DDL reorg does not respect @@timestamp variable

[lcwangchao]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

> set @@time_zone='UTC';
> create table t(a int);
> insert into t values(NULL);
> set @@timestamp=36000;
> alter table t modify column a timestamp not null;
> select a from t;

2. What did you expect to see? (Required)

In MySQL:

mysql> select a from t;
+---------------------+
| a                   |
+---------------------+
| 1970-01-01 10:00:00 |
+---------------------+

3. What did you see instead (Required)

In TiDB, it still use the system time ignoring the @@timestamp

> select a from t;
+---------------------+
| a                   |
+---------------------+
| 2024-09-13 12:42:49 |
+---------------------+

4. What is your TiDB version? (Required)

t> select tidb_version();
+-----------------------------------------------------------+
| tidb_version()                                            |
+-----------------------------------------------------------+
| Release Version: v8.2.0                                   |
| Edition: Community                                        |
| Git Commit Hash: 821e491a20fbab36604b36b647b5bae26a2c1418 |
| Git Branch: HEAD                                          |
| UTC Build Time: 2024-07-05 09:16:37                       |
| GoVersion: go1.21.10                                      |
| Race Enabled: false                                       |
| Check Table Before Drop: false                            |
| Store: tikv                                               |
+-----------------------------------------------------------+

But I think it affects all versions of tidb