feature: The query is not supported if use constant queries after union all.

[haitaoguan]

The engine we used here is : Tianmu

mysql> create table tt(id decimal(18,0));
Query OK, 0 rows affected (0.13 sec)

mysql> insert into tt values(1111.0);
Query OK, 1 row affected (0.01 sec)

mysql> select * from tt;
+------+
| id   |
+------+
| 1111 |
+------+
1 row in set (0.00 sec)

mysql> select id from tt union all select 2222 c1 from dual;
ERROR 6 (HY000): The query includes syntax that is not supported by the storage engine. Either restructure the query with supported syntax, or enable the MySQL core::Query Path in config file to execute the query with reduced performance.

Expected behavior
mysql> select id from tt union all select 2222 c1 from dual;
+------+
| id   |
+------+
| 1111 |
| 2222 |
+------+
2 rows in set (0.00 sec)

[haitaoguan]

mysql> create table ttt(name varchar(5));
Query OK, 0 rows affected (0.01 sec)

mysql> insert into ttt values('AAA');
Query OK, 1 row affected (0.00 sec)

mysql> select name from ttt union all select 'BBB' C1 from dual;
ERROR 6 (HY000): The query includes syntax that is not supported by the storage engine. Either restructure the query with supported syntax, or enable the MySQL core::Query Path in config file to execute the query with reduced performance.

[RingsC]

Ref to #1883

From the exception message indicated that the root cause maybe in the following reasons:
1: The table dual is not recongnized as a tianmu table, therefore, tianmu execution engine dont think that this sql statement can be executed in tianmu layer.

in Query::Compile fuction, it returns kToMySQL at processing the second statement of union. Diving into deeper, we find that it can not handle the join condition via AddJoins.

2: select xxx from dual, as we know that dual is a dummy table, which has not any real sql syntax roles in a statement. It's just only for usage convention. Therefore, in tianmu layer, it cannot deal with this sql pattern properly.

Incorrect query plan:

T:-1 = TABLE_ALIAS(T:0,"t1")
T:-2 = TMP_TABLE(T:4294967295)
VC:-2.0 = CREATE_VC(T:-2,PHYS_COL(T:-1,A:0))
A:-1 = T:-2.ADD_COLUMN(VC:-2.0,LIST,"a","ALL")
T:-2.APPLY_CONDS()
T:-3 = TMP_TABLE(T:4294967295)
VC:-3.0 = CREATE_VC(T:-3,EXPR("123"))
A:-1 = T:-3.ADD_COLUMN(VC:-3.0,LIST,"123","ALL")
MSG: T:-3.APPLY_CONDS()
T:-2 = UNION(T:-2,T:-3,0)
RESULT(T:-2)

[RingsC]

mysql>  select id from tt union all select 2222 c1 from dual;
+------+
| id   |
+------+
| 1111 |
| 2222 |
+------+

mysql>  select id from tt union all select 2222 ;
+------+
| id   |
+------+
| 1111 |
| 2222 |
+------+