code: func getPropByOrderByItems(items []*ByItems) (*property.PhysicalProperty, bool) {
  	propItems := make([]property.Item, 0, len(items))
  	for _, item := range items {
  		col, ok := item.Expr.(*expression.Column)
  		if !ok {
  			return nil, false
  		}
  		propItems = append(propItems, property.Item{Col: col, Desc: item.Desc})
  	}
  	return &property.PhysicalProperty{Items: propItems}, true
  }
docs: // getPropByOrderByItems will check if this sort property can be pushed or not. In order to simplify the problem, we only
  // consider the case that all expression are columns.