（六）MySQL三表索引优化

建表语句

CREATE TABLE IF NOT EXISTS `phone`(
`phoneid` INT(10) UNSIGNED NOT NULL AUTO_INCREMENT,
`card` INT(10) UNSIGNED NOT NULL,
PRIMARY KEY(`phoneid`)
)ENGINE = INNODB;

INSERT INTO phone(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO phone(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO phone(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO phone(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO phone(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO phone(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO phone(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO phone(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO phone(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO phone(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO phone(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO phone(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO phone(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO phone(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO phone(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO phone(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO phone(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO phone(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO phone(card) VALUES(FLOOR(1 + (RAND() * 20)));
INSERT INTO phone(card) VALUES(FLOOR(1 + (RAND() * 20)));

使用explain解读SQL

mysql> explain select * from class LEFT JOIN book on class.card=book.card LEFT JOIN phone on book.card = phone.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                                        |
|  1 | SIMPLE      | phone | NULL       | ALL   | NULL          | NULL | NULL    | NULL            |   20 |   100.00 | Using where; Using join buffer (Block Nested Loop) |
+----+-------------+-------+------------+-------+---------------+------+---------+-----------------+------+----------+----------------------------------------------------+
3 rows in set, 1 warning (0.00 sec)

• 可以看到phone表的type为all, Extra中显示使用了连接缓冲区区。

创建phone表的card索引

alter table `phone` add index Z(`card`);

• 再用explain查看sql语句

mysql> explain select * from class LEFT JOIN book on class.card=book.card LEFT JOIN phone on book.card = phone.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 |
|  1 | SIMPLE      | phone | NULL       | ref   | Z             | Z    | 4       | test.book.card  |    1 |   100.00 | Using index |
+----+-------------+-------+------------+-------+---------------+------+---------+-----------------+------+----------+-------------+

注意rows: 扫描的行数

• 其实可以做更好的优化:

mysql> explain select * from phone LEFT JOIN book on phone.card=book.card LEFT JOIN class on book.card = class.card;
+----+-------------+-------+------------+-------+---------------+------+---------+-----------------+------+----------+-------------+
| id | select_type | table | partitions | type  | possible_keys | key  | key_len | ref             | rows | filtered | Extra       |
+----+-------------+-------+------------+-------+---------------+------+---------+-----------------+------+----------+-------------+
|  1 | SIMPLE      | phone | NULL       | index | NULL          | Z    | 4       | NULL            |   20 |   100.00 | Using index |
|  1 | SIMPLE      | book  | NULL       | ref   | Y             | Y    | 4       | test.phone.card |    1 |   100.00 | Using index |
|  1 | SIMPLE      | class | NULL       | ref   | Y             | Y    | 4       | test.book.card  |    1 |   100.00 | Using index |
+----+-------------+-------+------------+-------+---------------+------+---------+-----------------+------+----------+-------------+
3 rows in set, 1 warning (0.00 sec)

思想: 用小表驱动大表, 能更好地减少行扫描次数。

• 结论:
  • 索引最好设置在需要经常查询的字段中
  • 尽可能减少Join语句中的NestedLoop(嵌套循环)的循环总数(优先优化NestedLoop内层循环)
  • 永远用小结果集驱动大的结果集

转载：https://www.cnblogs.com/ronnieyuan/p/12162240.html