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

    https://github.com/apache/spark/pull/9843#discussion_r46000917
  
    --- Diff: mllib/src/main/scala/org/apache/spark/mllib/feature/IDF.scala ---
    @@ -218,7 +218,7 @@ private object IDFModel {
               newValues(k) = values(k) * idf(indices(k))
               k += 1
             }
    -        Vectors.sparse(n, indices, newValues)
    +        Vectors.sparse(n, indices, newValues).toSparse
    --- End diff --
    
    I can see an argument for consistent behavior with respect to explicit 
zeros across the three versions of `Vectors.sparse`. Sorting only happens in 
one of the three (as a way of detecting duplicate identifiers), and while 
filtering doesn't seem that expensive in comparison, it would be significant 
extra work in the other two versions.
    
    The suggestion of optimizing performance with `idf.compressed` and extra 
cases in `transform` makes sense to me, but seems like more than is required to 
address this particular issue. The thought about `ArrayBuffer` suggests a 
reasonably small edit to the existing `SparseVector` case, though. I'll push a 
new commit with that change.


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