mysql> explain select 1 from sakila.actor as actor_1 union select 2 from sakila.actor as actor_2;
+----+--------------+------------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+--------------+------------+-------+---------------+---------+---------+------+------+-------------+
| 1 | PRIMARY | actor_1 | index | NULL | PRIMARY | 2 | NULL | 200 | Using index |
| 2 | UNION | actor_2 | index | NULL | PRIMARY | 2 | NULL | 200 | Using index |
| | UNION RESULT | <union1,2> | ALL | NULL | NULL | NULL | NULL | NULL | |
+----+--------------+------------+-------+---------------+---------+---------+------+------+-------------+
|