MySQL優化索引之三表優化

create table if not exists `phone`(

`phoneid` int(10) unsigned not null primary key auto_increment,
`card` int (10) unsigned not null
)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)));

drop index y on class;

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

解決方案是給後兩個表的字段加索引，永遠記住小表驅動大表

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

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

可以看到效能提公升非常明顯 

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