TiDB produces different results for SELECT statement when using transaction which uses update stmt

[JZuming]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

Setup the environment:

tidb/bin/tidb-server &
mysql -h "127.0.0.1" -u root -P 4000 -D testdb < mysql_bk.sql

mysql_bk.sql:
mysql_bk.sql.txt

Testcase 1

mysql -h "127.0.0.1" -u root -P 4000 -D testdb
mysql> update t_berydd set c_cdqetd = t_berydd.c_ioru4c;
mysql> select count(c_mp6ko) from t_berydd;

Testcase 2

mysql -h "127.0.0.1" -u root -P 4000 -D testdb
mysql> start transaction;
mysql> update t_berydd set c_cdqetd = t_berydd.c_ioru4c;
mysql> select count(c_mp6ko) from t_berydd;
mysql> commit;

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

The outputs of SELECT statements in Testcase 1 and Testcase 2 are same.

3. What did you see instead (Required)

Output of SELECT statement in Testcase 1

+----------------+
| count(c_mp6ko) |
+----------------+
|             39 |
+----------------+

Output of SELECT statement in Testcase 2

+----------------+
| count(c_mp6ko) |
+----------------+
|             36 |
+----------------+

Their results are different.

4. What is your TiDB version? (Required)

Release Version: v5.4.0-alpha-311-g28446605c
Edition: Community
Git Commit Hash: 28446605c2e019e84e934602f4bd136294a61ff3
Git Branch: master
UTC Build Time: 2021-12-03 08:12:32
GoVersion: go1.16
Race Enabled: false
TiKV Min Version: v3.0.0-60965b006877ca7234adaced7890d7b029ed1306
Check Table Before Drop: false

[JZuming]

Although issue#29947 has been fixed, these testcases still triggered the problem.

[zyguan]

Thanks, @JZuming ! Like #30361, this bug is also related to unistore.

[zyguan]

The count column processor of unistore is implemented incorrectly here, it always count the first column (c_ymgvrd in your case), that's why you got 39.
@JZuming can you test tidb with tikv? Unistore is only for test purpose.

[JZuming]

@zyguan OK. I have set up the environment for testing tidb with tikv (using "tiup playground"). Hope new bugs can be detected.