Research into reduce-side join [Spark Branch]

Hive's join operator is very sophisticated, especially for reduce-side join. While we expect that other types of join, such as map-side join and SMB map-side join, will work out of the box with our design, there may be some complication in reduce-side join, which extensively utilizes key tag and shuffle behavior. Our design principle prefers to making Hive implementation work out of box also, which might requires new functionality from Spark. The tasks is to research into this area, identifying requirements for Spark community and the work to be done on Hive to make reduce-side join work.

A design doc might be needed for this. For more information, please refer to the overall design doc on wiki.