Scan sharing can indeed be a useful optimization in spark, because you
amortize not only the time spent scanning over the data, but also time
spent in task launch and scheduling overheads.
Here's a trivial example in scala. I'm not aware of a place in SparkSQL
where this is used - I'd imagine that most development effort is being
placed on single-query optimization right now.
//This function takes a sequence of functions of type A => B and returns a
function of A => Seq[B] where each item in the input list corresponds to a
def combineFunctions[A,B](fns: Seq[A=>B]): A => Seq[B] = {
def combf(a: A): Seq[B] = {
fns.map(f => f(a))
}
combf
}
def plusOne(x: Int) = x + 1
def timesFive(x: Int) = x * 5
val sharedF = combineFunctions(Seq[Int => Int](plusOne, timesFive))
val data = sc.parallelize(Array(1,2,3,4,5,6,7))
//Apply this combine function to each of your data elements.
val res = data.map(sharedF)
res.take(5)
The result will look something like this.
res5: Array[Seq[Int]] = Array(List(2, 5), List(3, 10), List(4, 15), List(5,
20), List(6, 25))
On Tue, May 5, 2015 at 8:53 AM, Quang-Nhat HOANG-XUAN <hxquangn...@gmail.com
> wrote:
> Hi everyone,
>
> I have two Spark jobs inside a Spark Application, which read from the same
> input file.
> They are executed in 2 threads.
>
> Right now, I cache the input file into memory before executing these two
> jobs.
>
> Are there another ways to share their same input with just only one read?
> I know there is something called Multiple Query Optimization, but I don't
> know if it can be applicable on Spark (or SparkSQL) or not?
>
> Thank you.
>
> Quang-Nhat
>
