code: func (idx *Index) GroupBy(rowsQueries ...*PQLRowsQuery) *PQLBaseQuery {
  	if len(rowsQueries) < 1 {
  		return NewPQLBaseQuery("", idx, errors.New("there should be at least one rows query"))
  	}
  	text := fmt.Sprintf("GroupBy(%s)", strings.Join(serializeGroupBy(rowsQueries...), ","))
  	return NewPQLBaseQuery(text, idx, nil)
  }
docs: // GroupBy creates a GroupBy query with the given Rows queries