add generated stored column problem.

[crazycs520]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?
   If possible, provide a recipe for reproducing the error.

mysql root@127.0.0.1:test> create table t (a int)
Query OK, 0 rows affected
Time: 0.005s
mysql root@127.0.0.1:test> insert into t set a=1
Query OK, 1 row affected
Time: 0.003s
mysql root@127.0.0.1:test> alter table t add column b int as (a+1) stored;
Query OK, 0 rows affected
Time: 0.015s
mysql root@127.0.0.1:test> alter table t add column c int as (a+1);
Query OK, 0 rows affected
Time: 0.015s
mysql root@127.0.0.1:test> select * from t;
+---+--------+---+
| a | b      | c |
+---+--------+---+
| 1 | <null> | 2 |
+---+--------+---+
1 row in set
Time: 0.006s
mysql root@127.0.0.1:test>

2. What did you expect to see?

mysql root@127.0.0.1:test> select * from t;
+---+---+---+
| a | b | c |
+---+---+---+
| 1 | 2 | 2 |
+---+---+---+
1 row in set

3. What did you see instead?
   mysql root@127.0.0.1:test> select * from t;
   +---+--------+---+
   | a | b | c |
   +---+--------+---+
   | 1 | | 2 |
   +---+--------+---+

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

mysql root@127.0.0.1:test> select tidb_version();
+--------------------------------------------------------------------------+
| tidb_version()                                                           |
+--------------------------------------------------------------------------+
| Release Version: v3.0.0-beta-116-g9a8a0d88a                              |
| Git Commit Hash: 9a8a0d88a91c664398abfc2b0f93c40069a7adbe                |
| Git Branch: master                                                       |
| UTC Build Time: 2019-02-25 12:41:19                                      |
| GoVersion: go version go1.11.5 darwin/amd64                              |
| Race Enabled: false                                                      |
| TiKV Min Version: 2.1.0-alpha.1-ff3dd160846b7d1aed9079c389fc188f7f5ea13e |
| Check Table Before Drop: false                                           |
+--------------------------------------------------------------------------+
1 row in set
Time: 0.006s

[zimulala]

You cannot add the generated column in the storage type of STORED through ALTER TABLE.
You cannot create an index on the generated column through ALTER TABLE.

Refer to https://github.com/pingcap/docs/blob/master/sql/generated-columns.md#limitations
Related #9372

[crazycs520]

May be we can simply return error when add the generated column in the storage type of STORED through ALTER TABLE.

Because currently TiDB don't support backfill column value when do add column ddl.

[bb7133]

duplicate with #9288

[crazycs520]

fix in #10758.