Github user NarineK commented on a diff in the pull request:

    https://github.com/apache/spark/pull/12836#discussion_r62759544
  
    --- Diff: 
sql/core/src/main/scala/org/apache/spark/sql/execution/r/MapPartitionsRWrapper.scala
 ---
    @@ -25,6 +25,21 @@ import org.apache.spark.sql.Row
     import org.apache.spark.sql.types.{BinaryType, StructField, StructType}
     
     /**
    + * A function wrapper that applies the given R function to each partition 
of each group.
    + */
    +private[sql] case class MapGroupsRWrapper(
    --- End diff --
    
    Thank you for the explanation, @sun-rui !
    
    As I understand from previous notes:
    We want to pass multiple groups to a R worker and call the R function on 
each group separately, right ?
    
    I'm trying to understand what is the major difference between this approach 
and repartition .
    
    Because conceptually repartition was also passing multiple groups to a R 
worker (guaranteed that each group will be in one partition, but there can be 
multiple groups in a partition).   
    
    I'm fine with this optimization, just trying to compare.


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