Github user maropu commented on the pull request:
https://github.com/apache/spark/pull/10896#issuecomment-184248142
@yhuai The second approach's good to me though, I'm not exactly sure how to
remove unnecessary final aggregation covered in this pr. IMO these kinds of
partial aggregation optimization seem to be similar to `Filter` optmization
such as push-downs and pruning. So, it'd be better to get together these kinds
of optimization in a same file.
Even in the above example, we should push down the partial aggregation
below `TungstenExchange` generated by `DataFrame#repartition`.
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