter.createShuffleWriteMetrics(sparkContext)
+ private lazy val readMetrics =
+ SQLShuffleReadMetricsReporter.createShuffleReadMetrics(sparkContext)
+ override lazy val metrics = readMetrics ++ writeMetrics
protected override def doExecute(): RDD[InternalRow] = {
- // This is a bit hacky way to avoid a shuffle and scanning all data when
it performs
- // at `Dataset.tail`.
- // Since this execution plan and `execute` are currently called only when
- // `Dataset.tail` is invoked, the jobs are always executed when they are
supposed to be.
-
- // If we use this execution plan separately like `Dataset.limit` without
an actual
- // job launch, we might just have to mimic the implementation of
`CollectLimitExec`.
- sparkContext.parallelize(executeCollect().toImmutableArraySeq, numSlices =
1)
+ val childRDD = child.execute().map(_.copy())
+ if (childRDD.getNumPartitions == 0 || limit == 0) {
+ new ParallelCollectionRDD(sparkContext, Seq.empty[InternalRow], 1,
Map.empty)
+ } else {
+ val singlePartitionRDD = if (childRDD.getNumPartitions == 1) {
+ childRDD
+ } else {
+ val locallyLimited = childRDD.mapPartitionsInternal(takeRight)
+ new ShuffledRowRDD(
+ ShuffleExchangeExec.prepareShuffleDependency(
+ locallyLimited,
+ child.output,
+ SinglePartition,
+ serializer,
+ writeMetrics),
+ readMetrics)
+ }
+ singlePartitionRDD.mapPartitionsInternal(takeRight)
+ }
+ }
+
+ private def takeRight(iter: Iterator[InternalRow]): Iterator[InternalRow] = {
+ if (iter.isEmpty) {
+ Iterator.empty[InternalRow]
+ } else {
+ val queue = HybridRowQueue.apply(output.size)
+ while (iter.hasNext) {
+ queue.add(iter.next().asInstanceOf[UnsafeRow])
+ while (queue.size() > limit) {
+ queue.remove()
+ }
+ }
+ queue.destructiveIterator()
Review Comment:
Good point
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