Amend transaction can cause lost update

[zyguan]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

/* init */ drop table if exists accounts;
/* init */ create table accounts(id int primary key, balance bigint);
/* init */ insert into accounts values(1, 100), (2, 0), (3, 1);

/* s1 */ begin;
/* s2 */ alter table accounts add column bb int default 10;
/* s2 */ update accounts set bb = 100 where 1;
/* s1 */ update accounts set balance = id where 1;
/* s1 */ commit;
/* s2 */ select * from accounts;

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

/* s2 */ select * from accounts;
-- s2 >> +----+---------+-----+
-- s2    | ID | BALANCE | BB  |
-- s2    +----+---------+-----+
-- s2    |  1 |       1 | 100 |
-- s2    |  2 |       2 | 100 |
-- s2    |  3 |       3 | 100 |
-- s2    +----+---------+-----+

3. What did you see instead (Required)

/* s2 */ select * from accounts;
-- s2 >> +----+---------+----+
-- s2    | ID | BALANCE | BB |
-- s2    +----+---------+----+
-- s2    |  1 |       1 | 10 |
-- s2    |  2 |       2 | 10 |
-- s2    |  3 |       3 | 10 |
-- s2    +----+---------+----+

4. What is your TiDB version? (Required)

release-4.0

[zyguan]

cc @cfzjywxk

[cfzjywxk]

We'll document about this by now.

[cfzjywxk]

This issue is converted to a feature-request in #21069.

[Defined2014]

amend txn is deprecated in v6.5.0, so close this issue.