Github user rxin commented on the pull request:
https://github.com/apache/spark/pull/1254#issuecomment-47420413
The thing is we must scan data twice to make sure this actually works
(because we need to verify the number of partitions we checked is sufficient).
Usually users' specific use case can be solved with a very simple workaround
despite the lack of RDD.drop (e.g. for csv files with header that you want to
drop, you can just drop it at the first partition using an drop within a
mapPartitions).
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