dml: wrong result in subquery with order by clause

[PragmaTwice]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

create table a(b float);
SELECT avg(2) FROM(SELECT min(c) FROM a JOIN(SELECT 1 c) d ORDER BY b) e;

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

+--------+
| avg(2) |
+--------+
| 2.0000 |
+--------+
1 row in set (0.00 sec)

3. What did you see instead (Required)

+--------+
| avg(2) |
+--------+
|   NULL |
+--------+
1 row in set (0.00 sec)

4. What is your TiDB version? (Required)

+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| tidb_version()                                                                            |
+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Release Version: v5.1.0-alpha-167-ga8ad9b7d3
Edition: Community
Git Commit Hash: a8ad9b7d3aa53d0181422d2b22fa8d57b7d56db1
Git Branch: master
UTC Build Time: 2021-06-05 07:04:28
GoVersion: go1.16.4
Race Enabled: false
TiKV Min Version: v3.0.0-60965b006877ca7234adaced7890d7b029ed1306
Check Table Before Drop: false |
+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

[lzmhhh123]

It is because we transfer subquery to join, then the single line NULL result of the subquery disappears.

[ti-srebot]

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