void Field::register_field_in_read_map()
{
  if (vcol_info)
  {
    Item *vcol_item= vcol_info->expr;
    vcol_item->walk(&Item::register_field_in_read_map, 1, 0);
  }
  bitmap_set_bit(table->read_set, field_index);
}
void Field::register_field_in_read_map()
{
  if (vcol_info)
  {
    Item *vcol_item= vcol_info->expr;
    vcol_item->walk(&Item::register_field_in_read_map, 1, 0);
  }
  bitmap_set_bit(table->read_set, field_index);
}