Github user koertkuipers commented on the pull request:
https://github.com/apache/spark/pull/1698#issuecomment-50890613
redudeByKey being the same as reduce, and cartesian being the same as
broadcast is the whole point, the difference being that redudeByKey and
cartesian are evaluated lazily.
eager evaluation is often unexpected to the end user and can lead to
duplicate calculations (since the user does not anticipate them and deal with
them using rdd.cache calls)
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