[GitHub] spark pull request #17898: Optimize the CartesianRDD to reduce repeatedly da...

[srowen <gi...@git.apache.org>]

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

    https://github.com/apache/spark/pull/17898#discussion_r115210488
  
    --- Diff: core/src/main/scala/org/apache/spark/rdd/CartesianRDD.scala ---
    @@ -72,8 +72,10 @@ class CartesianRDD[T: ClassTag, U: ClassTag](
     
       override def compute(split: Partition, context: TaskContext): Iterator[(T, U)] = {
         val currSplit = split.asInstanceOf[CartesianPartition]
    -    for (x <- rdd1.iterator(currSplit.s1, context);
    -         y <- rdd2.iterator(currSplit.s2, context)) yield (x, y)
    +    val groupSize = 500;
    +    for (x <- rdd1.iterator(currSplit.s1, context).grouped(groupSize);
    +         y <- rdd2.iterator(currSplit.s2, context).grouped(groupSize);
    --- End diff --
    
    Pardon, doesn't this change the type of the result? you're iterating over groupings not elements, and emitting pairs of groups. As in below, but maybe I'm missing something.
    
    ```
    scala> val foo = List(1,2,3)
    foo: List[Int] = List(1, 2, 3)
    
    scala> val bar = List(4,5,6)
    bar: List[Int] = List(4, 5, 6)
    
    scala> for (x <- foo; y <- bar) yield (x, y)
    res0: List[(Int, Int)] = List((1,4), (1,5), (1,6), (2,4), (2,5), (2,6), (3,4), (3,5), (3,6))
    
    scala> (for (x <- foo.grouped(2); y <- bar.grouped(2)) yield (x, y)).foreach(println)
    (List(1, 2),List(4, 5))
    (List(1, 2),List(6))
    (List(3),List(4, 5))
    (List(3),List(6))
    ```


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