1. 索引的作用

类似于一本书的目录，起到优化查询的功能。

2. 索引类型(笔试)

BTREE(树)索引.
RTREE(树)索引
HASH索引
全文索引

3. BTREE的细分类(算法)

B-TREE
B+TREE
B*TREE

新新

4. Btree索引的功能分类

辅助索引(二级索引)
聚集索引(集群索引) Cluster Index
覆盖索引
前缀索引

5. B树是如何构建的

辅助索引 alter table t1 add index idx_name(name);

1. 将name列的所有值取出，进行自动排序
2. 将拍完序的值均匀的落到16KB叶子节点页中，并将索引键值所对应的数据行的聚集索引列值
3. 向上生成枝节点和根节点

聚集索引(IOT)

1. 默认安装主键生成聚集索引，没有主键，存储引擎会使用唯一键，如果都没有，会自动生成一个隐式的聚集索引。
2. 数据在存储时，就会按照聚集索引的顺序存储到磁盘的数据页
3. 由于本身数据时有序的，所以在聚集索引构建时，不需要进行排序。
4. 聚集索引直接将磁盘的数据页作为叶子节点。
5. 枝节点和根节点只会调取下层节点主键的最小值

辅助索引和聚集索引的区别：

(1).辅助索引

• 叶子节点只保存主键值+索引键值的有序存储
• 对索引键值会自动排序
• 需要手工创建
• 辅助索引可以有多个
• 任何列都可以创建

(2).聚集索引

• 只能在主键列生成，唯一且非空
• 数据存储时，就是按照聚集索引顺序进行有序存储

• 叶子节点不需要单独生成

  
  新新

6.辅助索引细分

(1) 单列辅助索引
select * from t1 where name=''
(2) 联合索引
select * from t1 where a and b and c
(3) 唯一索引
(4) 前缀索引

7. 索引树的高度(越低越好)

(1) 表的数据量级大
分区表(比较旧的方案)
分库分表(分布式架构)
(2) 索引键值的长度
① 尽可能选择列值短的的创建索引
② 采用前缀索引
(3) 数据类型的选择
varchar和char的选择
enum
(4) 聚集索引列的长度

8. 索引的管理

① 压力测试准备

create database test charset utf8mb4;
use test;
create table t100w (id int,num int,k1 char(2),k2 char(4),dt timestamp);
delimiter //
create  procedure rand_data(in num int)
begin
declare str char(62) default 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789';
declare str2 char(2);
declare str4 char(4);
declare i int default 0;
while i<num do
set str2=concat(substring(str,1+floor(rand()*61),1),substring(str,1+floor(rand()*61),1));
set str4=concat(substring(str,1+floor(rand()*61),2),substring(str,1+floor(rand()*61),2));
set i=i+1;
insert into t100w values (i,floor(rand()*num),str2,str4,now());
end while;
end;
//
delimiter ;
>>插入100w条数据：
call rand_data(1000000);
commit;

oldguo[test]>select count(*) from test.t100w; (查看完成率)

--- 压力测试命令
mysqlslap --defaults-file=/etc/my.cnf --concurrency=100 --iterations=1 --create-schema='test' --query="select * from test.t100w where k2='VWtu'" engine=innodb --number-of-queries=2000 -uroot -pwangxin -verbose
<查看进程列表>
show processlist;

② 索引命令操作

--- 查询索引
oldguo[school]>use school
oldguo[school]>desc student;
Key：PRI（主键），UNI（唯一索引），MUL（辅助索引）
show index from student\G

--- 创建索引
<创建单个索引>
alter table student add index idx_name(sname);
<创建联合索引>
alter table student add index idx_sname_sage_ssex(sname,sage,ssex);
idx_a_b_c ------> idx_a   +   idx_a_b   +   idx_a_b_c
<创建前缀索引(防止索引过长,取前五个字符创建索引)>
alter table student add index idx(sname(5));
<创建唯一索引>
alter table student add telnum char(11) not null;
alter table student add unique index idx_tell(telnum);
<删除索引>
alter table student drop index idx;
alter table student drop index idx_name;
alter table student add unique index idx_tell(telnum);

--- 查询索引
show index from student;

9. explain(desc)索引查询

explain select * from test.t100w where k2='VWtu';
>>或者
desc select * from test.t100w where k2='VWtu';

作用：抓取优化器优化过的执行计划

(1). 执行计划的分析

mysql[test]>explain select * from test.t100w where k2='VWtu';
+----+-------------+-------+------------+------+---------------+--------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key    | key_len | ref   | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+--------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | t100w | NULL       | ref  | idx_k2        | idx_k2 | 17      | const | 1116 |   100.00 | NULL  |
+----+-------------+-------+------------+------+---------------+--------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)
table   ：          以上SQL语句涉及到的表  ***
type    ：          查询的类型(全表扫描，索引扫描，查不到数据)  *****
possible_keys：     可能会用到的索引  ***
key     ：          使用到的索引  ****
key_len ：          索引的覆盖长度  *****
extra   ：          额外的信息  ****
--- 注：五星重点关注，四星熟悉，三星要清楚

新新

(2). type 详细说明
ALL ：全表扫描，不会走任何索引

• ① 查询条件，没建索引

mysql[test]>explain select * from test.t100w where k2='VWtu';
mysql[test]>desc t100w;
+-------+-----------+------+-----+-------------------+-----------------------------+
| Field | Type      | Null | Key | Default           | Extra                       |
+-------+-----------+------+-----+-------------------+-----------------------------+
| id    | int(11)   | YES  |     | NULL              |                             |
| num   | int(11)   | YES  |     | NULL              |                             |
| k1    | char(2)   | YES  |     | NULL              |                             |
| k2    | char(4)   | YES  | MUL | NULL              |                             |
| dt    | timestamp | NO   |     | CURRENT_TIMESTAMP | on update CURRENT_TIMESTAMP |
+-------+-----------+------+-----+-------------------+-----------------------------+
5 rows in set (0.01 sec)

• ② 有索引不走

mysql[test]>desc select * from t100w where k2 !='asdf';
+----+-------------+-------+------------+------+---------------+------+---------+------+--------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows   | filtered | Extra       |
+----+-------------+-------+------------+------+---------------+------+---------+------+--------+----------+-------------+
|  1 | SIMPLE      | t100w | NULL       | ALL  | idx_k2        | NULL | NULL    | NULL | 997537 |    88.45 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+--------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

>>避免出现的语句(有索引不走)：
desc select * from t100w where k2 like 'aa%';
desc select * from t100w where k2 not in ('asda','asas')
desc select * from t100w;
<注意：!=和not in如果是主键，则走range>

index：全索引扫描

mysql[test]>desc select k2 from t100w;
+----+-------------+-------+------------+-------+---------------+--------+---------+------+--------+----------+-------------+
| id | select_type | table | partitions | type  | possible_keys | key    | key_len | ref  | rows   | filtered | Extra       |
+----+-------------+-------+------------+-------+---------------+--------+---------+------+--------+----------+-------------+
|  1 | SIMPLE      | t100w | NULL       | index | NULL          | idx_k2 | 17      | NULL | 997537 |   100.00 | Using index |
+----+-------------+-------+------------+-------+---------------+--------+---------+------+--------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

从range开始，索引才是有价值的

range：索引范围查询

>>所有索引：
>>辅助索引
> , < , >= , <= , like , between and 
--- 示例：
mysql[world]>desc select * from city where id<10;
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type  | possible_keys | key     | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | city  | NULL       | range | PRIMARY       | PRIMARY | 4       | NULL |    9 |   100.00 | Using where |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

--- 示例：
mysql[world]>desc select * from city where countrycode like 'CH%';
+----+-------------+-------+------------+-------+---------------+-------------+---------+------+------+----------+-----------------------+
| id | select_type | table | partitions | type  | possible_keys | key         | key_len | ref  | rows | filtered | Extra                 |
+----+-------------+-------+------------+-------+---------------+-------------+---------+------+------+----------+-----------------------+
|  1 | SIMPLE      | city  | NULL       | range | CountryCode   | CountryCode | 3       | NULL |  397 |   100.00 | Using index condition |
+----+-------------+-------+------------+-------+---------------+-------------+---------+------+------+----------+-----------------------+
1 row in set, 1 warning (0.34 sec)



in() , or
--- 示例：
mysql[world]>desc select * from city where countrycode in ('CHN','USA');
+----+-------------+-------+------------+-------+---------------+-------------+---------+------+------+----------+-----------------------+
| id | select_type | table | partitions | type  | possible_keys | key         | key_len | ref  | rows | filtered | Extra                 |
+----+-------------+-------+------------+-------+---------------+-------------+---------+------+------+----------+-----------------------+
|  1 | SIMPLE      | city  | NULL       | range | CountryCode   | CountryCode | 3       | NULL |  637 |   100.00 | Using index condition |
+----+-------------+-------+------------+-------+---------------+-------------+---------+------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)

<说明：
B+tree 索引能额外优化到: > , < , >= , <= , like , between and  
in 和 or 享受不到b+tree额外的优化效果的,所以我一般情况会将in , or 进行改性
>



>>聚集索引：
!=  , not in
--- 示例：
mysql[world]>desc select * from world.city where id != 10;
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type  | possible_keys | key     | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | city  | NULL       | range | PRIMARY       | PRIMARY | 4       | NULL | 2103 |   100.00 | Using where |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.34 sec)

--- 示例：
mysql[world]>desc select * from world.city where id not in (10,20);
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type  | possible_keys | key     | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | city  | NULL       | range | PRIMARY       | PRIMARY | 4       | NULL | 2112 |   100.00 | Using where |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

ref：辅助索引等值查询

desc select * from city where countrycode = 'CHN';

eq_ref：多表连接查询中，非驱动表on的列条件列是主键或者唯一键

--- 示例：
mysql[world]>mysql[world]>desc select a.name,b.name from city as a join country as b on a.countrycode=b.code where a.population<100;
+----+-------------+-------+------------+--------+---------------+---------+---------+---------------------+------+----------+-------------+
| id | select_type | table | partitions | type   | possible_keys | key     | key_len | ref                 | rows | filtered | Extra       |
+----+-------------+-------+------------+--------+---------------+---------+---------+---------------------+------+----------+-------------+
|  1 | SIMPLE      | a     | NULL       | ALL    | CountryCode   | NULL    | NULL    | NULL                | 4188 |    33.33 | Using where |
|  1 | SIMPLE      | b     | NULL       | eq_ref | PRIMARY       | PRIMARY | 3       | world.a.CountryCode |    1 |   100.00 | NULL        |
+----+-------------+-------+------------+--------+---------------+---------+---------+---------------------+------+----------+-------------+
2 rows in set, 1 warning (0.00 sec)

const(system)：主键或唯一键的等值查询

mysql[world]>desc select * from city where id=10;
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type  | possible_keys | key     | key_len | ref   | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | city  | NULL       | const | PRIMARY       | PRIMARY | 4       | const |    1 |   100.00 | NULL  |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.01 sec)

NULL：获取不到数据(效果最好)

mysql[world]>desc select * from city where id=1000000000000000;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+--------------------------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra                          |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+--------------------------------+
|  1 | SIMPLE      | NULL  | NULL       | NULL | NULL          | NULL | NULL    | NULL | NULL |     NULL | no matching row in const table |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+--------------------------------+
1 row in set, 1 warning (0.00 sec)

(3). possible_keys：可能会用到的索引
NULL：没有和查询条件匹配的索引条目
有值： 有查询条件匹配的索引条目，但是没走，大部分原因是语句查询方式不合符索引应用条件
(4). key：使用的索引
最终使用的索引，可以帮助我们判断是否走了合适的索引
(5). key_len：索引的覆盖长度
在联合索引应用的判断时，会经常看
对于单列索引：

字符类型	长度	not null	没有指定not null
int	4个字节	4	4+1
tinyint	1个字节	1	2
utf8mb4	一个字符最大是4个字节	<==	<==
char(2)	^	2*4	2*4+1
varchar(2)	^	2*4+2	2*4+2+1

---

说明：

1. 有非空约束时，key_length就是最大字节长度
2. 在没有非空约束时：字符最大长度+1
3. varchar类型，需要额外在最大字符长度+2(存储字符长度的长度占位)

---

（1）最完美的查询情况
结论：当查询条件中，包含了索引列中的所有条件，并且都是等值的查询，name无关排列顺序，都可以走全联合索引优化。
（原因：优化器会自动调整顺序，来达到最佳的优化效果）
所以我们重点关注的是联合索引的建立顺序，从左到右，唯一值越多的列放在最左边。
（2）查询条件中，哪些因素会key_len长度
--- 按照索引的建立顺序，在查询条件中，少了任意一个中间列，后续的都无法走索引。
--- 在条件查询中间，出现不等值查询时，从不等值列开始后续列都无法使用联合索引。
优化方法：将不等值列放在最后。
（3）如果有多字句的条件查询(必须是联合索引)
按照子句的执行顺序，建立索引。

(6). Extra：额外的信息
Using filesort 额外的文件排序 在group by，order by，distinct等。
一般优化的方法，和where条件的列建立联合索引。

小结补充

如何判断某一列中是否有重复值

select name,count(id) from city group by name having count(id)>1 order by count(id) desc;

如果统计除了重复的行数

select count(distinct name) from city;