code: public Updater updaterFor(DecoratedKey key, ColumnFamily cf, OpOrder.Group opGroup)
      {
          return (indexesByColumn.isEmpty() && rowLevelIndexMap.isEmpty())
                  ? nullUpdater
                  : new StandardUpdater(key, cf, opGroup);
      }
docs: This helper acts as a closure around the indexManager
  and updated cf data to ensure that down in
  Memtable's ColumnFamily implementation, the index
  can get updated. Note: only a CF backed by AtomicSortedColumns implements
  this behaviour fully, other types simply ignore the index updater.