建表语句
CREATE TABLE IF NOT EXISTS `class`(
`id` INT(10) UNSIGNED NOT NULL AUTO_INCREMENT,
`card` INT(10) UNSIGNED NOT NULL,
PRIMARY KEY(`id`)
);
CREATE TABLE IF NOT EXISTS `book`(
`bookid` INT(10) UNSIGNED NOT NULL AUTO_INCREMENT,
`card` INT(10) UNSIGNED NOT NULL,
PRIMARY KEY(`bookid`)
);
INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO class(card) VALUES(FLOOR(1 + (RAND() * 20)));
使用explain对sql进行检查
mysql> explain SELECT * FROM class LEFT JOIN book ON class.card = book.card;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------------------------------------------+
| 1 | SIMPLE | class | NULL | ALL | NULL | NULL | NULL | NULL | 24 | 100.00 | NULL |
| 1 | SIMPLE | book | NULL | ALL | NULL | NULL | NULL | NULL | 1 | 100.00 | Using where; Using join buffer (Block Nested Loop) |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------------------------------------------+
2 rows in set, 1 warning (0.00 sec)
- 可以看到type都为all(执行全表查询)
- 使用了join缓冲区(其使用的算法为Block Nested-Loop(BNL))
添加索引
- 对book表的card属性添加索引
alter table `book` add index Y(`card`);
再次用explain查看之前的sql
mysql> explain SELECT * FROM class LEFT JOIN book ON class.card = book.card;
+----+-------------+-------+------------+------+---------------+------+---------+-----------------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+-----------------+------+----------+-------------+
| 1 | SIMPLE | class | NULL | ALL | NULL | NULL | NULL | NULL | 24 | 100.00 | NULL |
| 1 | SIMPLE | book | NULL | ref | Y | Y | 4 | test.class.card | 1 | 100.00 | Using index |
+----+-------------+-------+------------+------+---------------+------+---------+-----------------+------+----------+-------------+
2 rows in set, 1 warning (0.00 sec)
- 可以看到book表的type优化为了type, 且extra变为了using index, 不会再使用缓冲区了。
对class表的card属性添加索引
alter table `class` add index Y(`card`);
再次用explain查看之前的sql
mysql> explain SELECT * FROM class LEFT JOIN book ON class.card = book.card;
+----+-------------+-------+------------+-------+---------------+------+---------+-----------------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+------+---------+-----------------+------+----------+-------------+
| 1 | SIMPLE | class | NULL | index | NULL | Y | 4 | NULL | 24 | 100.00 | Using index |
| 1 | SIMPLE | book | NULL | ref | Y | Y | 4 | test.class.card | 1 | 100.00 | Using index |
+----+-------------+-------+------------+-------+---------------+------+---------+-----------------+------+----------+-------------+
2 rows in set, 1 warning (0.00 sec)- 可以看到type变为了index, ref 而 extra 都变为了Using index
可以得出的结论
- 左连接时, 对右表建索引进行优化更重要(左表都保留, 必然全表扫描)
- 右连接时, 对左表建索引进行优化更重要(右表都保留, 必然全表扫描)