code: func (c *Constant) EvalString(ctx sessionctx.Context, _ chunk.Row) (string, bool, error) {
  	if c.DeferredExpr != nil {
  		dt, err := c.DeferredExpr.Eval(chunk.Row{})
  		if err != nil {
  			return "", true, err
  		}
  		if dt.IsNull() {
  			return "", true, nil
  		}
  		val, err := dt.ToString()
  		if err != nil {
  			return "", true, err
  		}
  		c.Value.SetString(val)
  	} else {
  		if c.GetType().Tp == mysql.TypeNull || c.Value.IsNull() {
  			return "", true, nil
  		}
  	}
  	res, err := c.Value.ToString()
  	return res, err != nil, err
  }
docs: // EvalString returns string representation of Constant.