建表語句
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對sql進行檢查
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 | ****** | class | null | all | null | null | null | null | 24 | 100.00 | null |
| 1 | ****** | 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都為all(執行全表查詢)
使用了join緩衝區(其使用的演算法為block nested-loop(bnl))
新增索引
1.對book表的card屬性新增索引
alter table `book` add index y(`card`);
2.再次用explain檢視之前的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 | ****** | class | null | all | null | null | null | null | 24 | 100.00 | null |
| 1 | ****** | 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優化為了type, 且extra變為了using index, 不會再使用緩衝區了。
3.對class表的card屬性新增索引
alter table `class` add index y(`card`);
4.再次用explain檢視之前的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 | ****** | 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 |
2 rows in set, 1 warning (0.00 sec)
可以看到type變為了index, ref 而 extra 都變為了using index
可以得出的結論
左連線時, 對右表建索引進行優化更重要(左表都保留, 必然全表掃瞄)
右連線時, 對左表建索引進行優化更重要(右表都保留, 必然全表掃瞄)
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