code: func (b *builtinEltSig) evalString(row chunk.Row) (string, bool, error) {
  	idx, isNull, err := b.args[0].EvalInt(b.ctx, row)
  	if isNull || err != nil {
  		return "", true, err
  	}
  	if idx < 1 || idx >= int64(len(b.args)) {
  		return "", true, nil
  	}
  	arg, isNull, err := b.args[idx].EvalString(b.ctx, row)
  	if isNull || err != nil {
  		return "", true, err
  	}
  	return arg, false, nil
  }
docs: // evalString evals a builtinEltSig.
  // See https://dev.mysql.com/doc/refman/5.7/en/string-functions.html#function_elt