change generated column expression bug when have index on this generated column

[crazycs520]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?
   planner: allow queries on virtual columns to use index #10802 : This PR make virtual columns can be indexed.
   Next reproduce is base on this PR.

mysql>create table t1 (a int, b int as (a+1), index idx(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+2)
Query OK, 0 rows affected
Time: 0.072s
mysql>select b from t1 use index(idx);
+---+
| b |
+---+
| 2 |
+---+
1 row in set
Time: 0.007s
mysql>select b from t1 ignore index(idx);
+---+
| b |
+---+
| 3 |
+---+
mysql>admin check table t1;
(8003, u't1 err:[admin:1]index:&admin.RecordData{Handle:1, Values:[]types.Datum{types.Datum{k:0x1, collation:0x0, decimal:0x0, length:0x0, i:2, b:[]uint8(nil), x:interface {}(nil)}}} != record:&admin.RecordData{Handle:1, Values:[]types.Datum{types.Datum{k:0x1, collation:0x0, decimal:0x0, length:0x0, i:3, b:[]uint8(nil), x:interface {}(nil)}}}')

2. How to fix it?
   When modify generated column generated expression and there is a index on the column, TiDB should do at least 1 thing on the below:

• backfill the index data on the generated column again.
• prohibit modify virtual generated column generated expression and there is a index on the column

Maybe we can simply prohibit this action first. But this behaviour is not compatible with mysql.

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.1-236-g45f437eef                              |
| Git Commit Hash: 45f437eef076ff7bb6591d05589522eb2cf8d495                |
| Git Branch: 10802                                                        |
| UTC Build Time: 2019-06-24 08:44:22                                      |
| GoVersion: go version go1.12 darwin/amd64                                |
| Race Enabled: false                                                      |
| TiKV Min Version: 2.1.0-alpha.1-ff3dd160846b7d1aed9079c389fc188f7f5ea13e |
| Check Table Before Drop: false                                           |
+--------------------------------------------------------------------------+