MySql索引優化案例

-- 建立新錶

create table article(
id int unsigned not null primary key auto_increment,
author_id int unsigned not null,
category_id int unsigned not null,
views int unsigned not null,
comments int unsigned not null,
title varchar(255) not null,
content text not null
)character set utf8 collate utf8_general_ci;
--插入資料
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');
--查詢category_id為1且comments大於1的情況下,views最多的article_id
explain select * from article where category_id=1 and comments>1 order by views desc limit 1;

type 為 all，且出現檔案內排序(using filesort),也沒有索引(沒建)

-- 建立索引

create index idx_article_views on article(`views`);
-- 再分析
explain select * from article where category_id=1 and comments>1 order by views desc limit 1;
*************************** 1. row ***************************
id: 1
select_type: ******
table: article
partitions: null
type: index
possible_keys: null
key: idx_article_views
key_len: 4
ref: null
rows: 1
filtered: 33.33
extra: using where; backward index scan
1 row in set, 1 warning (0.00 sec)
-- 刪除索引
drop index idx_article_views on article;

-- 建表

create table class(
id int unsigned not null primary key auto_increment,
card int unsigned not null
)character set utf8 collate utf8_general_ci;
create table book(
bookid int unsigned not null auto_increment primary key,
card int unsigned not null
)character set utf8 collate utf8_general_ci;
-- 向兩個表中插入隨機資料，各插入20行資料
insert into class(card) values(round(rand()*100));
insert into book(`card`) values(round(rand()*100));
explain select * from class inner join book on class.card=book.card;
*************************** 1. row ***************************
id: 1
select_type: ******
table: class
partitions: null
type: all
possible_keys: null
key: null
key_len: null
ref: null
rows: 20
filtered: 100.00
extra: null
*************************** 2. row ***************************
id: 1
select_type: ******
table: book
partitions: null
type: all
possible_keys: null
key: null
key_len: null
ref: null
rows: 20
filtered: 10.00
extra: using where; using join buffer (block nested loop)

round()表示向下取證

-- 建立索引

create index idx_cc on class(card);
explain select * from book left join class on class.card=book.card\g
*************************** 1. row ***************************
id: 1
select_type: ******
table: book
partitions: null
type: all
possible_keys: null
key: null
key_len: null
ref: null
rows: 20
filtered: 100.00
extra: null
*************************** 2. row ***************************
id: 1
select_type: ******
table: class
partitions: null
type: ref
possible_keys: idx_cc
key: idx_cc
key_len: 4
ref: demo.book.card
rows: 1
filtered: 100.00
extra: using index
2 rows in set, 1 warning (0.00 sec)

驅動表的概念，mysql中inner join指定了連線條件時，滿足查詢條件的記錄行數少的表為驅動表；如未指定查詢條件，則掃瞄行數少的為驅動表。

left join 寫在左邊的是驅動表，join後的是被驅動表

三個表的關聯與雙表是類似的，以小表驅動大表

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