| 2012-11-20 00:06:59 |
Igor Babaev |
description |
If to create and populate tables t1,t2 with the commands
CREATE TABLE t1(a int PRIMARY KEY, b int) ENGINE=myisam;
INSERT INTO t1 VALUES
(5, 10), (2, 70), (7, 80), (6, 20), (1, 50), (9, 40), (8, 30), (3, 60);
CREATE TABLE t2 (p int, a int, INDEX i_a(a)) ENGINE=myisam;
INSERT INTO t2 VALUES
(103, 7), (109, 3), (102, 3), (108, 1), (106, 3),
(107, 7), (105, 1), (101, 3), (100, 7), (110, 1);
then Mariadb 5.5 will return the following explain output for the query
SELECT t1.a FROM t1 LEFT JOIN t2 ON t1.a=t2.a ORDER BY t1.a
MariaDB [test]> EXPLAIN
-> SELECT t1.a FROM t1 LEFT JOIN t2 ON t1.a=t2.a ORDER BY t1.a;
+------+-------------+-------+-------+---------------+---------+---------+-----------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+------+-------------+-------+-------+---------------+---------+---------+-----------+------+-------------+
| 1 | SIMPLE | t1 | index | NULL | PRIMARY | 4 | NULL | 4 | Using index |
| 1 | SIMPLE | t2 | ref | i_a | i_a | 5 | test.t1.a | 2 | Using index |
+------+-------------+-------+-------+---------------+---------+---------+-----------+------+-------------+
The expected number of the examined rows from t1 is 4 though it should be 8 as for the following query:
MariaDB [test]> EXPLAIN
-> SELECT t1.a FROM t1 LEFT JOIN t2 ON t1.a=t2.a ORDER BY t1.a;
+------+-------------+-------+-------+---------------+---------+---------+-----------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+------+-------------+-------+-------+---------------+---------+---------+-----------+------+-------------+
| 1 | SIMPLE | t1 | index | NULL | PRIMARY | 4 | NULL | 4 | Using index |
| 1 | SIMPLE | t2 | ref | i_a | i_a | 5 | test.t1.a | 2 | Using index |
+------+-------------+-------+-------+---------------+---------+---------+-----------+------+-------------+ |
If to create and populate tables t1,t2 with the commands
CREATE TABLE t1(a int PRIMARY KEY, b int) ENGINE=myisam;
INSERT INTO t1 VALUES
(5, 10), (2, 70), (7, 80), (6, 20), (1, 50), (9, 40), (8, 30), (3, 60);
CREATE TABLE t2 (p int, a int, INDEX i_a(a)) ENGINE=myisam;
INSERT INTO t2 VALUES
(103, 7), (109, 3), (102, 3), (108, 1), (106, 3),
(107, 7), (105, 1), (101, 3), (100, 7), (110, 1);
then Mariadb 5.5 will return the following explain output for the query
SELECT t1.a FROM t1 LEFT JOIN t2 ON t1.a=t2.a ORDER BY t1.a
MariaDB [test]> EXPLAIN
-> SELECT t1.a FROM t1 LEFT JOIN t2 ON t1.a=t2.a ORDER BY t1.a;
+------+-------------+-------+-------+---------------+---------+---------+-----------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+------+-------------+-------+-------+---------------+---------+---------+-----------+------+-------------+
| 1 | SIMPLE | t1 | index | NULL | PRIMARY | 4 | NULL | 4 | Using index |
| 1 | SIMPLE | t2 | ref | i_a | i_a | 5 | test.t1.a | 2 | Using index |
+------+-------------+-------+-------+---------------+---------+---------+-----------+------+-------------+
The expected number of the examined rows from t1 is 4 though it should be 8 as for the following query:
MariaDB [test]> explain SELECT t1.a FROM t1 LEFT JOIN t2 ON t1.a=t2.a ORDER BY t1.a LIMIT 1000;
+------+-------------+-------+-------+---------------+---------+---------+-----------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+------+-------------+-------+-------+---------------+---------+---------+-----------+------+-------------+
| 1 | SIMPLE | t1 | index | NULL | PRIMARY | 4 | NULL | 8 | Using index |
| 1 | SIMPLE | t2 | ref | i_a | i_a | 5 | test.t1.a | 2 | Using index |
+------+-------------+-------+-------+---------------+---------+---------+-----------+------+-------------+ |
|
