Github user erikerlandson commented on the pull request:
https://github.com/apache/spark/pull/1254#issuecomment-47426628
It will scan one partition twice: the one containing the "boundary"
between things dropped and not-dropped. Any partitions prior to that boundary
are ignored by the resulting RDD (so they are scanned once), and any partitions
after the boundary are not examined unless/until the result RDD is evaluated.
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