code: func builtinFindInSet(args []types.Datum, _ context.Context) (d types.Datum, err error) {
  	// args[0] -> Str
  	// args[1] -> StrList
  	if args[0].IsNull() || args[1].IsNull() {
  		return
  	}
  
  	str, err := args[0].ToString()
  	if err != nil {
  		return d, errors.Trace(err)
  	}
  	strlst, err := args[1].ToString()
  	if err != nil {
  		return d, errors.Trace(err)
  	}
  
  	d.SetInt64(0)
  	if len(strlst) == 0 {
  		return
  	}
  	for i, s := range strings.Split(strlst, ",") {
  		if s == str {
  			d.SetInt64(int64(i + 1))
  			return
  		}
  	}
  	return
  }
docs: // See http://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.