All, I noticed that while some operations that return RDDs are very cheap, such as map and flatMap, some are quite expensive, such as union and groupByKey. I'm referring here to the cost of constructing the RDD scala value, not the cost of collecting the values contained in the RDD. This does not match my understanding that RDD transformations only set up a computation without actually running it. Oh, Spark developers, can you please provide some clarity?
Alex
