mysql> explain select count(*) from geonames_features f left join geonames_timezones tz on f.country_code = tz.country_code and f.country_code in (select country_code from geonames_timezones group by country_code having count(*) = 1);
+----+-------------+--------------------+-------+---------------+--------------+---------+----------------------------------------+---------+--------------------------+
| id | select_type | table              | type  | possible_keys | key          | key_len | ref                                    | rows    | Extra                    |
+----+-------------+--------------------+-------+---------------+--------------+---------+----------------------------------------+---------+--------------------------+
|  1 | PRIMARY     | f                  | index | NULL          | utc_offset   | 785     | NULL                                   | 9668878 | Using index              |
|  1 | PRIMARY     | tz                 | ref   | country_code  | country_code | 768     | blopboard_production_v1.f.country_code |       1 | Using where; Using index |
|  2 | SUBQUERY    | geonames_timezones | index | country_code  | country_code | 768     | NULL                                   |     417 | Using index              |
+----+-------------+--------------------+-------+---------------+--------------+---------+----------------------------------------+---------+--------------------------+
3 rows in set (0.11 sec)