execute_simple_calculation

# File activerecord/lib/active_record/relation/calculations.rb, line 243
    def execute_simple_calculation(operation, column_name, distinct) #:nodoc:
      # Postgresql doesn't like ORDER BY when there are no GROUP BY
      relation = unscope(:order)

      column_alias = column_name

      bind_values = nil

      if operation == "count" && (relation.limit_value || relation.offset_value)
        # Shortcut when limit is zero.
        return 0 if relation.limit_value == 0

        query_builder = build_count_subquery(relation, column_name, distinct)
        bind_values = query_builder.bind_values + relation.bind_values
      else
        column = aggregate_column(column_name)

        select_value = operation_over_aggregate_column(column, operation, distinct)

        column_alias = select_value.alias
        column_alias ||= @klass.connection.column_name_for_operation(operation, select_value)
        relation.select_values = [select_value]

        query_builder = relation.arel
        bind_values = query_builder.bind_values + relation.bind_values
      end

      result = @klass.connection.select_all(query_builder, nil, bind_values)
      row    = result.first
      value  = row && row.values.first
      column = result.column_types.fetch(column_alias) do
        type_for(column_name)
      end

      type_cast_calculated_value(value, column, operation)
    end