perform_calculation

# File activerecord/lib/active_record/relation/calculations.rb, line 410
      def perform_calculation(operation, column_name)
        operation = operation.to_s.downcase

        # If #count is used with #distinct (i.e. `relation.distinct.count`) it is
        # considered distinct.
        distinct = distinct_value

        if operation == "count"
          column_name ||= select_for_count
          if column_name == :all
            if !distinct
              distinct = distinct_select?(select_for_count) if group_values.empty?
            elsif group_values.any? || select_values.empty? && order_values.empty?
              column_name = primary_key
            end
          elsif distinct_select?(column_name)
            distinct = nil
          end
        end

        if group_values.any?
          execute_grouped_calculation(operation, column_name, distinct)
        else
          execute_simple_calculation(operation, column_name, distinct)
        end
      end