code: func GetStringFromConstant(ctx sessionctx.Context, value Expression) (string, bool, error) {
  	con, ok := value.(*Constant)
  	if !ok {
  		err := errors.Errorf("Not a Constant expression %+v", value)
  		return "", true, err
  	}
  	str, isNull, err := con.EvalString(ctx, chunk.Row{})
  	if err != nil || isNull {
  		return "", true, err
  	}
  	return str, false, nil
  }
docs: // GetStringFromConstant gets a string value from the Constant expression.