incompatible behavior when modifying a stored generated column

[crazycs520]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?

In TiDB Master

mysql>create table t1 (a int, b int as (a+1) stored, index idx(a,b));
Query OK, 0 rows affected
Time: 0.071s
mysql>insert into t1 set a=1
Query OK, 1 row affected
Time: 0.008s
mysql>alter table t1 modify column b int as (a+5) stored;
Query OK, 0 rows affected
Time: 0.071s
mysql>insert into t1 set a=1
Query OK, 1 row affected
Time: 0.002s
mysql>select a,b from t1;
+---+---+
| a | b |
+---+---+
| 1 | 2 |
| 1 | 6 |
+---+---+

In mysql

mysql>create table t1 (a int, b int as (a+1) stored, index idx(a,b));
Query OK, 0 rows affected
Time: 0.005s
mysql>insert into t1 set a=1
Query OK, 1 row affected
Time: 0.001s
mysql>alter table t1 modify column b int as (a+5) stored;
Query OK, 1 row affected
Time: 0.006s
mysql>insert into t1 set a=1
Query OK, 1 row affected
Time: 0.001s
mysql>select a,b from t1;
+---+---+
| a | b |
+---+---+
| 1 | 6 |
| 1 | 6 |
+---+---+

2. What did you expect to see?

3. What did you see instead?

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

mysql>select tidb_version();
+--------------------------------------------------------------------------+
| tidb_version()                                                           |
+--------------------------------------------------------------------------+
| Release Version: v3.0.0-rc.1-235-ga57b6e60a                              |
| Git Commit Hash: a57b6e60a6f7d5f36e5fdc81cc8d82c3eeab464f                |
| Git Branch: master                                                       |
| UTC Build Time: 2019-06-25 04:56:41                                      |
| GoVersion: go version go1.12 darwin/amd64                                |
| Race Enabled: false                                                      |
| TiKV Min Version: 2.1.0-alpha.1-ff3dd160846b7d1aed9079c389fc188f7f5ea13e |
| Check Table Before Drop: false                                           |
+--------------------------------------------------------------------------+

[zimulala]

Related to #9372

[tangenta]

Here is a similar issue:

create table t (a int, b int);
insert into t value (1, 0);
alter table t modify column b int as (a + 5) stored;
select * from t;

In MySQL :

+---+---+
| a | b |
+---+---+
| 1 | 6 |
+---+---+

In TiDB:

+---+---+
| a | b |
+---+---+
| 1 | 0 |
+---+---+