code: func (p *PhysicalTopN) ExplainInfo() string {
  	buffer := bytes.NewBufferString("")
  	for i, item := range p.ByItems {
  		order := "asc"
  		if item.Desc {
  			order = "desc"
  		}
  		fmt.Fprintf(buffer, "%s:%s", item.Expr.ExplainInfo(), order)
  		if i+1 < len(p.ByItems) {
  			buffer.WriteString(", ")
  		}
  	}
  	fmt.Fprintf(buffer, ", offset:%v, count:%v", p.Offset, p.Count)
  	return buffer.String()
  }
docs: // ExplainInfo implements PhysicalPlan interface.