executor: for global index update partition keys but returns "Duplicate Key" error

[L-maple]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

drop table if exists test_global;
create table test_global ( a int, b int, c int)
partition by range( a ) (
	partition p1 values less than (10),
	partition p2 values less than (20),
	partition p3 values less than (30)
);
alter table test_global add unique index idx_b (b);
insert into test_global values (1, 1, 1), (8, 8, 8), (11, 11, 11), (12, 12, 12);
update test_global set a = 2 where a = 11;

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

update successfully.

3. What did you see instead (Required)

Duplicate Key error.

4. What is your TiDB version? (Required)