execute_simple_calculation

# File activerecord/lib/active_record/relation/calculations.rb, line 198
    def execute_simple_calculation(operation, column_name, distinct) #:nodoc:
      column = aggregate_column(column_name)

      # Postgresql doesn't like ORDER BY when there are no GROUP BY
      relation = except(:order)
      select_value = operation_over_aggregate_column(column, operation, distinct)

      relation.select_values = [select_value]

      type_cast_calculated_value(@klass.connection.select_value(relation.to_sql), column_for(column_name), operation)
    end