code: func (b *builtinLocate2ArgsSig) 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
  	}
  	if int64(len([]rune(subStr))) == 0 {
  		return 1, false, nil
  	}
  	slice := string([]rune(strings.ToLower(str)))
  	ret, idx := 0, strings.Index(slice, strings.ToLower(subStr))
  	if idx != -1 {
  		ret = utf8.RuneCountInString(slice[:idx]) + 1
  	}
  	return int64(ret), false, nil
  }
docs: // evalInt evals LOCATE(substr,str), non case-sensitive.
  // See https://dev.mysql.com/doc/refman/5.7/en/string-functions.html#function_locate