Github user liancheng commented on the pull request:
https://github.com/apache/spark/pull/5714#issuecomment-97335761
@cloud-fan Yeah, that's a good point. @rxin also mentioned this offline
just now. However, hm... I guess I should argue that `DataFrame` isn't RDD, it
just looks like RDD ;-)
I think the problem here is that although in most cases RDD is immutable,
it does has mutable state like storage level. And this single mutable state
makes RDD identities (references) matter. Personally I think `RDD.persist()`
should also return a new RDD instance instead of rely on side effect from the
very beginning. But we can't change that part of API now.
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