Support nullable group by

[sundy-li]

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Summary

Improve performance for group by nullable column

Changelog

• Improvement

Related Issues

Continue: #4158
Fixes #4079

Test Plan

Unit Tests

Stateless Tests

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[mergify]

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[BohuTANG]

💯

[sundy-li]

Ontime datasets benchmark (Both run on the second query with metacache)：

Before:

MySQL [(none)]> SELECT DayOfWeek, count(*) AS c FROM ontime GROUP BY DayOfWeek ORDER BY c DESC;
+-----------+----------+
| DayOfWeek | c        |
+-----------+----------+
|         3 | 23513895 |
|         2 | 23384295 |
|         4 | 23113350 |
|         7 | 21270195 |
|         1 | 20861550 |
|         6 | 19360215 |
|         5 | 18896085 |
+-----------+----------+
7 rows in set (2.578 sec)

After:

MySQL [(none)]> SELECT DayOfWeek, count(*) AS c FROM ontime GROUP BY DayOfWeek ORDER BY c DESC;
+-----------+----------+
| DayOfWeek | c        |
+-----------+----------+
|         3 | 23513895 |
|         2 | 23384295 |
|         4 | 23113350 |
|         7 | 21270195 |
|         1 | 20861550 |
|         6 | 19360215 |
|         5 | 18896085 |
+-----------+----------+
7 rows in set (0.896 sec)

[BohuTANG]

Also cc @wubx @dantengsky for the performance improvement.

It's time to change the subject/ontime table schema:
#4342

[wangzhen11aaa]

C~l