code: func (b *builtinDecodeSig) evalString(row chunk.Row) (string, bool, error) {
  	dataStr, isNull, err := b.args[0].EvalString(b.ctx, row)
  	if isNull || err != nil {
  		return "", true, err
  	}
  
  	passwordStr, isNull, err := b.args[1].EvalString(b.ctx, row)
  	if isNull || err != nil {
  		return "", true, err
  	}
  
  	decodeStr, err := encrypt.SQLDecode(dataStr, passwordStr)
  	return decodeStr, false, err
  }
docs: // evalString evals DECODE(str, password_str).
  // See https://dev.mysql.com/doc/refman/5.7/en/encryption-functions.html#function_decode