Decreasing the hash partition count in TiDB does not automatically delete old statistics and update the global stats

[Rustin170506]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

1. Create a table: create table ht(a int, b varchar(10), index idx_b (b)) partition by hash(a) partitions 3;
2. Insert some data: insert into ht values (1, "a"), (2, "b"), (3, "c");
3. Check the meta:

mysql> select * from mysql.stats_meta;
+--------------------+----------+--------------+-------+----------+
| version            | table_id | modify_count | count | snapshot |
+--------------------+----------+--------------+-------+----------+
| 445359571104169990 |      103 |            1 |     1 |        0 |
| 445359571104169998 |      104 |            1 |     1 |        0 |
| 445359571104170003 |      105 |            1 |     1 |        0 |
| 445359571104170003 |      102 |            3 |     3 |        0 |
+--------------------+----------+--------------+-------+----------+
4 rows in set (0.00 sec)

4. Decrease the partitions number to 2: alter table ht partition by hash(a) partitions 2;
5. Check the meta again:

mysql> select * from mysql.stats_meta;
+--------------------+----------+--------------+-------+----------+
| version            | table_id | modify_count | count | snapshot |
+--------------------+----------+--------------+-------+----------+
| 445359865298681862 |      103 |            1 |     1 |        0 |
| 445359865298681870 |      104 |            1 |     1 |        0 |
| 445359865298681875 |      105 |            1 |     1 |        0 |
| 445359865298681875 |      109 |            3 |     3 |        0 |
| 445359872520486920 |      107 |            0 |     0 |        0 |
| 445359872520486923 |      108 |            0 |     0 |        0 |
+--------------------+----------+--------------+-------+----------+
6 rows in set (0.00 sec)

mysql> show stats_meta;
+---------+------------+----------------+---------------------+--------------+-----------+
| Db_name | Table_name | Partition_name | Update_time         | Modify_count | Row_count |
+---------+------------+----------------+---------------------+--------------+-----------+
| test    | ht         | global         | 2023-11-02 16:17:40 |            3 |         3 |
| test    | ht         | p0             | 2023-11-02 16:18:07 |            0 |         0 |
| test    | ht         | p1             | 2023-11-02 16:18:07 |            0 |         0 |
+---------+------------+----------------+---------------------+--------------+-----------+
3 rows in set (0.01 sec)

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

1. Drop the old stats.

3. What did you see instead (Required)

mysql> select * from mysql.stats_meta;
+--------------------+----------+--------------+-------+----------+
| version            | table_id | modify_count | count | snapshot |
+--------------------+----------+--------------+-------+----------+
| 445359865298681862 |      103 |            1 |     1 |        0 |
| 445359865298681870 |      104 |            1 |     1 |        0 |
| 445359865298681875 |      105 |            1 |     1 |        0 |
| 445359865298681875 |      109 |            3 |     3 |        0 |
| 445359872520486920 |      107 |            0 |     0 |        0 |
| 445359872520486923 |      108 |            0 |     0 |        0 |
+--------------------+----------+--------------+-------+----------+
6 rows in set (0.00 sec)

4. What is your TiDB version? (Required)

mysql> select tidb_version();
+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| tidb_version()                                                                                                                                                                                                                                                      |
+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Release Version: v7.6.0-alpha-69-gbf8c728934
Edition: Community
Git Commit Hash: bf8c728934c7376765ddd943fd302fcb65af4bf0
Git Branch: master
UTC Build Time: 2023-11-01 08:29:41
GoVersion: go1.21.1
Race Enabled: false
Check Table Before Drop: false
Store: tikv |
+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)