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mysql> create view aavv as select userid from user union select userid from user;
Query OK, 0 rows affected (0.05 sec)

mysql> explain select userid from aavv;
+----+--------------+------------+-------+---------------+---------+---------+------+------+-
| id | select_type | table | type | possible_keys | key | key_len | ref | rows |
+----+--------------+------------+-------+---------------+---------+---------+------+------+-
| 1 | PRIMARY | <derived2> | ALL | NULL | NULL | NULL | NULL | 8 |
| 2 | DERIVED | user | index | NULL | PRIMARY | 2 | NULL | 8 |
| 3 | UNION | us


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