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If the join condition is of the form <t2.key>=<t1.no_key>, then the server performs no index look-ups for the values of t1,no_key that are NULLS when joining tables t1 and t2.
However if the condition is of the form <t2.key>=<expression>(<t1.no_key>) the look-ups for NULL keys are
performed.
This can be seen from the following example:
CREATE TABLE t1 (a int) ;
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INSERT INTO t1 VALUES (NULL), (1), (NULL), (NULL), (2);
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CREATE TABLE t2 (a int, INDEX idx(a)) ;
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INSERT INTO t2 VALUES (7), (4), (1), (NULL), (5), (2), (1), (NULL), (9);
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MariaDB [test]> EXPLAIN SELECT * FROM t1,t2 WHERE t2.a=t1.a;
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+----+-------------+-------+------+---------------+------+---------+-----------+------+-------------+
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| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
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+----+-------------+-------+------+---------------+------+---------+-----------+------+-------------+
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| 1 | SIMPLE | t1 | ALL | NULL | NULL | NULL | NULL | 5 | Using where |
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| 1 | SIMPLE | t2 | ref | idx | idx | 5 | test.t1.a | 2 | Using index |
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+----+-------------+-------+------+---------------+------+---------+-----------+------+-------------+
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2 rows in set (0.00 sec)
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MariaDB [test]> FLUSH STATUS;
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Query OK, 0 rows affected (0.00 sec)
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MariaDB [test]> SELECT * FROM t1,t2 WHERE t2.a=t1.a;
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+------+------+
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| a | a |
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+------+------+
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| 1 | 1 |
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| 1 | 1 |
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| 2 | 2 |
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+------+------+
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3 rows in set (0.00 sec)
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MariaDB [test]> SHOW STATUS LIKE 'Handler_read%';
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+-----------------------+-------+
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| Variable_name | Value |
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+-----------------------+-------+
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| Handler_read_first | 0 |
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| Handler_read_key | 2 |
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| Handler_read_next | 3 |
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| Handler_read_prev | 0 |
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| Handler_read_rnd | 0 |
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| Handler_read_rnd_next | 6 |
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+-----------------------+-------+
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6 rows in set (0.00 sec)
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MariaDB [test]> EXPLAIN SELECT * FROM t1,t2 WHERE t2.a=t1.a+0;
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+----+-------------+-------+------+---------------+------+---------+------+------+--------------------------+
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| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
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+----+-------------+-------+------+---------------+------+---------+------+------+--------------------------+
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| 1 | SIMPLE | t1 | ALL | NULL | NULL | NULL | NULL | 5 | |
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| 1 | SIMPLE | t2 | ref | idx | idx | 5 | func | 2 | Using where; Using index |
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+----+-------------+-------+------+---------------+------+---------+------+------+--------------------------+
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2 rows in set (0.00 sec)
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MariaDB [test]> FLUSH STATUS;
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Query OK, 0 rows affected (0.00 sec)
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MariaDB [test]> SELECT * FROM t1,t2 WHERE t2.a=t1.a+0;
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+------+------+
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| a | a |
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+------+------+
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| 1 | 1 |
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| 1 | 1 |
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| 2 | 2 |
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+------+------+
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3 rows in set (0.00 sec)
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MariaDB [test]> SHOW STATUS LIKE 'Handler_read%';
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+-----------------------+-------+
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| Variable_name | Value |
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+-----------------------+-------+
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| Handler_read_first | 0 |
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| Handler_read_key | 5 |
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| Handler_read_next | 9 |
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| Handler_read_prev | 0 |
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| Handler_read_rnd | 0 |
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| Handler_read_rnd_next | 6 |
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+-----------------------+-------+
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6 rows in set (0.00 sec)
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LPexportBug731069.xml