DROP TABLE IF EXISTS t;

CREATE TABLE t (pk INT PRIMARY KEY, i INT) ENGINE=MyISAM;
INSERT INTO t VALUES (1,8),(2,0),(3,9),(4,3),(5,4);

( SELECT t1.i FROM t AS t1, t AS t2 WHERE t1.pk = t2.pk ORDER BY i LIMIT 2 ) 
UNION 
( SELECT t1.i FROM t AS t1, t AS t2 WHERE t1.pk = t2.pk ORDER BY i LIMIT 2 )
; 

MariaDB [test]> ( SELECT t1.i FROM t AS t1, t AS t2 WHERE t1.pk = t2.pk ORDER BY i LIMIT 2 ) 
    -> UNION 
    -> ( SELECT t1.i FROM t AS t1, t AS t2 WHERE t1.pk = t2.pk ORDER BY i LIMIT 2 )
    -> ; 
+------+
| i    |
+------+
|    0 |
|    3 |
|    4 |
|    8 |
+------+
4 rows in set (0.00 sec)

MariaDB [test]> EXPLAIN EXTENDED ( SELECT t1.i FROM t AS t1, t AS t2 WHERE t1.pk = t2.pk ORDER BY i LIMIT 2 )  UNION  ( SELECT t1.i FROM t AS t1, t AS t2 WHERE t1.pk = t2.pk ORDER BY i LIMIT 2 );
+------+--------------+------------+--------+---------------+---------+---------+------------+------+----------+----------------+
| id   | select_type  | table      | type   | possible_keys | key     | key_len | ref        | rows | filtered | Extra          |
+------+--------------+------------+--------+---------------+---------+---------+------------+------+----------+----------------+
|    1 | PRIMARY      | t1         | ALL    | PRIMARY       | NULL    | NULL    | NULL       |    5 |   100.00 | Using filesort |
|    1 | PRIMARY      | t2         | eq_ref | PRIMARY       | PRIMARY | 4       | test.t1.pk |    1 |   100.00 | Using index    |
|    2 | UNION        | t1         | ALL    | PRIMARY       | NULL    | NULL    | NULL       |    5 |   100.00 | Using filesort |
|    2 | UNION        | t2         | eq_ref | PRIMARY       | PRIMARY | 4       | test.t1.pk |    1 |   100.00 | Using index    |
| NULL | UNION RESULT | <union1,2> | ALL    | NULL          | NULL    | NULL    | NULL       | NULL |     NULL |                |
+------+--------------+------------+--------+---------------+---------+---------+------------+------+----------+----------------+
5 rows in set, 1 warning (0.00 sec)

| Note  | 1003 | (select `test`.`t1`.`i` AS `i` from `test`.`t` `t1` join `test`.`t` `t2` where (`test`.`t2`.`pk` = `test`.`t1`.`pk`) order by `test`.`t1`.`i` limit 2) union (select `test`.`t1`.`i` AS `i` from `test`.`t` `t1` join `test`.`t` `t2` where (`test`.`t2`.`pk` = `test`.`t1`.`pk`) order by `test`.`t1`.`i` limit 2) |