code: func (b *builtinFindInSetSig) evalInt(row chunk.Row) (int64, bool, error) {
  	str, isNull, err := b.args[0].EvalString(b.ctx, row)
  	if isNull || err != nil {
  		return 0, isNull, err
  	}
  
  	strlist, isNull, err := b.args[1].EvalString(b.ctx, row)
  	if isNull || err != nil {
  		return 0, isNull, err
  	}
  
  	if len(strlist) == 0 {
  		return 0, false, nil
  	}
  
  	for i, strInSet := range strings.Split(strlist, ",") {
  		if str == strInSet {
  			return int64(i + 1), false, nil
  		}
  	}
  	return 0, false, nil
  }
docs: // evalInt evals FIND_IN_SET(str,strlist).
  // See https://dev.mysql.com/doc/refman/5.7/en/string-functions.html#function_find-in-set
  // TODO: This function can be optimized by using bit arithmetic when the first argument is
  // a constant string and the second is a column of type SET.