spark 2.1 SizeEstimator

/** * A trait that allows a class to give [[SizeEstimator]] more accurate size estimation. * When a class extends it, [[SizeEstimator]] will query the `estimatedSize` first. * If `estimatedSize` does not return [[None]], [[SizeEstimator]] will use the returned size * as the size of the object. Otherwise, [[SizeEstimator]] will do the estimation work. * The difference between a [[KnownSizeEstimation]] and * [[org.apache.spark.util.collection.SizeTracker]] is that, a * [[org.apache.spark.util.collection.SizeTracker]] still uses [[SizeEstimator]] to * estimate the size. However, a [[KnownSizeEstimation]] can provide a better estimation without * using [[SizeEstimator]]. */private[spark] trait KnownSizeEstimation {  def estimatedSize: Long}