Suppose you have a key like (page, geo, day) and you want to make rollups/datacube so you can query for all pages, or all geos or all days.
Here is how you do it:
def opts[T](t: T): Seq[Option[T]] = Seq(Some(t), None)
val p: TypedPipe[(String, String, Int)] = ...
p.sumByLocalKeys