perform_calculation

# File activerecord/lib/active_record/relation/calculations.rb, line 195
    def perform_calculation(operation, column_name, options = {})
      operation = operation.to_s.downcase

      # If #count is used in conjuction with #uniq it is considered distinct. (eg. relation.uniq.count)
      distinct = options[:distinct] || self.uniq_value

      if operation == "count"
        column_name ||= (select_for_count || :all)

        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