code: @Override
    public List<AggregatorFactory> getRequiredColumns()
    {
      return Collections.singletonList(new HllSketchBuildAggregatorFactory(fieldName, fieldName, lgK, tgtHllType.toString()));
    }
docs: This is a convoluted way to return a list of input field names this aggregator needs.
  Currently the returned factories are only used to obtain a field name by calling getName() method.