3.查询优化

创建一张测试表并插入数据：

drop table if exists students;
CREATE TABLE students (id INT PRIMARY KEY AUTO_INCREMENT COMMENT "主键id",sname VARCHAR (24) COMMENT '学生姓名',age INT COMMENT '年龄',score INT COMMENT '分数',time TIMESTAMP COMMENT '入学时间'
);INSERT INTO students(sname,age,score,time) VALUES('小明',22,100,now());
INSERT INTO students(sname,age,score,time) VALUES('小红',23,80,now());
INSERT INTO students(sname,age,score,time) VALUES('小绿',24,80,now());
INSERT INTO students(sname,age,score,time) VALUES('黑',23,70,now());

3.1索引失效

1）最佳左前缀法则：如果索引了多列，要遵循最左前缀法则，指的是查询从索引的最左前列开始并且不跳过索引中的列。

alter table students add index idx_sname_age_score(sname,age,score);-- 索引失效情况
① explain select * from students where sname="小明" and age = 22 and score = 100;
② explain select * from students where sname="小明" and age = 22;
③ explain select * from students where sname="小明";
④ explain select * from students where sname="小明" and score = 80;

可以看到①②都遵守最佳左前缀法则，但是②的where条件没有完全覆盖包含索引的列，所以①的key_len相对较高，即索引使用率高
③遵守但是④不遵守，两者都锁定了sname，但是score使索引不再生效，所以③④的索引使用率一样

2）不在索引列上做任何计算、函数操作，会导致索引失效而转向全表扫描。

-- 不在索引列上做任何计算、函数操作，会导致索引失效而转向全表扫描。
explain select * from students where left(sname,2) = "小明";

3）存储引擎不能使用索引中范围条件右边的列。

-- 存储引擎不能使用索引中范围条件右边的列。
explain select * from students where sname="小明" and age > 22 and score = 100;

4）Mysql在使用不等于时无法使用索引会导致全表扫描。

-- Mysql在使用不等于时无法使用索引会导致全表扫描。
explain select * from students where sname!="小明";

5）is null可以使用索引，但是is not null无法使用索引。

-- is null可以使用索引，但是is not null无法使用索引。
explain select * from students where sname is not null;

6）like以通配符开头会使索引失效导致全表扫描。

-- like以通配符开头会使索引失效导致全表扫描。
explain select * from students where sname like "%黑%";

7）字符串不加单引号索引会失效。

-- 字符串不加单引号索引会失效。
explain select * from students where sname = 123;

8）使用or连接时索引失效。

-- 使用or连接时索引失效。
explain select * from students where sname="小明" or age = 22;

建议：
1.对于单值索引，尽量选择针对当前查询字段过滤性更好的索引。
2.对于组合索引，当前where查询中过滤性更好的字段在索引字段顺序中位置越靠前越好。
3.对于组合索引，尽量选择能够包含在当前查询中where子句中更多字段的索引。
4.尽可能通过分析统计信息和调整query的写法来达到选择合适索引的目的。

3.2单表查询优化

例子：

-- 单表查询优化
CREATE TABLE IF NOT EXISTS article (
id INT(10) PRIMARY KEY AUTO_INCREMENT,
author_id INT(10) NOT NULL,
category_id INT(10) NOT NULL,
views INT(10) NOT NULL,
comments INT(10) 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');#1.查询category_id为1的，且comments大于1的情况下，views最多的id和author_id的信息
explain select id,author_id 
from article
where category_id=1 and comments>1 order by views desc limit 1;
#2.建立索引
alter table article add index idx_ccv(category_id,comments,views);
#3.再次测试
explain select id,author_id 
from article
where category_id=1 and comments>1 order by views desc limit 1;
#4.重新创建索引
drop index idx_ccv on article;
alter table article add index idx_cv(category_id,views);
#5.再次测试
explain select id,author_id 
from article
where category_id=1 and comments>1 order by views desc limit 1;

3.3关联查询优化

例子：

CREATE TABLE IF NOT EXISTS class (
id INT(10) AUTO_INCREMENT,
card INT(10),
PRIMARY KEY (id)
);
CREATE TABLE IF NOT EXISTS book (
bookid INT(10) AUTO_INCREMENT,
card INT(10),
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 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)));#1.联表查询
explain select *
from class
left join book
on class.card = book.card;
#2.建立索引
alter table book add index idx_card(card);
#3.测试
explain select *
from class
left join book
on class.card = book.card;

内连接时，mysql会自动把小结果集的选为驱动表，所以大表的字段最好加上索引。左外连接时，左表会全表扫描，所以右边大表字段最好加上索引，右外连接同理。我们最好保证被驱动表上的字段建立了索引。

3.4排序优化

1.尽量避免使用Using FileSort方式排序。
2.order by语句使用索引最左前列或使用where子句与order by子句条件组合满足索引最左前列。
3.where子句中如果出现索引范围查询会导致order by索引失效。

3.5分组优化

drop table if exists students;
CREATE TABLE students (id INT PRIMARY KEY AUTO_INCREMENT COMMENT "主键id",sname VARCHAR (24) COMMENT '学生姓名',age INT COMMENT '年龄',score INT COMMENT '分数',time TIMESTAMP COMMENT '入学时间'
);INSERT INTO students(sname,age,score,time) VALUES('小明',22,100,now());
INSERT INTO students(sname,age,score,time) VALUES('小红',23,80,now());
INSERT INTO students(sname,age,score,time) VALUES('小绿',24,80,now());
INSERT INTO students(sname,age,score,time) VALUES('黑',23,70,now());-- 分组优化
alter table students add index idx_sas(sname,age,score);
explain select count(*),sname 
from students 
where sname="小明" and age > 22
GROUP BY score;