ALTER VIEW not supported

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?

use test;
DROP VIEW IF EXISTS v1;
CREATE VIEW v1 AS SELECT 1;
SELECT * FROM v1;
ALTER VIEW v1 AS SELECT 2;
SELECT * FROM v1;

2. What did you expect to see?

mysql [localhost:8015] {msandbox} (test) > use test;
Database changed
mysql [localhost:8015] {msandbox} (test) > DROP VIEW IF EXISTS v1;
Query OK, 0 rows affected (0.01 sec)

mysql [localhost:8015] {msandbox} (test) > CREATE VIEW v1 AS SELECT 1;
Query OK, 0 rows affected (0.01 sec)

mysql [localhost:8015] {msandbox} (test) > SELECT * FROM v1;
+---+
| 1 |
+---+
| 1 |
+---+
1 row in set (0.00 sec)

mysql [localhost:8015] {msandbox} (test) > ALTER VIEW v1 AS SELECT 2;
Query OK, 0 rows affected (0.01 sec)

mysql [localhost:8015] {msandbox} (test) > SELECT * FROM v1;
+---+
| 2 |
+---+
| 2 |
+---+
1 row in set (0.00 sec)

3. What did you see instead?

mysql> use test;
Database changed
mysql> DROP VIEW IF EXISTS v1;
Query OK, 0 rows affected (0.22 sec)

mysql> CREATE VIEW v1 AS SELECT 1;
Query OK, 0 rows affected (0.09 sec)

mysql> SELECT * FROM v1;
+------+
| 1    |
+------+
|    1 |
+------+
1 row in set (0.00 sec)

mysql> ALTER VIEW v1 AS SELECT 2;
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your TiDB version for the right syntax to use line 1 column 10 near "VIEW v1 AS SELECT 2" 
mysql> SELECT * FROM v1;
+------+
| 1    |
+------+
|    1 |
+------+
1 row in set (0.00 sec)

4. What version of TiDB are you using (tidb-server -V or run select tidb_version(); on TiDB)?

mysql> SELECT tidb_version()\G
*************************** 1. row ***************************
tidb_version(): Release Version: v3.0.0-beta-239-g1ef9dae1a-dirty
Git Commit Hash: 1ef9dae1ac189bf45123639aee005f6608e7ba82
Git Branch: master
UTC Build Time: 2019-03-19 07:39:53
GoVersion: go version go1.12.1 linux/amd64
Race Enabled: false
TiKV Min Version: 2.1.0-alpha.1-ff3dd160846b7d1aed9079c389fc188f7f5ea13e
Check Table Before Drop: false
1 row in set (0.00 sec)