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explain
select actor_id from (
select actor_id from sakila.actor
) as der_1
union
select film_id from (
select film_id from sakila.film
) as der_2;
+----+--------------+------------+-------+---------------+--------------------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+--------------+------------+-------+---------------+--------------------+---------+------+------+-------------+
| 1 | PRIMARY | <derived2> | ALL | NULL | NULL | NULL | NULL | 200 | |
| 2 | DERIVED | actor | index | NULL | PRIMARY | 2 | NULL | 200 | Using index |
| 3 | UNION | <derived4> | ALL | NULL | NULL | NULL | NULL | 1000 | |
| 4 | DERIVED | film | index | NULL | idx_fk_language_id | 1 | NULL | 951 | Using index |
| NULL | UNION RESULT | <union1,3> | ALL | NULL | NULL | NULL | NULL | NULL | |
+----+--------------+------------+-------+---------------+--------------------+---------+------+------+-------------+
|
