mysql 高级（基础四索引优化 join 案例）

索引分析：单表
建表sql

create table if not exists article(
     id INT(10) UNSIGNED NOT NULL PRIMARY KEY AUTO_INCREMENT,
     author_id INT(10) UNSIGNED NOT NULL,
     category_id INT(10) UNSIGNED NOT NULL,
     views INT(10) UNSIGNED NOT NULL,
     comments INT(10) UNSIGNED NOT NULL,
     title VARBINARY(255) NOT NULL,
     content TEXT NOT NULL
    );
INSERT INTO article (author_id , category_id,views ,comments ,title,content) VALUES
(1,1,1,1,'1','1'),
(2,2,2,2,'2','2'),
(1,1,3,3,'3','3');

查询category_id 为1 且 comments 大于1 的情况下，views 最多的article_id

select id,author_id from article where category_id =1 and comments>1 order by views desc limit 1;

explain 分析
这里写图片描述
显然，type是all ,即最坏的情况，Extra 里还出现了Using filesort,也是最坏的情况。优化是必须的。

show index from article ;

这里写图片描述

开始优化：
1.1新建索引+删除索引

ALTER TABLE article add index index_article_ccv(category_id,comments,views);
create index idx_article_ccv on article(category_id,comments,views);
DROP INDEX idx_article_ccv ON  article; //删除索引

第一次想到的是这样创建索引。

create index idx_article_ccv on article(category_id,comments,views);

但是分析三个字段，category_id ,comments ,views
按照BTree 索引建立的工作原理，
先排序category_id，
如果遇到相同的category_id 则再排序comments，如果遇到相同的comments则再排序views。
当comments 字段在联合索引中间位置时，
因为comments>1是范围值(range) ,
mysql 无法利用索引再对后面的view部分进行检索，即range类型查询字段后面的索引无效。
所以，创建索引，选择

create index idx_article_ccv on article(category_id,views);

这里写图片描述
查看添加索引后的查询效果：

type为ref extra 为Using where
That’s beautiful

两表：

create table if not exists clazz (
    id INT(10) UNSIGNED NOT NULL PRIMARY KEY AUTO_INCREMENT,
    card INT(10) UNSIGNED NOT NULL
);

create table if not exists book (
    bookid INT(10) UNSIGNED NOT NULL PRIMARY KEY AUTO_INCREMENT,
    card INT(10) UNSIGNED NOT NULL
);

insert into clazz(card) values(FLOOR(1+(RAND()*20)));
insert into clazz(card) values(FLOOR(1+(RAND()*20)));
insert into clazz(card) values(FLOOR(1+(RAND()*20)));
insert into clazz(card) values(FLOOR(1+(RAND()*20)));
insert into clazz(card) values(FLOOR(1+(RAND()*20)));
insert into clazz(card) values(FLOOR(1+(RAND()*20)));
insert into clazz(card) values(FLOOR(1+(RAND()*20)));
insert into clazz(card) values(FLOOR(1+(RAND()*20)));
insert into clazz(card) values(FLOOR(1+(RAND()*20)));
insert into clazz(card) values(FLOOR(1+(RAND()*20)));
insert into clazz(card) values(FLOOR(1+(RAND()*20)));
insert into clazz(card) values(FLOOR(1+(RAND()*20)));
insert into clazz(card) values(FLOOR(1+(RAND()*20)));
insert into clazz(card) values(FLOOR(1+(RAND()*20)));
insert into clazz(card) values(FLOOR(1+(RAND()*20)));
insert into clazz(card) values(FLOOR(1+(RAND()*20)));
insert into clazz(card) values(FLOOR(1+(RAND()*20)));
insert into clazz(card) values(FLOOR(1+(RAND()*20)));
insert into clazz(card) values(FLOOR(1+(RAND()*20)));
insert into clazz(card) values(FLOOR(1+(RAND()*20)));

insert into book (card) values(FLOOR(1+(RAND()*20)));
insert into book (card) values(FLOOR(1+(RAND()*20)));
insert into book (card) values(FLOOR(1+(RAND()*20)));
insert into book (card) values(FLOOR(1+(RAND()*20)));
insert into book (card) values(FLOOR(1+(RAND()*20)));
insert into book (card) values(FLOOR(1+(RAND()*20)));
insert into book (card) values(FLOOR(1+(RAND()*20)));
insert into book (card) values(FLOOR(1+(RAND()*20)));
insert into book (card) values(FLOOR(1+(RAND()*20)));
insert into book (card) values(FLOOR(1+(RAND()*20)));
insert into book (card) values(FLOOR(1+(RAND()*20)));
insert into book (card) values(FLOOR(1+(RAND()*20)));
insert into book (card) values(FLOOR(1+(RAND()*20)));
insert into book (card) values(FLOOR(1+(RAND()*20)));
insert into book (card) values(FLOOR(1+(RAND()*20)));
insert into book (card) values(FLOOR(1+(RAND()*20)));
insert into book (card) values(FLOOR(1+(RAND()*20)));
insert into book (card) values(FLOOR(1+(RAND()*20)));
insert into book (card) values(FLOOR(1+(RAND()*20)));
insert into book (card) values(FLOOR(1+(RAND()*20)));

如果是左连接就将索引建到右表；
如果是右连接就将索引建到左表；
事例：
这里写图片描述
可以看出来book在左索引生效，book在右不走索引；

三表：
再加一张表：

create table if not exists phone (
phoneid INT(10) UNSIGNED NOT NULL PRIMARY KEY AUTO_INCREMENT,
    card INT(10) UNSIGNED NOT NULL
);

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)));

这里写图片描述
book phone 为全表扫描
为book phone 建立索引后的结果

结论：
join语句的优化
尽可能减少join语句中的NestedLoop 的循环总次数；”永远用小结果集驱动大结果集”
优先优化NestedLoop 的内层循环：
保证join语句中被驱动表上join条件字段已经被索引；
当无法保证被驱动表的join条件字段被索引且内存资源充足的前提下，不要太吝啬joinbuffer的设置。

mysql 高级（基础四 索引优化 join 案例）

猜你喜欢

mysql 高级（基础四索引优化 join 案例）