code: func (c *Constant) String() string {
  	if c.DeferredExpr != nil {
  		dt, err := c.Eval(chunk.Row{})
  		if err != nil {
  			logutil.Logger(context.Background()).Error("eval constant failed", zap.Error(err))
  			return ""
  		}
  		c.Value.SetValue(dt.GetValue())
  	}
  	return fmt.Sprintf("%v", c.Value.GetValue())
  }
docs: // String implements fmt.Stringer interface.