mysql三表關聯加索引 mysql 三表索引優化

建表語句

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)));

使用explain解讀sql

mysql> explain select * from class left join book on class.card=book.card left join phone on book.card = phone.card;

| 1 | ****** | class | null | index | null | y | 4 | null | 24 | 100.00 | using index |

| 1 | ****** | book | null | ref | y | y | 4 | test.class.card | 1 | 100.00 | using index |

| 1 | ****** | 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為all, extra中顯示使用了連線緩衝區區。

建立phone表的card索引

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

再用explain檢視sql語句 mysql> explain select * from class left join book on class.card=book.card left join phone on book.card = phone.card;

| 1 | ****** | class | null | index | null | y | 4 | null | 24 | 100.00 | using index |

| 1 | ****** | book | null | ref | y | y | 4 | test.class.card | 1 | 100.00 | using index |

| 1 | ****** | phone | null | ref | z | z | 4 | test.book.card | 1 | 100.00 | using index |

注意rows: 掃瞄的行數

其實可以做更好的優化: mysql> explain select * from phone left join book on phone.card=book.card left join class on book.card = class.card;

| 1 | ****** | phone | null | index | null | z | 4 | null | 20 | 100.00 | using index |

| 1 | ****** | book | null | ref | y | y | 4 | test.phone.card | 1 | 100.00 | using index |

| 1 | ****** | 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: 用小表驅動大表, 能更好地減少行掃瞄次數。

結論:索引最好設定在需要經常查詢的字段中

盡可能減少join語句中的nestedloop的迴圈總數

永遠用小結果集驅動大的結果集