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This is a legacy defect: can be reproduced in mariadb-5.1, mariadb-5.2 and all versions of mysql.
Let's use these 3 tables and a view:
create table t1 (a int, b int, key(a)) engine=myisam;
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insert into t1 values
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(3,20), (7,10), (2,10), (4,30), (8,70),
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(7,70), (9,100), (9,60), (8,80), (7,60);
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create table t2 (c int, d int, key (c)) engine=myisam;
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insert into t2 values
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(50,100), (20, 200), (10,300),
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(150,100), (120, 200), (110,300),
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(250,100), (220, 200), (210,300);
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create table t3(e int, f int not null, key(e), unique (f)) engine=myisam;
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insert into t3 values
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(100, 3), (300, 5), (400, 4), (300,7),
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(300,2), (600, 13), (800, 15), (700, 14),
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(600, 23), (800, 25), (700, 24);
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create view v1 as
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select * from t2 left join t3 on t3.e=t2.d where t3.f is not null;
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The view is mergeable .
Let's execute the following query:
MariaDB [test]> select *
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-> from t1 left join v1
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-> on v1.c=t1.b
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-> where t1.a < 5;
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+------+------+------+------+------+------+
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| a | b | c | d | e | f |
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+------+------+------+------+------+------+
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| 2 | 10 | 10 | 300 | 300 | 5 |
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| 2 | 10 | 10 | 300 | 300 | 7 |
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| 2 | 10 | 10 | 300 | 300 | 2 |
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| 3 | 20 | NULL | NULL | NULL | NULL |
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| 4 | 30 | NULL | NULL | NULL | NULL |
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+------+------+------+------+------+------+
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Here the optimizer does not convert LEFT JOIN of the view into an INNER JOIN,
though it can be converted, as the WHERE condition of the view does not accept
NULL complemented rows.
MariaDB [test]> explain extended
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-> select *
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-> from t1 left join v1
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-> on v1.c=t1.b
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-> where t1.a < 5;
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+------+-------------+-------+-------+---------------+------+---------+-----------+------+----------+-----------------------+
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| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
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+------+-------------+-------+-------+---------------+------+---------+-----------+------+----------+-----------------------+
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| 1 | SIMPLE | t1 | range | a | a | 5 | NULL | 3 | 100.00 | Using index condition |
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| 1 | SIMPLE | t2 | ref | c | c | 5 | test.t1.b | 2 | 100.00 | Using where |
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| 1 | SIMPLE | t3 | ref | e | e | 5 | test.t2.d | 2 | 100.00 | Using where |
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+------+-------------+-------+-------+---------------+------+---------+-----------+------+----------+-----------------------+
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MariaDB [test]> show warnings;
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+-------+------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
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| Level | Code | Message |
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+-------+------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
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| Note | 1003 | select `test`.`t1`.`a` AS `a`,`test`.`t1`.`b` AS `b`,`test`.`t2`.`c` AS `c`,`test`.`t2`.`d` AS `d`,`test`.`t3`.`e` AS `e`,`test`.`t3`.`f` AS `f` from `test`.`t1` left join (`test`.`t2` left join `test`.`t3` on(((`test`.`t3`.`e` = `test`.`t2`.`d`) and (`test`.`t2`.`d` is not null)))) on(((`test`.`t2`.`c` = `test`.`t1`.`b`) and (`test`.`t3`.`f` is not null) and (`test`.`t1`.`b` is not null))) where (`test`.`t1`.`a` < 5) |
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+-------+------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
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Yet, if we manually merge the view we get the following:
MariaDB [test]> select *
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-> from t1 left join (t2 left join t3 on t3.e=t2.d )
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-> on t2.c=t1.b and t3.f is not null
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-> where t1.a < 5;
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+------+------+------+------+------+------+
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| a | b | c | d | e | f |
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+------+------+------+------+------+------+
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| 2 | 10 | 10 | 300 | 300 | 5 |
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| 2 | 10 | 10 | 300 | 300 | 7 |
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| 2 | 10 | 10 | 300 | 300 | 2 |
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| 3 | 20 | NULL | NULL | NULL | NULL |
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| 4 | 30 | NULL | NULL | NULL | NULL |
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+------+------+------+------+------+------+
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MariaDB [test]> explain extended
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-> select *
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-> from t1 left join (t2 left join t3 on t3.e=t2.d )
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-> on t2.c=t1.b and t3.f is not null
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-> where t1.a < 5;
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+------+-------------+-------+-------+---------------+------+---------+-----------+------+----------+-----------------------+
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| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
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+------+-------------+-------+-------+---------------+------+---------+-----------+------+----------+-----------------------+
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| 1 | SIMPLE | t1 | range | a | a | 5 | NULL | 3 | 100.00 | Using index condition |
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| 1 | SIMPLE | t2 | ref | c | c | 5 | test.t1.b | 2 | 100.00 | Using where |
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| 1 | SIMPLE | t3 | ref | f,e | e | 5 | test.t2.d | 2 | 100.00 | Using where |
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+------+-------------+-------+-------+---------------+------+---------+-----------+------+----------+-----------------------+
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MariaDB [test]> show warnings;
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+-------+------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
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| Level | Code | Message |
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+-------+------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
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| Note | 1003 | select `test`.`t1`.`a` AS `a`,`test`.`t1`.`b` AS `b`,`test`.`t2`.`c` AS `c`,`test`.`t2`.`d` AS `d`,`test`.`t3`.`e` AS `e`,`test`.`t3`.`f` AS `f` from `test`.`t1` left join (`test`.`t2` join `test`.`t3`) on(((`test`.`t2`.`c` = `test`.`t1`.`b`) and (`test`.`t3`.`e` = `test`.`t2`.`d`) and (`test`.`t3`.`f` is not null) and (`test`.`t1`.`b` is not null) and (`test`.`t2`.`d` is not null))) where (`test`.`t1`.`a` < 5) |
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+-------+------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
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Here we see that the optimizer converted the embedded LEFT JOIN into an INNER JOIN.
It gives the optimizer more freedom when choosing a better execution plan.
The following query shows that the optimizer really can choose a different plan with
INNER JOIN instead of LEFT JOIN.
MariaDB [test]> explain extended
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-> select *
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-> from t1 left join v1
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-> on v1.c=t1.b and v1.f=t1.a
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-> where t1.a < 5;
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+------+-------------+-------+-------+---------------+------+---------+-----------+------+----------+-----------------------+
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| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
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+------+-------------+-------+-------+---------------+------+---------+-----------+------+----------+-----------------------+
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| 1 | SIMPLE | t1 | range | a | a | 5 | NULL | 3 | 100.00 | Using index condition |
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| 1 | SIMPLE | t2 | ref | c | c | 5 | test.t1.b | 2 | 100.00 | Using where |
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| 1 | SIMPLE | t3 | ref | e | e | 5 | test.t2.d | 2 | 100.00 | Using where |
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+------+-------------+-------+-------+---------------+------+---------+-----------+------+----------+-----------------------+
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MariaDB [test]> show warnings;
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+-------+------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
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| Level | Code | Message |
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+-------+------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
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| Note | 1003 | select `test`.`t1`.`a` AS `a`,`test`.`t1`.`b` AS `b`,`test`.`t2`.`c` AS `c`,`test`.`t2`.`d` AS `d`,`test`.`t3`.`e` AS `e`,`test`.`t3`.`f` AS `f` from `test`.`t1` left join (`test`.`t2` left join `test`.`t3` on(((`test`.`t3`.`e` = `test`.`t2`.`d`) and (`test`.`t2`.`d` is not null)))) on(((`test`.`t2`.`c` = `test`.`t1`.`b`) and (`test`.`t3`.`f` = `test`.`t1`.`a`) and (`test`.`t1`.`a` is not null) and (`test`.`t1`.`b` is not null))) where (`test`.`t1`.`a` < 5) |
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+-------+------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
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MariaDB [test]> explain extended
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-----+-------------+-------+--------+---------------+------+---------+-----------+------+----------+-----------------------+
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| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
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+------+-------------+-------+--------+---------------+------+---------+-----------+------+----------+-----------------------+
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| 1 | SIMPLE | t1 | range | a | a | 5 | NULL | 3 | 100.00 | Using index condition |
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| 1 | SIMPLE | t3 | eq_ref | f,e | f | 4 | test.t1.a | 1 | 100.00 | Using where |
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| 1 | SIMPLE | t2 | ref | c | c | 5 | test.t1.b | 2 | 100.00 | Using where |
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+------+-------------+-------+--------+---------------+------+---------+-----------+------+----------+-----------------------+
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MariaDB [test]> show warnings;
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+-------+------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
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| Level | Code | Message |
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+-------+------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
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| Note | 1003 | select `test`.`t1`.`a` AS `a`,`test`.`t1`.`b` AS `b`,`test`.`t2`.`c` AS `c`,`test`.`t2`.`d` AS `d`,`test`.`t3`.`e` AS `e`,`test`.`t3`.`f` AS `f` from `test`.`t1` left join (`test`.`t2` join `test`.`t3`) on(((`test`.`t2`.`c` = `test`.`t1`.`b`) and (`test`.`t3`.`f` = `test`.`t1`.`a`) and (`test`.`t2`.`d` = `test`.`t3`.`e`) and (`test`.`t1`.`a` is not null) and (`test`.`t1`.`a` is not null) and (`test`.`t1`.`b` is not null))) where (`test`.`t1`.`a` < 5) |
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+-------+------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
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