[GitHub] [spark] amaliujia commented on a diff in pull request #39057: [SPARK-41513][SQL] Implement an accumulator to collect per mapper row count metrics

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

amaliujia commented on code in PR #39057:
URL: https://github.com/apache/spark/pull/39057#discussion_r1048850181


##########
core/src/main/scala/org/apache/spark/util/AccumulatorV2.scala:
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@@ -513,3 +513,81 @@ class CollectionAccumulator[T] extends AccumulatorV2[T, java.util.List[T]] {
     getOrCreate.addAll(newValue)
   }
 }
+
+
+/**
+ * An [[AccumulatorV2 counter]] for collecting a list of (mapper id, row count).
+ *
+ * @since 3.4.0
+ */
+class MapperRowCounter extends AccumulatorV2[jl.Long, java.util.List[java.util.List[jl.Long]]] {
+
+  private var _agg: java.util.List[java.util.List[jl.Long]] = _

Review Comment:
   Which of the following that you are thinking:
   1. a list of integers where every two forms a pair for partition id and its row count?
   2. a list of integers that the index is the mapper/partition id and the value is the row count?



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