plan :fix the compatibility problem of UNION statement

[spongedu]

In MySQL, LIMIT, ORDER BY is not allowed in the first few SelectStmts. For example:

mysql> CREATE TABLE IF NOT EXISTS `t` (
    ->   `id` int(6) unsigned NOT NULL,
    ->   `tn` int(3) unsigned NOT NULL,
    ->   `type` varchar(200) NOT NULL,
    ->   PRIMARY KEY (`id`,`tn`)
    -> ) DEFAULT CHARSET=utf8;
Query OK, 0 rows affected (0.05 sec)

mysql> INSERT INTO `t` (`id`, `tn`, `type`) VALUES
    ->   ('1', '5', 'cat'),
    ->   ('2', '6', 'dog'),
    ->   ('3', '7', 'duck'),
    ->   ('4', '8', 'chicken');
Query OK, 4 rows affected (0.02 sec)
Records: 4  Duplicates: 0  Warnings: 0
mysql> select * from t limit 1 offset 2 union select 1,2,3 from dual;
ERROR 1221 (HY000): Incorrect usage of UNION and LIMIT
mysql> select * from t order by id  union select 1,2,3 from dual;
ERROR 1221 (HY000): Incorrect usage of UNION and ORDER BY

[zz-jason]

LGTM

[zz-jason]

/run-all-tests

[shenli]

@spongedu Thanks!
Please merge the latest master to fix the CI.

[shenli]

/run-all-tests

[shenli]

LGTM

[zz-jason]

I think we can cherry pick this patch to release 2.0

[shenli]

@zz-jason Agree