use user variables in [order by] with union got error

[irene-coming]

• dble version：
  5.6.29-dble-9.9.9.9-78a278906811927e31ad0cdc0cc19c2379963992-20191012125722
• preconditions ：

drop table if exists sharding_2_t1;
drop table if exists schema2.sharding_3_t1;
create table sharding_2_t1(id int(4), B float(8,2)) ;
insert into sharding_2_t1 values(1,234.25),(2,67.29),(3,1.25),(12,1),(1,234.25) ;
create table schema2.sharding_3_t1(id int(4), B int(4)) ;
insert into schema2.sharding_3_t1 values (10, 1),(11, 2),(10,2) ;

• configs：

schema.xml

rule.xml

server.xml

• steps：
  step1. query with dble

mysql> set @id_a=2,@id_b=10;
Query OK, 0 rows affected (0.00 sec)

mysql> SELECT id FROM sharding_2_t1 WHERE id=@id_a AND B=67.29 order by @id_a;
+------+
| id   |
+------+
|    2 |
+------+
1 row in set (0.01 sec)
mysql> (SELECT id FROM sharding_2_t1 WHERE id=@id_a AND B=67.29) UNION (SELECT id FROM schema2.sharding_3_t1 WHERE id=@id_b AND B=2);
+------+
| id   |
+------+
|    2 |
|   10 |
+------+
2 rows in set (0.01 sec)
mysql> (SELECT id FROM sharding_2_t1 WHERE id=@id_a AND B=67.29) UNION (SELECT id FROM schema2.sharding_3_t1 WHERE id=@id_b AND B=2) order by @id_a;
ERROR 1003 (HY000): Union field merge error, field not found:@id_a

step2 query with dble

mysql> set @id_a=2,@id_b=10;                                                                                                                                                                    Query OK, 0 rows affected (0.01 sec)

mysql> (SELECT id FROM sharding_2_t1 WHERE id=@id_a AND B=67.29) UNION (SELECT id FROM schema2.sharding_3_t1 WHERE id=@id_b AND B=2) UNION (SELECT id FROM sharding_2_t1 WHERE id=@id_a+@id_b AND B=1) ;
+------+
| id   |
+------+
|    2 |
|   10 |
|   12 |
+------+
3 rows in set (0.03 sec)

mysql> (SELECT id FROM sharding_2_t1 WHERE id=@id_a AND B=67.29) UNION (SELECT id FROM schema2.sharding_3_t1 WHERE id=@id_b AND B=2) UNION (SELECT id FROM sharding_2_t1 WHERE id=@id_a+@id_b AND B=1) order by @id_a ;

• expect result：
  1.step1 the last query success with resultset:

+------+
| id   |
+------+
|    2 |
|   10 |
+------+

2.step2 the last query success with resultset

+------+
| id   |
+------+
|    2 |
|   10 |
|   12 |
+------+

• real result：
  1.step1 the last union query got error
  2.step2 the last union query got hang
• supplements：
  1.

[irene-coming]

the expect need to be updated as:
select ... order by [constant]
constant is the column index, when the index out of range expect get error message like:
ERROR 4004 (HY000): Unknown column '2' in order statement
this is fixed
verified on dble: 5.6.29-dble-9.9.9.9-96c130230a2ef9268bdb3605e190d559b0439625-20200203081515