code: func (b *builtinLocateBinary2ArgsSig) evalInt(row chunk.Row) (int64, bool, error) {
  	subStr, isNull, err := b.args[0].EvalString(b.ctx, row)
  	if isNull || err != nil {
  		return 0, isNull, err
  	}
  	str, isNull, err := b.args[1].EvalString(b.ctx, row)
  	if isNull || err != nil {
  		return 0, isNull, err
  	}
  	subStrLen := len(subStr)
  	if subStrLen == 0 {
  		return 1, false, nil
  	}
  	ret, idx := 0, strings.Index(str, subStr)
  	if idx != -1 {
  		ret = idx + 1
  	}
  	return int64(ret), false, nil
  }
docs: // evalInt evals LOCATE(substr,str), case-sensitive.
  // See https://dev.mysql.com/doc/refman/5.7/en/string-functions.html#function_locate