add index row count doesn't match the actual row count

[crazycs520]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

create table t (id bigint key,b int);
split table t by (10),(20),(30);
 insert into t values (0,0),(10,10),(20,20),(30,30);
alter table t add index idx1(b);
admin show ddl jobs;
select count(*) from t;

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

add index row count match the actual row count.

3. What did you see instead (Required)

doesn't match.

4. What is your TiDB version? (Required)

test> select tidb_version()\G
***************************[ 1. row ]***************************
tidb_version() | Release Version: v5.0.2
Edition: Community
Git Commit Hash: dc40a093a0058b95fc859f60d4b84337dbdff9f6
Git Branch: HEAD
UTC Build Time: 2021-07-05 08:12:38
GoVersion: go1.16
Race Enabled: false
TiKV Min Version: v3.0.0-60965b006877ca7234adaced7890d7b029ed1306
Check Table Before Drop: false

[wjhuang2016]

Region：      [0, 10)   [10, 20),  [20, 30),  [30, ...)
Record:         0,          10,      20,        30
------------------------------------------------
startKey: 0 endKey: 30
addIndex Splited KV range(according to region):
[0: 10],  [10, 20], [20, 30]
  2     +     2      +   2          =   6

I found the root cause, but it's not easy to fix it.

[bb7133]

Mark the issue as 'enhancement' because the 'add index row' is now an estimation, it is not accurate by design.

[dcswinner01]

When will this bug be fixed?