| Bug #63868 | Left outer join with user defined function on NULL column gives wrong results | ||
|---|---|---|---|
| Submitted: | 28 Dec 2011 19:34 | Modified: | 29 Dec 2011 4:24 |
| Reporter: | Pim VAN DER WAL | Email Updates: | |
| Status: | Verified | Impact on me: | |
| Category: | MySQL Server: Optimizer | Severity: | S3 (Non-critical) |
| Version: | 5.1.60, 5.5.20 | OS: | Linux (Centos 5 x64) |
| Assigned to: | CPU Architecture: | Any | |
| Tags: | FUNCTION, LEFT OUTER JOIN, udf | ||
[29 Dec 2011 4:24]
Valeriy Kravchuk
Thank you for the problem report. Verified with 5.5.20 also:
macbook-pro:5.5 openxs$ bin/mysql -uroot test
Reading table information for completion of table and column names
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Server version: 5.5.20-debug Source distribution
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Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.
mysql> CREATE TABLE tbl_1 (a INT) ENGINE=INNODB;
Query OK, 0 rows affected (0.31 sec)
mysql> CREATE TABLE tbl_2 (b INT) ENGINE=INNODB;
Query OK, 0 rows affected (0.20 sec)
mysql> INSERT INTO tbl_1 (a) VALUES (1);
Query OK, 1 row affected (0.14 sec)
mysql> DELIMITER $$
mysql> DROP FUNCTION IF EXISTS fn_1 $$
Query OK, 0 rows affected, 1 warning (0.00 sec)
mysql> CREATE FUNCTION fn_1 (p_1 INT) RETURNS INT
-> BEGIN
-> RETURN 1;
-> END $$
Query OK, 0 rows affected (0.15 sec)
mysql> DELIMITER ;
mysql> SELECT *
-> FROM tbl_1
-> LEFT JOIN (SELECT b FROM tbl_2) AS tmp ON tbl_1.a = tmp.b
-> WHERE fn_1(tmp.b) = 1;
Empty set (0.06 sec)
mysql> SELECT * FROM tbl_1 LEFT JOIN tbl_2 AS tmp ON tbl_1.a = tmp.b WHERE fn_1(tmp.b) = 1;
Empty set (0.04 sec)
mysql> SELECT * FROM tbl_1 LEFT JOIN tbl_2 AS tmp ON tbl_1.a = tmp.b WHERE fn_1(tmp.b) <=> 1;
+------+------+
| a | b |
+------+------+
| 1 | NULL |
+------+------+
1 row in set (0.00 sec)
Here is what happens internally:
mysql> explain extended SELECT * FROM tbl_1 LEFT JOIN tbl_2 AS tmp ON tbl_1.a = tmp.b WHERE fn_1(tmp.b) = 1;
+----+-------------+-------+------+---------------+------+---------+------+------+----------+--------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+----------+--------------------------------+
| 1 | SIMPLE | tbl_1 | ALL | NULL | NULL | NULL | NULL | 1 | 100.00 | Using where |
| 1 | SIMPLE | tmp | ALL | NULL | NULL | NULL | NULL | 1 | 100.00 | Using where; Using join buffer |
+----+-------------+-------+------+---------------+------+---------+------+------+----------+--------------------------------+
2 rows in set, 1 warning (0.10 sec)
mysql> show warnings\G
*************************** 1. row ***************************
Level: Note
Code: 1003
Message: select `test`.`tbl_1`.`a` AS `a`,`test`.`tmp`.`b` AS `b` from `test`.`tbl_1` join `test`.`tbl_2` `tmp` where ((`test`.`tmp`.`b` = `test`.`tbl_1`.`a`) and (`fn_1`(`test`.`tbl_1`.`a`) = 1))
1 row in set (0.00 sec)
So, query is (mistakenly) rewritten as JOIN instead of LEFT JOIN. Compare to this:
mysql> explain extended SELECT * FROM tbl_1 LEFT JOIN tbl_2 AS tmp ON tbl_1.a = tmp.b WHERE fn_1(tmp.b) <=> 1;
+----+-------------+-------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+----------+-------------+
| 1 | SIMPLE | tbl_1 | ALL | NULL | NULL | NULL | NULL | 1 | 100.00 | |
| 1 | SIMPLE | tmp | ALL | NULL | NULL | NULL | NULL | 1 | 100.00 | Using where |
+----+-------------+-------+------+---------------+------+---------+------+------+----------+-------------+
2 rows in set, 1 warning (0.00 sec)
mysql> show warnings\G
*************************** 1. row ***************************
Level: Note
Code: 1003
Message: select `test`.`tbl_1`.`a` AS `a`,`test`.`tmp`.`b` AS `b` from `test`.`tbl_1` left join `test`.`tbl_2` `tmp` on((`test`.`tmp`.`b` = `test`.`tbl_1`.`a`)) where (`fn_1`(`test`.`tmp`.`b`) <=> 1)
1 row in set (0.00 sec)
Description: A query with a left join and a user defined function in the where clause using a column from the outer table of the join produces the wrong results if the outer table does not have a matching row. In this case the value passed to the function should be NULL but it apparently has some indeterminate value. The example script below has a function that always returns 1 even if the parameter is NULL. The where clause should therefore be true for every row but in the main query no rows are returned indicating that a different value is returned. The alternative is that an error or a warning is produced but that doesn't happen either and the error also shows no message. How to repeat: CREATE TABLE tbl_1 (a INT) ENGINE=INNODB; CREATE TABLE tbl_2 (b INT) ENGINE=INNODB; INSERT INTO tbl_1 (a) VALUES (1); DELIMITER $$ DROP FUNCTION IF EXISTS fn_1 $$ CREATE FUNCTION fn_1 (p_1 INT) RETURNS INT BEGIN RETURN 1; END $$ DELIMITER ; SELECT * FROM tbl_1 LEFT JOIN (SELECT b FROM tbl_2) AS tmp ON tbl_1.a = tmp.b WHERE fn_1(tmp.b) = 1; Does not produce any results whereas it should produce 1 row since fn_1 always produces 1 even if called with NULL. Strange enough the following where clause does produce the correct result which seems to indicate the return value is actually NULL: WHERE FN_1(tmp.b) <=> 1; However the following where clause does not produce any results again. WHERE FN_1(tmp.b) IS NULL;