Details
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Bug
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Status: Confirmed (View Workflow)
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Trivial
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Resolution: Unresolved
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5.3.12, 5.5.36, 10.0.10, 5.5(EOL), 10.0(EOL), 10.1(EOL)
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None
Description
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) |