Github user cloud-fan commented on a diff in the pull request:

    https://github.com/apache/spark/pull/21859#discussion_r211167603
  
    --- Diff: core/src/main/scala/org/apache/spark/Partitioner.scala ---
    @@ -166,9 +169,20 @@ class RangePartitioner[K : Ordering : ClassTag, V](
           // Assume the input partitions are roughly balanced and over-sample 
a little bit.
           val sampleSizePerPartition = math.ceil(3.0 * sampleSize / 
rdd.partitions.length).toInt
           val (numItems, sketched) = RangePartitioner.sketch(rdd.map(_._1), 
sampleSizePerPartition)
    +      val numSampled = sketched.map(_._3.length).sum
           if (numItems == 0L) {
             Array.empty
           } else {
    +        // already got the whole data
    +        if (sampleCacheEnabled && numItems == numSampled) {
    +          // get the sampled data
    +          sampledArray = new Array[K](numSampled)
    +          var curPos = 0
    +          sketched.foreach(_._3.foreach(sampleRow => {
    --- End diff --
    
    can't we just keep the `sketched` instead of copying it to a new array?


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