Github user jerryshao commented on the pull request:
https://github.com/apache/spark/pull/8881#issuecomment-143269767
I cannot remember very clearly, `count()` may introduce incorrect result
when you use `None` instead of `RDD.empty` as I remembered.
In the early version some operators actually return `None` instead of
`RDD.empty`, that's why this interface is designed like this. But `None` will
introduce some problem, I remembered TD fixed several this kind of bugs by
changing to empty RDD instead of None. TD might better know this issue.
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