wrong result when evaluating correlated aggregate functions

[XuHuaiyu]

Description

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

CREATE TABLE tab(i INT); 
CREATE TABLE tab2(j INT); 

insert into tab values(1),(2),(3);
insert into tab2 values(1),(2),(3),(15);

SELECT m.i, 
       (SELECT COUNT(n.j) 
           FROM tab2 WHERE j=15) AS o 
    FROM tab m, tab2 n GROUP BY 1 order by m.i;

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

mysql> SELECT m.i,
         (SELECT COUNT(n.j)
             FROM tab2 WHERE j=15) AS o
      FROM tab m, tab2 n GROUP BY 1 order by m.i;
+------+------+
| i    | o    |
+------+------+
|    1 |    4 |
|    2 |    4 |
|    3 |    4 |
+------+------+
3 rows in set (0.00 sec)

3. What did you see instead (Required)

tidb> SELECT m.i,
         (SELECT COUNT(n.j)
             FROM tab2 WHERE j=15) AS o
      FROM tab m, tab2 n GROUP BY 1 order by m.i;
+------+---+
| i    | o |
+------+---+
|    1 | 1 |
|    2 | 1 |
|    3 | 1 |
+------+---+
3 rows in set (0.00 sec)

4. Affected version (Required)

all the versions

5. Root Cause Analysis

TiDB does not handle correlated aggregate in subqueries correctly. I do not find any references from MySQL official manual, but this doc can be referred.

SIG slack channel

#sig-planner

Score

• 900

Mentor

• @lzmhhh123

[XuHuaiyu]

We may rewrite the SQL to

SELECT m.i, 
       (SELECT (select COUNT(tab2.j) from tab2) 
           FROM tab2 WHERE j=15) AS o 
    FROM tab m, tab2 n GROUP BY 1 order by m.i;

[dyzsr]

/assign

[ti-srebot]

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