value of INDEX_LENGTH column is incorrect with global index

[Defined2014]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

create table t(a int, b int, c int) partition by hash(c) partitions 4;
alter table t add unique index b(b);
insert into t values (1,1,1);
analyze table t;
select * from information_schema.partitions where table_name like 't';

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

mysql> select partition_name, index_length from information_schema.partitions where table_name like 't';
+----------------+--------------+
| partition_name | index_length |
+----------------+--------------+
| p0             |            0 |
| p1             |            0 |
| p2             |            0 |
| p3             |            0 |
+----------------+--------------+
4 rows in set (0.01 sec)

3. What did you see instead (Required)

mysql> select partition_name, index_length from information_schema.partitions where table_name like 't';
+----------------+--------------+
| partition_name | index_length |
+----------------+--------------+
| p0             |            0 |
| p1             |            8 |
| p2             |            0 |
| p3             |            0 |
+----------------+--------------+
4 rows in set (0.01 sec)

4. What is your TiDB version? (Required)

[Defined2014]

@mjonss Do you think this was expected or a bug? Should we change it?