expression: Support distinct in select stddev_pop() (#19680)

[ti-srebot]

cherry-pick #19680 to release-3.0

---

What problem does this PR solve?

Issue Number: close #19676

Problem Summary:

What is changed and how it works?

Proposal: xxx

What's Changed:

How it Works:

Related changes

• Need to cherry-pick to the release branch

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)

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) |
+-------------------------+
|                     0.5 |
+-------------------------+
1 row in set (0.00 sec)

Side effects

Release note

• No release note.

[ti-srebot]

/run-all-tests

[SunRunAway]

LGTM

[wshwsh12]

lgtm

[wshwsh12]

/run-all-tests

[Win-Man]

/run-all-tests

[lilinghai]

/build

[lilinghai]

/run-all-tests