[Improve]For different query, merge has different meanings, so we need to split to MERGE and MERGE_AND_ORDER

[yanhuqing666]

We split old MERGE to new MERGE AND MERGE_AND_ORDER
New MERGE means just send all rowsResponse from differnt date node to next step to handle.

for exmaple:

mysql> explain select count(*) from sharding_two_node;
+-----------------+---------------+--------------------------------------------------------------------------+
| DATA_NODE       | TYPE          | SQL/REF                                                                  |
+-----------------+---------------+--------------------------------------------------------------------------+
| dn1_0           | BASE SQL      | select COUNT(*) as `_$COUNT$_rpda_0` from  `sharding_two_node` LIMIT 100 |
| dn2_0           | BASE SQL      | select COUNT(*) as `_$COUNT$_rpda_0` from  `sharding_two_node` LIMIT 100 |
| merge_1         | MERGE         | dn1_0; dn2_0                                                             |
| aggregate_1     | AGGREGATE     | merge_1                                                                  |
| limit_1         | LIMIT         | aggregate_1                                                              |
| shuffle_field_1 | SHUFFLE_FIELD | limit_1                                                                  |
+-----------------+---------------+--------------------------------------------------------------------------+
6 rows in set (1.32 sec)

MERGE_AND_ORDER mean order all rowsResponse from differnt date node and used them to join in dble or return to client.
for exmaple:

mysql> explain select a.id as c,* from sharding_two_node a inner join sharding_four_node b on a.id =b.id ;
+-------------------+-----------------+--------------------------------------------------------------------------------+
| DATA_NODE         | TYPE            | SQL/REF                                                                        |
+-------------------+-----------------+--------------------------------------------------------------------------------+
| dn1_0             | BASE SQL        | select `a`.`id`,`a`.`name` from  `sharding_two_node` `a` ORDER BY `a`.`id` ASC |
| dn2_0             | BASE SQL        | select `a`.`id`,`a`.`name` from  `sharding_two_node` `a` ORDER BY `a`.`id` ASC |
| merge_and_order_1 | MERGE_AND_ORDER | dn1_0; dn2_0                                                                   |
| shuffle_field_1   | SHUFFLE_FIELD   | merge_and_order_1                                                              |
| dn1_1             | BASE SQL        | select `b`.`id`,`b`.`c` from  `sharding_four_node` `b` ORDER BY `b`.`id` ASC   |
| dn2_1             | BASE SQL        | select `b`.`id`,`b`.`c` from  `sharding_four_node` `b` ORDER BY `b`.`id` ASC   |
| dn3_0             | BASE SQL        | select `b`.`id`,`b`.`c` from  `sharding_four_node` `b` ORDER BY `b`.`id` ASC   |
| dn4_0             | BASE SQL        | select `b`.`id`,`b`.`c` from  `sharding_four_node` `b` ORDER BY `b`.`id` ASC   |
| merge_and_order_2 | MERGE_AND_ORDER | dn1_1; dn2_1; dn3_0; dn4_0                                                     |
| shuffle_field_3   | SHUFFLE_FIELD   | merge_and_order_2                                                              |
| join_1            | JOIN            | shuffle_field_1; shuffle_field_3                                               |
| shuffle_field_2   | SHUFFLE_FIELD   | join_1                                                                         |
+-------------------+-----------------+--------------------------------------------------------------------------------+
12 rows in set (0.01 sec)

[FlyingMao]

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