code: public Query<T> setColumnNames(Collection<N> columnNames) {
      if ( columnNames != null && columnNames.size() > 0) {
        slicePredicate.setColumnNames(columnNames);
      } else {
        slicePredicate.setKeysOnlyPredicate();
      }
      return this;
    }
docs: Sets the column names to be retrieved by this query
  @param columns a Collection of column names