Result set is not merged cause of union query on different global tables

[FlyingMao]

dble result:

mysql> (select * from test_global where id=2) union (select * from global_table2 where id=2);
+----+------+--------+-----+---------------+
| id | t_id | name   | pad | _dble_op_time |
+----+------+--------+-----+---------------+
|  2 |    2 | test_2 |   2 | 1506066633776 |
|  2 |    2 | test_2 |   2 | 1506066633796 |
+----+------+--------+-----+---------------+
2 rows in set (0.02 sec)

mysql result:

mysql> (select * from test_global where id=2) union (select * from global_table2 where id=2);
+----+------+--------+-----+
| id | t_id | name   | pad |
+----+------+--------+-----+
|  2 |    2 | test_2 |   2 |
+----+------+--------+-----+
1 row in set (0.00 sec)

[FlyingMao]

natural join has the similar problem
version: 5.6.29-dble-2.18.02.1-dev-20180428023831
steps
step1:prepare table

mysql> drop table if exists test_global;
$order$4',4),(5,5,'order...5',1);
Query OK, 0 rows affected (0.03 sec)

mysql> drop table if exists global_table2;
Query OK, 0 rows affected (0.22 sec)

mysql> CREATE TABLE test_global(`id` int(10) unsigned NOT NULL,`t_id` int(10) unsigned NOT NULL DEFAULT '0',`name` char(120) NOT NULL DEFAULT '',`pad` int(11) NOT NULL,PRIMARY KEY (`id`),KEY `k_1` (`t_id`))DEFAULT CHARSET=UTF8;
Query OK, 0 rows affected (0.46 sec)

mysql> CREATE TABLE global_table2(`id` int(10) unsigned NOT NULL,`o_id` int(10) unsigned NOT NULL DEFAULT '0',`name` char(120) NOT NULL DEFAULT '',`pad` int(11) NOT NULL,PRIMARY KEY (`id`),KEY `k_1` (`o_id`))DEFAULT CHARSET=UTF8;
Query OK, 0 rows affected (0.46 sec)

mysql> insert into test_global values(1,1,'test中id为1',1),(2,2,'test_2',2),(3,3,'test中id为3',4),(4,4,'$test$4',3),(5,5,'test...5',1),(6,6,'test6',6);
Query OK, 6 rows affected (0.10 sec)
Records: 6  Duplicates: 0  Warnings: 0

mysql> insert into global_table2 values(1,1,'order中id为1',1),(2,2,'test_2',2),(3,3,'order中id为3',3),(4,4,'$order$4',4),(5,5,'order...5',1);
Query OK, 5 rows affected (0.12 sec)
Records: 5  Duplicates: 0  Warnings: 0

step2:execute select in dble

mysql> select a.id,a.t_id,a.name,a.pad,b.id,b.o_id,b.name,b.pad from test_global a natural left join global_table2 b order by a.id,b.id;
+----+------+---------------+-----+------+------+------+------+
| id | t_id | name          | pad | id   | o_id | name | pad  |
+----+------+---------------+-----+------+------+------+------+
|  1 |    1 | test中id为1   |   1 | NULL | NULL | NULL | NULL |
|  2 |    2 | test_2        |   2 | NULL | NULL | NULL | NULL |
|  3 |    3 | test中id为3   |   4 | NULL | NULL | NULL | NULL |
|  4 |    4 | $test$4       |   3 | NULL | NULL | NULL | NULL |
|  5 |    5 | test...5      |   1 | NULL | NULL | NULL | NULL |
|  6 |    6 | test6         |   6 | NULL | NULL | NULL | NULL |
+----+------+---------------+-----+------+------+------+------+
6 rows in set (0.00 sec)

step3:execute select in mysql

mysql> select a.id,a.t_id,a.name,a.pad,b.id,b.o_id,b.name,b.pad from test_global a natural left join global_table2 b order by a.id,b.id;
+----+------+---------------+-----+------+------+--------+------+
| id | t_id | name          | pad | id   | o_id | name   | pad  |
+----+------+---------------+-----+------+------+--------+------+
|  1 |    1 | test中id为1   |   1 | NULL | NULL | NULL   | NULL |
|  2 |    2 | test_2        |   2 |    2 |    2 | test_2 |    2 |
|  3 |    3 | test中id为3   |   4 | NULL | NULL | NULL   | NULL |
|  4 |    4 | $test$4       |   3 | NULL | NULL | NULL   | NULL |
|  5 |    5 | test...5      |   1 | NULL | NULL | NULL   | NULL |
|  6 |    6 | test6         |   6 | NULL | NULL | NULL   | NULL |
+----+------+---------------+-----+------+------+--------+------+
6 rows in set (0.00 sec)

step4:explain in dble

mysql> explain select a.id,a.t_id,a.name,a.pad,b.id,b.o_id,b.name,b.pad from test_global a natural left join global_table2 b order by a.id,b.id;
+-----------+----------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| DATA_NODE | TYPE     | SQL/REF                                                                                                                                                                                                                                                                       |
+-----------+----------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| dn4.0     | BASE SQL | select `a`.`id`,`a`.`t_id`,`a`.`name`,`a`.`pad`,`b`.`id`,`b`.`o_id`,`b`.`name`,`b`.`pad` from  `test_global` `a` left join  `global_table2` `b` on a.id = b.id and a.name = b.name and a.pad = b.pad and a._dble_op_time = b._dble_op_time order by `a`.`id` ASC,`b`.`id` ASC |
| merge.1   | MERGE    | dn4.0                                                                                                                                                                                                                                                                         |
+-----------+----------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
2 rows in set (0.00 sec)