@Override
abstract public DistinctValueWithCountClientAggregator newClientAggregator();
@Override
public Aggregator newServerAggregator(Configuration config, ImmutableBytesWritable ptr) {
DistinctValueWithCountClientAggregator clientAgg = newClientAggregator();
clientAgg.aggregate(null, ptr);
return new DistinctValueWithCountServerAggregator(config, clientAgg);
}