planner, executor: correlated select items should return NULL if the outer table is empty

[fzhedu]

Description

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?

create table t (a int);
create table s (a int);
insert into t values(1);
insert into s values(1);
select count(t.a), (select count(s.a) from s where s.a > t.a) as field from t;
select count(t.a), (select count(s.a) from s where s.a > t.a) as field from t where t.a=100;

2. What did you expect to see?
   Results from MySQL:

mysql> select count(t.a), (select count(s.a) from s where s.a > t.a) as field from t;
+------------+-------+
| count(t.a) | field |
+------------+-------+
|          1 |     0 |
+------------+-------+
1 row in set (0.00 sec)

mysql> select count(t.a), (select count(s.a) from s where s.a > t.a) as field from t where t.a=100;
+------------+-------+
| count(t.a) | field |
+------------+-------+
|          0 |  NULL |
+------------+-------+
1 row in set (0.00 sec)

mysql> select version();
+-----------+
| version() |
+-----------+
| 8.0.18    |
+-----------+
1 row in set (0.00 sec)

3. What did you see instead?
   Results from TiDB:

mysql> select count(t.a), (select count(s.a) from s where s.a > t.a) as field from t;
+------------+-------+
| count(t.a) | field |
+------------+-------+
|          1 |     0 |
+------------+-------+
1 row in set (0.00 sec)

mysql> select count(t.a), (select count(s.a) from s where s.a > t.a) as field from t where t.a=100;
+------------+-------+
| count(t.a) | field |
+------------+-------+
|          0 |     0 |
+------------+-------+
1 row in set (0.00 sec)

mysql> select version();
+-----------------------------------------+
| version()                               |
+-----------------------------------------+
| 5.7.25-TiDB-v4.0.0-alpha-959-gaf2a2c532 |
+-----------------------------------------+
1 row in set (0.00 sec)

4. What version of TiDB are you using (tidb-server -V or run select tidb_version(); on TiDB)?

5.7.25-TiDB-v4.0.0-alpha-959-gaf2a2c532

SIG slack channel

#sig-exec

Score

• 300

Mentor

• @SunRunAway

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The issue has been removed from the challenge program.

[ti-challenge-bot]

The issue has been removed from the challenge program.

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The issue has been removed from the challenge program.

[ti-challenge-bot]

The issue has been removed from the challenge program.

[ti-srebot]

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Bug

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1. Root Cause Analysis (RCA) (optional)

2. Symptom (optional)

3. All Trigger Conditions (optional)

4. Workaround (optional)

5. Affected versions

6. Fixed versions

[eurekaka]

The fix is not complete, reopen this issue first.

[wjhuang2016]

@eurekaka If the fix is not complete, is the bug still critical?

[eurekaka]

Yes