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

    https://github.com/apache/spark/pull/15484#discussion_r84001582
  
    --- Diff: 
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/Analyzer.scala
 ---
    @@ -216,10 +216,16 @@ class Analyzer(
          *  Group Count: N + 1 (N is the number of group expressions)
          *
          *  We need to get all of its subsets for the rule described above, 
the subset is
    -     *  represented as the bit masks.
    +     *  represented as sequence of expressions.
          */
    -    def bitmasks(r: Rollup): Seq[Int] = {
    -      Seq.tabulate(r.groupByExprs.length + 1)(idx => (1 << idx) - 1)
    +    def selectGroupExprsRollup(exprs: Seq[Expression]): 
Seq[Seq[Expression]] = {
    --- End diff --
    
    This is too complex. It is hard to grasp what is going on here. Lets make 
this imperative:
    ```scala
    val buffer = mutable.Buffer.empty[Seq[Expression]]
    var current = exprs
    while (current.nonEmpty) {
      buffer += current
      current = current.init
    }
    buffer
    ```


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