perform_calculation

# File activerecord/lib/active_record/relation/calculations.rb, line 161
    def perform_calculation(operation, column_name, options = {})
      operation = operation.to_s.downcase

      distinct = nil

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

        if arel.join_sql =~ /LEFT OUTER/
          distinct = true
          column_name = @klass.primary_key if column_name == :all
        end

        distinct = nil if column_name =~ /\s*DISTINCT\s+/
      end

      distinct = options[:distinct] || distinct

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