测试数据初始化sql
#单表 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'); #两表 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 ( book_id int(10) not null auto_increment, card int(10) unsigned not null, primary key(book_id) ); 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))); #三表 create table if not exists phone ( phone_id int(10) not null auto_increment, card int(10) unsigned not null, primary key(phone_id) )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)));
执行案列
#查询category_id 为1 且comments大于1的情况下，views最多的article_id. explain select id,author_id from article where category_id =1 and comments > 1 order by views desc limit 1; #结论：很显然，type是all 即最坏的情况，Extra中还出现了Using filesort,也是最坏的情况，优化是必须的 #开始优化 #1.1 新建索引 + 删除索引 create index idx_article_ccv on article(category_id,comments,views); #范围之后全失效--后面会说 explain select id,author_id from article where category_id =1 and comments =1 order by views desc limit 1; #1.2 第二次explain drop index idx_article_ccv on article; explain select id,author_id from article where category_id =1 and comments > 1 order by views desc limit 1; ##################### #下面开始explain 分析 explain select * from class left join book on class.card=book.card; #结论 type 有all #添加索引优化 alter table book add index Y(card); #第二次解析explain explain select * from class left join book on class.card=book.card; #可以看到第二行的type变成了ref,rows也变少了优化比较明显 #这是由左连接的特性决定的。left join 条件用于确定如何从右表搜索行，左表一定都有 #所以右边是我们的关注点，一定要建立索引 #删除旧索引+ 新建 第三次explain drop index Y on book; alter table class add index Y(card); explain select * from class left join book on class.card=book.card; #然后来看一个右连接查询： #优化比较明显，这是因为Right join条件用于确定如何从左表搜索行，右表一定都有，所以左表是我们关注点，一定要建立索引 explain select * from class right join book on class.card=book.card; drop index X on class; alter table class add index Y(card); #################### explain select * from class left join book on class.card=book.card left join phone on book.card=phone.card; alter table phone add index Z(card); alter table book add index Y(card); #后两行的type 都是ref 且中rows优化较好，效果不错。因此索引最好设置在需要经常查询的字段中。
Join语句的优化
注意: 尽可能的减少Join语句中的NestedLoop的循环总次数："永远用小结果集驱动大的结果集" 优先优化NestedLoop的内层循环 保证Join语句中被驱动的表上Join条件字段已被索引 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，不要太吝啬使用JoinBuffer的设置 优化需要优化的Query 定位优化对象性能瓶颈 从Explain入手 永远小结果集驱动大结果集# 尽可能在索引中完成排序 只取自己需要的Column 仅仅使用最有效的过滤条件 尽可能避免复杂的join和子查询
create table staffs ( id int primary key auto_increment, name varchar(24) not null default '' comment '姓名', age int not null default 0 comment '年龄', pos varchar(20) not null default '' comment '职位', add_time timestamp not null default current_timestamp comment '入职时间' )charset utf8 comment '员工记录表';insert into staffs(name,age,pos,add_time) values('wxt',22,'manager',now()); insert into staffs(name,age,pos,add_time) values('wxt2',23,'test',now()); insert into staffs(name,age,pos,add_time) values('wxt3',24,'dev',now()); insert into staffs(name,age,pos,add_time) values('wxt4',25,'dev',now()); select * from staffs; alter table staffs add index index_staff_nameAgePos(name,age,pos);
#1.全值匹配我最爱 explain select * from staffs where name="wxt3"; explain select * from staffs where name="wxt3" and age=24; explain select * from staffs where name="wxt3" and age=24 and pos='dev'; explain select * from staffs where age=24 and pos='dev'; explain select * from staffs where pos='dev'; #2.最佳左前缀法则 如果索引了多例，要遵守最左前缀法则。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。 explain select * from staffs where name="wxt3" and pos='dev'; explain select * from staffs where age=24 and pos='dev'; explain select * from staffs where pos='dev'; #3.不在索引列上做任何操作（计算、函数、（自动or手动）类型转换），会导致索引失效而转向全表扫描 explain select * from staffs where left(name,4)='wxt3'; explain select * from staffs where name='wxt3'; #4.存储引擎不能使用索引中范围条件右边的列 explain select * from staffs where name="wxt3" and age=24 and pos='dev'; explain select * from staffs where name="wxt3" and age>23 and pos='dev'; #范围之后全失效（注意key-len的变化） #5.尽量使用覆盖索引（只访问索引的查询（索引列和查询列一致）），减少select* explain select * from staffs where name="wxt3" and age=24 and pos='dev'; explain select name,age,pos from staffs where name="wxt3" and age=24 and pos='dev'; explain select name,age,pos from staffs where name="wxt3" and age >23 and pos='dev'; explain select name,age,pos from staffs where name="wxt3" and age =24; explain select name from staffs where name="wxt3" and age >23 and pos='dev'; #6.mysql在使用不等于（！=或者<>）的时候无法使用索引会导致全表扫描 explain select * from staffs where name="wxt3"; explain select * from staffs where name!="wxt3"; explain select * from staffs where name<>"wxt3"; #7.is null,is not null 也无法使用索引 explain select * from staffs where name is null; explain select * from staffs where name is not null; #8.like以通配符开头（'%abc...'）mysql索引失效会变成全表扫描操作 explain select * from staffs where name like '%wxt3%'; explain select * from staffs where name like '%wxt3'; explain select * from staffs where name like 'wxt3%'; #9.字符串不加单引号索引失效(如果列类型是字符串，那一定要在条件中将数据使用引号引用起来。否则不使用索引) explain select * from staffs where name=200; explain select * from staffs where name='200'; #10.少用or,用它连接时会索引失效 explain select * from staffs where name='wxt' or name='wxt3';
问题：解决like'%字符串%'索引不被使用的方法
1、可以使用主键索引 2、使用覆盖索引，查询字段必须是建立覆盖索引字段 3、当覆盖索引指向的字段是varchar(380)及380以上的字段时，覆盖索引会失效！ create table tbl_user( id int(11) not null auto_increment, name varchar(20) default null, age int(11) default null, email varchar(20) default null, primary key(id) )engine=innodb auto_increment=1 default charset=utf8; insert into tbl_user(name,age,email) values('1aa1',21,'aa@163.com'); insert into tbl_user(name,age,email) values('2aa2',21,'bb@163.com'); insert into tbl_user(name,age,email) values('3aa3',21,'cc@163.com'); insert into tbl_user(name,age,email) values('4aa4',21,'dd@163.com'); #before index explain select name ,age from tbl_user where name like '%aa%'; explain select id from tbl_user where name like '%aa%'; explain select name from tbl_user where name like '%aa%'; explain select age from tbl_user where name like '%aa%'; explain select id, name from tbl_user where name like '%aa%'; explain select id,name ,age from tbl_user where name like '%aa%'; explain select name ,age from tbl_user where name like '%aa%'; explain select * from tbl_user where name like '%aa%'; explain select id, name,age,email from tbl_user where name like '%aa%'; #create index 后 alter table tbl_user add index idx_user_nameAge(name,age); explain select id from tbl_user where name like '%aa%'; explain select name from tbl_user where name like '%aa%'; explain select age from tbl_user where name like '%aa%'; explain select * from tbl_user where name like '%aa%'; explain select id, name,age,email from tbl_user where name like '%aa%'; .....
总结
like KK%相当于=常量 %KK和%KK% 相当于范围 1.带头大哥不能死 2.中间兄弟不能断 3.索引列上无计算 4.like %加右边 5.范围之后全失效 6.字符串中有引号 【优化总结口诀】 全值匹配我最爱，最左前缀要遵守； 带头大哥不能死，中间兄弟不能断； 索引列上少计算，范围之后全失效； like百分写最右，覆盖索引不写星； 不等空值还有or,索引失效要减少； var引号不能丢，sql高级也不难； 小总结 假设index(a,b,c) where 语句 索引是否被使用 where a=3 Y,使用到a where a=3 and b=5 Y，使用到a,b where a=3 and b=5 and c=4 Y，使用到a,b,c where b=3 / where b=3 and c=4 或者 where c=4 N where a=3 and c=5 使用到a,但是c不可以，b中间断了 where a=3 and b>4 and c=5 使用到a和b ,c不能使用在范围之后，b断了 where a=3 and b like 'kk%' and c=4 Y,使用到a,b,c where a=3 and b like '%kk' and c=4 Y,只使用到a where a=3 and b like '%kk%' and c=4 Y,只使用到a where a=3 and b like 'k%kk%' and c=4 Y,使用到a,b,c like 结合test03表 select * from test03 where c1='a1' and c2 like 'kk%' and c3='c3';
一般性建议：
对于单键索引，尽量选择针对当前query过滤性更好的索引 在选择组合索引的时候，当前Query中过滤性最好的字段在索引字段顺序中，位置越靠前越好。 在选择组合索引的时候，尽量选择可以能包含当前query中的where子句中更多字段的索引 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
当只一行数据使用limit 1
面试题讲解
定值、范围还是排序，一般order by是给个范围 group by 基本上都需要进行排序，会有临时表产生
题目1： 【建表语句】 create table test03( id int primary key not null auto_increment, c1 char(10), c2 char(10), c3 char(10), c4 char(10), c5 char(10) ); insert into test03(c1,c2,c3,c4,c5) values('a1','a2','a3','a4','a5'); insert into test03(c1,c2,c3,c4,c5) values('b1','b2','b3','b4','b5'); insert into test03(c1,c2,c3,c4,c5) values('c1','c2','c3','c4','c5'); insert into test03(c1,c2,c3,c4,c5) values('d1','d2','d3','d4','d5'); insert into test03(c1,c2,c3,c4,c5) values('e1','e2','e3','e4','e5'); select * from test03; 【建立索引】 create index idx_test03_c1234 on test03(c1,c2,c3,c4); show index from test03; explain select * from test03 where c1='a1' ; explain select * from test03 where c1='a1' and c2='a2'; explain select * from test03 where c1='a1' and c2='a2' and c3='a3'; explain select * from test03 where c1='a1' and c2='a2' and c3='a3' and c4='a4'; 问题：我们创建符合索引idx_test03_c1234，根据以下的sql分析索引的使用情况 1) explain select * from test03 where c1='a1' and c2='a2' and c3='a3' and c4='a4'; 2) explain select * from test03 where c1='a1' and c2='a2' and c4='a4' and c3='a3'; 3) explain select * from test03 where c1='a1' and c2='a2' and c3 > 'a3' and c4='a4'; 4) explain select * from test03 where c1='a1' and c2='a2' and c4 > 'a4' and c3='a3'; 5) explain select * from test03 where c1='a1' and c2='a2' and c4 = 'a4' order by c3; #c3的作用在排序而不是查找 6) explain select * from test03 where c1='a1' and c2='a2' order by c3; 7) explain select * from test03 where c1='a1' and c2='a2' order by c4; #出现filesort 8.1) explain select * from test03 where c1='a1' and c5='a5' order by c2,c3;#只用c1一个字段索引，但是c2,c3用于排序，无filesort 8.2) explain select * from test03 where c1='a1' and c5='a5' order by c3,c2;#出现了filesort,我们建立的索引是1234，但是没有按照顺序，32颠倒了 9) explain select * from test03 where c1='a1' and c2='a2' order by c2,c3; 10) explain select * from test03 where c1='a1' and c2='a2' and c5='c5' order by c2,c3;#用c1 c2两个字段索引，但是c2,c3用于排序，无filesort explain select * from test03 where c1='a1' and c2='a2' and c5='c5' order by c3,c2; #本案例有常量c2的情况，注意和8.2的对比 explain select * from test03 where c1='a1' and c5='a5' order by c3,c2;#filesort 11) explain select * from test03 where c1='a1' and c4='a4' group by c2,c3; 12) explain select * from test03 where c1='a1' and c4='a4' group by c3,c2;#usering where ;usering filesort;usering temporary
show status like 'Handler_read%'; Handler_read_first 41 Handler_read_key 16075067 Handler_read_last 0 Handler_read_next 9534050 Handler_read_prev 0 Handler_read_rnd 411 Handler_read_rnd_next 126758972 #大家可以注意：handler_read_key:这个值越高越好，越高表示使用索引查询到的次数。#handler_read_rnd_next:这个值越高，说明查询低效。 #这时我们会看到handler_read_rnd_next值很高，这是因为我们前面没有加索引的时候，做过多次查询的原因