code: func (b *builtinLengthSig) evalInt(row chunk.Row) (int64, bool, error) {
  	val, isNull, err := b.args[0].EvalString(b.ctx, row)
  	if isNull || err != nil {
  		return 0, isNull, err
  	}
  	return int64(len([]byte(val))), false, nil
  }
docs: // evalInt evaluates a builtinLengthSig.
  // See https://dev.mysql.com/doc/refman/5.7/en/string-functions.html