code: func (b *builtinFormatSig) evalString(row chunk.Row) (string, bool, error) {
  	x, d, isNull, err := evalNumDecArgsForFormat(b, row)
  	if isNull || err != nil {
  		return "", isNull, err
  	}
  	formatString, err := mysql.GetLocaleFormatFunction("en_US")(x, d)
  	return formatString, false, err
  }
docs: // evalString evals FORMAT(X,D).
  // See https://dev.mysql.com/doc/refman/5.7/en/string-functions.html#function_format