Github user scwf commented on the pull request:
https://github.com/apache/spark/pull/3531#issuecomment-65167159
hi @marmbrus, the old version also eliminate the parallelism to 1 by a
shuffledRDD, the diff is this PR using ```coalesce``` to the same thing but
avoid shuffle(which will write and read file from disk).
In my test with the new version it can speed up 3X on a 95g dataset.
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