execute_simple_calculation

# File activerecord/lib/active_record/relation/calculations.rb, line 289
      def execute_simple_calculation(operation, column_name, distinct) #:nodoc:
        if operation == "count" && (column_name == :all && distinct || has_limit_or_offset?)
          # Shortcut when limit is zero.
          return 0 if limit_value == 0

          query_builder = build_count_subquery(spawn, column_name, distinct)
        else
          # PostgreSQL doesn't like ORDER BY when there are no GROUP BY
          relation = unscope(:order).distinct!(false)

          column = aggregate_column(column_name)
          select_value = operation_over_aggregate_column(column, operation, distinct)
          select_value.distinct = true if operation == "sum" && distinct

          relation.select_values = [select_value]

          query_builder = relation.arel
        end

        result = skip_query_cache_if_necessary { @klass.connection.select_all(query_builder) }

        type_cast_calculated_value(result.cast_values.first, operation) do |value|
          type = column.try(:type_caster) ||
            lookup_cast_type_from_join_dependencies(column_name.to_s) || Type.default_value
          type.deserialize(value)
        end
      end