[MDEV-2550] LP:884184 - Wrong result with RIGHT JOIN + derived_merge Created: 2011-10-31  Updated: 2015-02-02  Resolved: 2012-10-04

Status: Closed
Project: MariaDB Server
Component/s: None
Affects Version/s: None
Fix Version/s: None

Type: Bug Priority: Minor
Reporter: Philip Stoev (Inactive) Assignee: Sergei Petrunia
Resolution: Fixed Votes: 0
Labels: Launchpad

Attachments: XML File LPexportBug884184.xml    

 Description   

The following query:

SELECT alias1.*
FROM (
SELECT t1.*
FROM t1
LEFT JOIN t2
ON ( t1.a = t2.a )
WHERE t2.a <> 0
) AS alias1
RIGHT JOIN t3
ON ( t3.a = alias1.b );

returns the following incorrect result when executed with derived_merge=on:

a b
0 g

the correct result is (NULL , NULL) as the subquery does not return any rows and therefore the RIGHT JOIN between the subquery and a table can not include any rows from the subquery.

explain:

id select_type table type possible_keys key key_len ref rows filtered Extra
1 SIMPLE t3 system NULL NULL NULL NULL 1 100.00
1 SIMPLE t1 ALL NULL NULL NULL NULL 1 100.00 Using where

select `test`.`t1`.`a` AS `a`,`test`.`t1`.`b` AS `b` from `test`.`t3` left join (`test`.`t1`) on(((`test`.`t2`.`a` <> 0) and (`test`.`t1`.`b` = 'g'))) where 1

minimal switch: derived_merge=on
full switch: index_merge=on,index_merge_union=on,index_merge_sort_union=on,index_merge_intersection=on,index_merge_sort_intersection=off,index_condition_pushdown=off,derived_merge=on,derived_with_keys=off,firstmatch=off,loosescan=off,materialization=off,in_to_exists=on,semijoin=off,partial_match_rowid_merge=on,partial_match_table_scan=on,subquery_cache=on,mrr=off,mrr_cost_based=off,mrr_sort_keys=off,outer_join_with_cache=off,semijoin_with_cache=off,join_cache_incremental=on,join_cache_hashed=on,join_cache_bka=on,optimize_join_buffer_size=off,table_elimination=on

bzr version-info
revision-id: <email address hidden>
date: 2011-10-28 12:38:36 +0400
build-date: 2011-10-31 13:55:00 +0200
revno: 3258
branch-nick: maria-5.3

test case:

DROP TABLE IF EXISTS t1;
CREATE TABLE t1 ( a int(11), b varchar(1)) ;
INSERT IGNORE INTO t1 VALUES (0,'g');

DROP TABLE IF EXISTS t3;
CREATE TABLE t3 ( a varchar(1)) ;
INSERT IGNORE INTO t3 VALUES ('g');

DROP TABLE IF EXISTS t2;
CREATE TABLE t2 ( a int(11) NOT NULL, PRIMARY KEY (a)) ;

SET SESSION optimizer_swtich='derived_merge=on';

SELECT alias1.*
FROM (
SELECT t1.*
FROM t1
LEFT JOIN t2
ON ( t1.a = t2.a )
WHERE t2.a <> 0
) AS alias1
RIGHT JOIN t3
ON ( t3.a = alias1.b );

 Comments   
Comment by Sergei Petrunia [ 2011-11-01 ]

Re: Wrong result with RIGHT JOIN + derived_merge
Changed query from RIGHT JOIN to LEFT JOIN (the bug repeats):

SELECT alias1.*
FROM
t3
LEFT JOIN
(
SELECT t1.*
FROM t1 LEFT JOIN t2 ON ( t1.a = t2.a )
WHERE t2.a <> 0
) AS alias1
ON ( t3.a = alias1.b );

Comment by Igor Babaev [ 2011-11-01 ]

Re: Wrong result with RIGHT JOIN + derived_merge
This bug can be reproduced as well with:

  • LEFT JOIN instead of RIGHT JOIN
  • view instead of derived table.

The bug is reproducible in 5.2 (and most probably in 5.1):

igor@sophia:~/maria/maria-5.2/mysql-test> ../client/mysql test
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

MariaDB [test]> DROP TABLE IF EXISTS t1;
Query OK, 0 rows affected, 1 warning (0.00 sec)

MariaDB [test]> CREATE TABLE t1 ( a int(11), b varchar(1)) ;
Query OK, 0 rows affected (0.01 sec)

MariaDB [test]> INSERT IGNORE INTO t1 VALUES (0,'g');
Query OK, 1 row affected (0.00 sec)

MariaDB [test]>
MariaDB [test]> DROP TABLE IF EXISTS t3;
Query OK, 0 rows affected, 1 warning (0.00 sec)

MariaDB [test]> CREATE TABLE t3 ( a varchar(1)) ;
Query OK, 0 rows affected (0.01 sec)

MariaDB [test]> INSERT IGNORE INTO t3 VALUES ('g');
Query OK, 1 row affected (0.00 sec)

MariaDB [test]>
MariaDB [test]> DROP TABLE IF EXISTS t2;
Query OK, 0 rows affected, 1 warning (0.00 sec)

MariaDB [test]> CREATE TABLE t2 ( a int(11) NOT NULL, PRIMARY KEY (a)) ;
Query OK, 0 rows affected (0.01 sec)

MariaDB [test]> create view v1 as SELECT t1.* FROM t1 LEFT JOIN t2 ON ( t1.a = t2.a ) WHERE t2.a <> 0;
Query OK, 0 rows affected (0.01 sec)

MariaDB [test]> SELECT alias1.* FROM t3 LEFT JOIN v1 as alias1 ON ( t3.a = alias1.b );
----------+

a b

----------+

0 g

----------+
1 row in set (0.00 sec)

MariaDB [test]> EXPLAIN SELECT alias1.* FROM t3 LEFT JOIN v1 as alias1 ON ( t3.a = alias1.b );
---------------------------------------------------------------------+

id select_type table type possible_keys key key_len ref rows Extra

---------------------------------------------------------------------+

1 SIMPLE t3 system NULL NULL NULL NULL 1  
1 SIMPLE t1 ALL NULL NULL NULL NULL 1 Using where

---------------------------------------------------------------------+
2 rows in set (0.00 sec)

This is a bug of table elimination:

MariaDB [test]> set optimizer_switch='table_elimination=off';
Query OK, 0 rows affected (0.00 sec)

MariaDB [test]> EXPLAIN SELECT alias1.* FROM t3 LEFT JOIN v1 as alias1 ON ( t3.a = alias1.b );
------------------------------------------------------------------------------------------+

id select_type table type possible_keys key key_len ref rows Extra

------------------------------------------------------------------------------------------+

1 SIMPLE t3 system NULL NULL NULL NULL 1  
1 SIMPLE t1 ALL NULL NULL NULL NULL 1 Using where
1 SIMPLE t2 eq_ref PRIMARY PRIMARY 4 test.t1.a 1 Using where; Using index

------------------------------------------------------------------------------------------+
3 rows in set (0.00 sec)

MariaDB [test]> SELECT alias1.* FROM t3 LEFT JOIN v1 as alias1 ON ( t3.a = alias1.b );
----------+

a b

----------+

NULL NULL

----------+
1 row in set (0.00 sec)

Comment by Rasmus Johansson (Inactive) [ 2011-12-13 ]

Launchpad bug id: 884184

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