truncate partition will not split a new region when tidb_scatter_region is on

[lcwangchao]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

set @@global.tidb_scatter_region='ON';
CREATE TABLE tp (id int, c varchar(128), key c(c)) partition by range (id) (partition p0 values less than (10), partition p1 values less than MAXVALUE);
show table tp regions;
alter table tp truncate partition p0;
show table tp regions;

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

There will be at least two regions for table tp after tuncate partition.

3. What did you see instead (Required)

after create table:

TiDB root@127.0.0.1:test> show table tp regions;
+-----------+-----------+----------+-----------+-----------------+------------+------------+---------------+------------+----------------------+------------------+------------------------+------------------+
| REGION_ID | START_KEY | END_KEY  | LEADER_ID | LEADER_STORE_ID | PEERS      | SCATTERING | WRITTEN_BYTES | READ_BYTES | APPROXIMATE_SIZE(MB) | APPROXIMATE_KEYS | SCHEDULING_CONSTRAINTS | SCHEDULING_STATE |
+-----------+-----------+----------+-----------+-----------------+------------+------------+---------------+------------+----------------------+------------------+------------------------+------------------+
| 2549      | t_177_    | t_178_   | 2551      | 2               | 2551       | 0          | 121           | 0          | 1                    | 0                |                        |                  |
| 2516      | t_178_    | 78000000 | 2517      | 1               | 2517, 2552 | 0          | 0             | 0          | 1                    | 346              |                        |                  |
+-----------+-----------+----------+-----------+-----------------+------------+------------+---------------+------------+----------------------+------------------+------------------------+------------------+
2 rows in set
Time: 0.009s

after truncate partition p0

TiDB root@127.0.0.1:test> show table tp regions;
+-----------+-----------+----------+-----------+-----------------+-------+------------+---------------+------------+----------------------+------------------+------------------------+------------------+
| REGION_ID | START_KEY | END_KEY  | LEADER_ID | LEADER_STORE_ID | PEERS | SCATTERING | WRITTEN_BYTES | READ_BYTES | APPROXIMATE_SIZE(MB) | APPROXIMATE_KEYS | SCHEDULING_CONSTRAINTS | SCHEDULING_STATE |
+-----------+-----------+----------+-----------+-----------------+-------+------------+---------------+------------+----------------------+------------------+------------------------+------------------+
| 2516      | t_178_    | 78000000 | 2552      | 2               | 2552  | 0          | 115           | 0          | 1                    | 0                |                        |                  |
+-----------+-----------+----------+-----------+-----------------+-------+------------+---------------+------------+----------------------+------------------+------------------------+------------------+
1 row in set
Time: 0.006s

There is only one region left.

4. What is your TiDB version? (Required)

TiDB root@127.0.0.1:test> select tidb_version();
+-----------------------------------------------------------+
| tidb_version()                                            |
+-----------------------------------------------------------+
| Release Version: v7.1.0-alpha                             |
| Edition: Community                                        |
| Git Commit Hash: 8e7ebb330fd8152f7b7a3db84ee77e72c1078a30 |
| Git Branch: heads/refs/tags/v7.1.0-alpha                  |
| UTC Build Time: 2023-04-17 14:26:49                       |
| GoVersion: go1.20.3                                       |
| Race Enabled: false                                       |
| TiKV Min Version: 6.2.0-alpha                             |
| Check Table Before Drop: false                            |
| Store: tikv                                               |
+-----------------------------------------------------------+
1 row in set
Time: 0.002s