The `sub_part` in information_schema.statistics table incorrect

[Defined2014]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

create table t (c text);
alter table t add index idx_t (c(16));
select SUB_PART from information_schema.statistics where TABLE_SCHEMA = 'test' and TABLE_NAME = 't';

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

mysql> select SUB_PART from information_schema.statistics where TABLE_SCHEMA = 'test' and TABLE_NAME = 't';
+----------+
| SUB_PART |
+----------+
|       16 |
+----------+
1 row in set (0.00 sec)

3. What did you see instead (Required)

mysql> select SUB_PART from information_schema.statistics where TABLE_SCHEMA = 'test' and TABLE_NAME = 't';
+----------+
| SUB_PART |
+----------+
|     NULL |
+----------+
1 row in set (0.00 sec)

4. What is your TiDB version? (Required)