（五）MySQL两表索引优化

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
1.对book表的card属性添加索引

alter table `book` add index Y(`card`);

2.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。

3.对class表的card属性添加索引

alter table `class` add index Y(`card`);

4.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)

转载：https://www.cnblogs.com/ronnieyuan/p/12160863.html