This question explains how Spark's random split works, How does Sparks RDD.randomSplit actually split the RDD, but I don't understand how spark keeps track of what values went to one split so that those same values don't go to the second split.
If we look at the implementation of randomSplit:
def randomSplit(weights: Array[Double], seed: Long): Array[DataFrame] = {
// It is possible that the underlying dataframe doesn't guarantee the ordering of rows in its
// constituent partitions each time a split is materialized which could result in
// overlapping splits. To prevent this, we explicitly sort each input partition to make the
// ordering deterministic.
val sorted = Sort(logicalPlan.output.map(SortOrder(_, Ascending)), global = false, logicalPlan)
val sum = weights.sum
val normalizedCumWeights = weights.map(_ / sum).scanLeft(0.0d)(_ + _)
normalizedCumWeights.sliding(2).map { x =>
new DataFrame(sqlContext, Sample(x(0), x(1), withReplacement = false, seed, sorted))
}.toArray
}
we can see that it creates two DataFrames that share the same sqlContext and with two different Sample(rs).
How are these two DataFrame(s) communicating with each other so that a value that fell in the first one is not included in the second one?
And is the data being fetched twice? (Assume the sqlContext is selecting from a DB, is the select being executed twice?).
