perform_calculation

# File activerecord/lib/active_record/relation/calculations.rb, line 188
    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 = self.distinct_value

      if operation == "count"
        column_name ||= select_for_count

        unless arel.ast.grep(Arel::Nodes::OuterJoin).empty?
          distinct = true
        end

        column_name = primary_key if column_name == :all && distinct
        distinct = nil if column_name =~ /\s*DISTINCT[\s(]+/
      end

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