execute_simple_calculation

# File activerecord/lib/active_record/relation/calculations.rb, line 259
      def execute_simple_calculation(operation, column_name, distinct) #:nodoc:
        column_alias = column_name

        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)
          if operation == "sum" && distinct
            select_value.distinct = true
          end

          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
        end

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

        type_cast_calculated_value(value, type, operation)
      end