Github user cloud-fan commented on the issue:
https://github.com/apache/spark/pull/13526
OK now I agree this is a useful API.
For performance, I would expect that
`ds.groupByKey(_._1).mapValues(_._2).mapGroups { case (k, vs) => (k, vs.sum) }`
should be at least as fast as `ds.groupByKey(_._1).mapGroups { case (k, vs) =>
(k, vs.map(_._2).sum) }`. But the current implementation looks not?
I'll take a closer look tomorrow, and let's discuss what's the best way to
do it.
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