Hey All, There is a small API change that we are considering for the external sort patch. Previously we allowed mergeCombiner to be null when map side aggregation was not enabled. This is because it wasn't necessary in that case since mappers didn't ship pre-aggregated values to reducers.
Because the external sort capability also relies on the mergeCombiner function to merge partially-aggregated on-disk segments, we now need it all the time, even if map side aggregation is enabled. This is a fairly esoteric thing that I'm not sure anyone other than Shark ever used, but I want to check in case anyone had feelings about this. The relevant code is here: https://github.com/apache/incubator-spark/pull/303/files#diff-f70e97c099b5eac05c75288cb215e080R72 - Patrick
