support GROUP_CONCAT(ORDER BY)

[WangXiangUSTC]

this is the data in my table:

mysql> select * from test;
+------+------+
| id        | name |
+------+------+
|    2      | bcd   |
|    1      | abc    |
+------+------+
2 rows in set (0.20 sec)

And then I execute this sql:

mysql> SELECT GROUP_CONCAT(id ORDER BY id ASC) AS tmp FROM `test1`.`test`;
+-----------+
| tmp             |
+-----------+
| 2,1               |
+-----------+
1 row in set (0.00 sec)

But I want to see:

+-----------+
| tmp             |
+-----------+
| 1,2               |
+-----------+

it seems that tidb don't support ORDER BY in GROUP_CONCAT now.

the tidb version:

mysql> select tidb_version();
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| tidb_version()                                                                                                                                                                                                                                                                                     |
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Release Version: v2.1.0-alpha-45-g390e08b
Git Commit Hash: 390e08bb87cf98d6efe15ccf2882cdcdbc17e252
Git Branch: master
UTC Build Time: 2018-06-12 11:55:43
GoVersion: go version go1.10.2 linux/amd64
Race Enabled: false
TiKV Min Version: 2.1.0-alpha.1-c4133d3ef0f099f8716a916c198390b26293ec00