Major Description
In testing out the SummingCombiner, I set the Combiner#COLUMNS_OPTION which led me to believe that the SummingCombiner would limit columns to the set that is provided as an option through the IteratorSetting.
In practice, this was not the case as Combiner#findTop isn't used, and thus that column list is not adhered to. I can get the expected functionality out of SummingCombiner by calling ScannerBase#fetchColumn or ScannerBase#fetchColumnFamily.