The result of stddev_pop(distinct xxx) is wrong

[Win-Man]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

mysql> create table t(id int);
Query OK, 0 rows affected (0.00 sec)

mysql> insert into t values(1),(2);
Query OK, 2 rows affected (0.00 sec)
Records: 2  Duplicates: 0  Warnings: 0

mysql> select stddev_pop(id) from t;
+----------------+
| stddev_pop(id) |
+----------------+
|            0.5 |
+----------------+
1 row in set (0.00 sec)

mysql> insert into t values(1);
Query OK, 1 row affected (0.00 sec)

mysql> select stddev_pop(distinct id) from t;
+-------------------------+
| stddev_pop(distinct id) |
+-------------------------+
|      0.4714045207910317 |
+-------------------------+
1 row in set (0.00 sec)

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

mysql> select stddev_pop(distinct id) from t;
+----------------+
| stddev_pop(id) |
+----------------+
|            0.5 |
+----------------+
1 row in set (0.00 sec)

3. What did you see instead (Required)

4. What is your TiDB version? (Required)

Release Version: v4.0.0-beta.2-1111-gbdb673f5a
Edition: Community
Git Commit Hash: bdb673f5a2d5f8fb1602fcf121c3332b973a06d7
Git Branch: master
UTC Build Time: 2020-09-01 12:42:19
GoVersion: go1.13
Race Enabled: false
TiKV Min Version: v3.0.0-60965b006877ca7234adaced7890d7b029ed1306
Check Table Before Drop: false

[ZenoTan]

/label status/PTAL