Details
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Type:
Bug
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Status: Closed
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Priority:
Major
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Resolution: Fixed
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Affects Version/s: 5.5.25
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Fix Version/s: 5.5.27
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Component/s: None
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Labels:None
Description
In MariaDB 5.3 the output from the EXPLAIN of the test case for lp bug #714999 is
MariaDB [test]> EXPLAIN SELECT f2 FROM t3 WHERE (
-> SELECT MAX( pk ) FROM t1
-> WHERE EXISTS (
-> SELECT max(f1)
-> FROM t2 GROUP BY f1
-> )
-> ) IS NULL ;
-------------------------------------------------------------------------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
-------------------------------------------------------------------------------------------------------------+
| 1 | PRIMARY | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Impossible WHERE noticed after reading const tables |
| 2 | SUBQUERY | t1 | system | NULL | NULL | NULL | NULL | 1 | |
| 3 | SUBQUERY | t2 | ALL | NULL | NULL | NULL | NULL | 2 | Using temporary; Using filesort |
-------------------------------------------------------------------------------------------------------------+
So the expected number of examined rows from t2 is 2 and that is correct.
Yet in MariaDB 5.5 the EXPLAIN output for the same test case is
MariaDB [test]> EXPLAIN SELECT f2 FROM t3 WHERE (
-> SELECT MAX( pk ) FROM t1
-> WHERE EXISTS (
-> SELECT max(f1)
-> FROM t2 GROUP BY f1
-> )
-> ) IS NULL ;
----------------------------------------------------------------------------------------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
----------------------------------------------------------------------------------------------------------------------------+
| 1 | PRIMARY | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Impossible WHERE noticed after reading const tables |
| 2 | SUBQUERY | t1 | system | NULL | NULL | NULL | NULL | 1 | |
| 3 | SUBQUERY | internal_tmp_table | ALL | group_key | NULL | NULL | NULL | 1 | Using temporary; Using filesort |
----------------------------------------------------------------------------------------------------------------------------+
Here the name of the table in the third line is 'internal_tmp_table' and the expected number of the examined rows is 1.
Apparently this is not correct. This line of the EXPLAIN output is for the plan of the uncorrelated sub-query 'SELECT max(f1)FROM t2 GROUP BY f1'.
This change in the plan appeared after the patch that fixes lp bug #944706.
I consider this as a serious regression bug.
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Got approval from SergeyP, Merged, tested, pushed.
This should be fixed by backporting the relevant part of SHOW EXPLAIN.