Needless look-ups for NULL keys in equi-joins

[Igor Babaev]

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) ;
INSERT INTO t1 VALUES (NULL), (1), (NULL), (NULL), (2);
CREATE TABLE t2 (a int, INDEX idx(a)) ;
INSERT INTO t2 VALUES (7), (4), (1), (NULL), (5), (2), (1), (NULL), (9);

MariaDB [test]> EXPLAIN SELECT * FROM t1,t2 WHERE t2.a=t1.a;
+----+-------------+-------+------+---------------+------+---------+-----------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+-----------+------+-------------+
| 1 | SIMPLE | t1 | ALL | NULL | NULL | NULL | NULL | 5 | Using where |
| 1 | SIMPLE | t2 | ref | idx | idx | 5 | test.t1.a | 2 | Using index |
+----+-------------+-------+------+---------------+------+---------+-----------+------+-------------+
2 rows in set (0.00 sec)

MariaDB [test]> FLUSH STATUS;
Query OK, 0 rows affected (0.00 sec)

MariaDB [test]> SELECT * FROM t1,t2 WHERE t2.a=t1.a;
+------+------+
| a | a |
+------+------+
| 1 | 1 |
| 1 | 1 |
| 2 | 2 |
+------+------+
3 rows in set (0.00 sec)

MariaDB [test]> SHOW STATUS LIKE 'Handler_read%';
+-----------------------+-------+
| Variable_name | Value |
+-----------------------+-------+
| Handler_read_first | 0 |
| Handler_read_key | 2 |
| Handler_read_next | 3 |
| Handler_read_prev | 0 |
| Handler_read_rnd | 0 |
| Handler_read_rnd_next | 6 |
+-----------------------+-------+
6 rows in set (0.00 sec)

MariaDB [test]> EXPLAIN SELECT * FROM t1,t2 WHERE t2.a=t1.a+0;
+----+-------------+-------+------+---------------+------+---------+------+------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+--------------------------+
| 1 | SIMPLE | t1 | ALL | NULL | NULL | NULL | NULL | 5 | |
| 1 | SIMPLE | t2 | ref | idx | idx | 5 | func | 2 | Using where; Using index |
+----+-------------+-------+------+---------------+------+---------+------+------+--------------------------+
2 rows in set (0.00 sec)

MariaDB [test]> FLUSH STATUS;
Query OK, 0 rows affected (0.00 sec)

MariaDB [test]> SELECT * FROM t1,t2 WHERE t2.a=t1.a+0;
+------+------+
| a | a |
+------+------+
| 1 | 1 |
| 1 | 1 |
| 2 | 2 |
+------+------+
3 rows in set (0.00 sec)

MariaDB [test]> SHOW STATUS LIKE 'Handler_read%';
+-----------------------+-------+
| Variable_name | Value |
+-----------------------+-------+
| Handler_read_first | 0 |
| Handler_read_key | 5 |
| Handler_read_next | 9 |
| Handler_read_prev | 0 |
| Handler_read_rnd | 0 |
| Handler_read_rnd_next | 6 |
+-----------------------+-------+
6 rows in set (0.00 sec)

[Igor Babaev (igorb-seattle)]

This is an optimization problem. That's why I set the importance to 'Low'.