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MySQL索引介绍

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2020-06-14

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MySQL索引介绍
目录

一、MySQL索引介绍

1.1.1 MySQL索引介绍

  1. 什么是MySQL索引？

索引类似大学图书馆建书目索引，可以提高数据检索的效率，降低数据库的IO成本。MySQL在300万条记录左右性能开始逐渐下降，虽然官方文档说500~800w记录，所以大数据量建立索引是非常有必要的。MySQL提供了Explain，用于显示SQL执行的详细信息，可以进行索引的优化。MySQL官方对索引的定义为：索引(Index)是帮助MySQL高效获取数据的数据结构。我们以简单理解为：快速查找排好序的一种数据结构。Mysql索引主要有两种结构：B+Tree引和Hash索引。我们平常所说的索引，如果没有特别指明，一般都是指B树结构组织的索引(B+Tree索引)。
MySQL索引介绍

  1. 索引的种类（算法）
1、B树索引   
 * B-tree 
 * B+tree
 * B*Tree
2、Hash索引
3、R树
4、Full text
5、GIS

3.辅助索引(S索引)怎么构建B树结构的?

(1). 辅助索引是基于表中,列(索引键)的值生成的B树结构
(2). 首先提取此列所有的值,进行自动排序
(3). 将排好序的值,将所有的键值按顺序落到BTree索引的叶子节点上，均匀的分布到索引树的叶子节点中(16K)
(4). 然后生成此索引键值所对应得后端数据页的指针
(5). 生成枝节点和根节点,根据数据量级和索引键长度,生成合适的索引树高度
(6)叶子节点除了存储键值之外，还存储了相邻叶子节点的指针，另外还会保存原表数据的指针
id  name  age  gender
select  *  from  t1 where id=10;
问题: 基于索引键做where查询,对于id列是顺序IO,但是对于其他列的查询,可能是随机IO.（聚集索引解决此问题）
  1. 聚集索引(C)怎么构建B树结构的?
1、前提条件
(1)建表时有主键列（ID）,主键列就会自动被作为聚集索引.
(2)如果没有主键,会选择唯一键作为聚集索引.
(3)聚集索引必须在建表时才有意义,一般是表的无关列(ID)

2、怎么构建B树结构的?
(1)在建表时,设置了主键列(ID)
(2) 表中进行数据存储，会按照ID列的顺序，有序的存储一行一行的数据到数据页上(这个动作叫做聚集索引组织表)
(3) 将排好序的整行数据,生成叶子节点.可以理解为,磁盘的数据页就是叶子节点,表的数据页被作为聚集索引的叶子节点
(4) 把叶子节点的主键值生成上层枝节点和根节点
  1. 聚集索引和辅助索引构成区别总结
(1)聚集索引只能有一个,非空唯一,一般时主键
(2)辅助索引,可以有多个,是配合聚集索引使用的
(3)聚集索引叶子节点,就是磁盘的数据行存储的数据页
(4)MySQL是根据聚集索引,组织存储数据,数据存储时就是按照聚集索引的顺序进行存储数据
(5)辅助索引,只会提取索引键值,进行自动排序生成B树结构
  1. 辅助索引的细分
(1)单列的辅助索引
(2)联合多列辅助索引(覆盖索引)
(3)唯一索引

7.关于索引树的高度受什么影响？

(1)数据行多， 分表
(2)索引列字符长度 ，前缀索引
(3)char varchar  ，表设计 
(4)enum 优化索引高度，能用则用。

二、索引的管理

2.1.1 索引的管理

  1. 添加索引
mysql> use test
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Database changed

mysql> alter table t100w add index idx_k2(k2);
Query OK, 0 rows affected (3.84 sec)
Records: 0  Duplicates: 0  Warnings: 0
#注意建索引的时候会出现短暂的锁表，所以在创建索引的时候要考虑在不忙的时候创建。

#查看索引
mysql> show index from t100w\G;
*************************** 1. row ***************************
        Table: t100w
   Non_unique: 1
     Key_name: idx_k2
 Seq_in_index: 1
  Column_name: k2
    Collation: A
  Cardinality: 1222
     Sub_part: NULL
       Packed: NULL
         Null: YES
   Index_type: BTREE
      Comment: 
Index_comment: 
1 row in set (0.00 sec)

mysql> 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 (0.00 sec)
  1. 创建唯一索引
#检查是否有重复项
mysql> select count(distinct(k1)) from t100w;
+---------------------+
| count(distinct(k1)) |
+---------------------+
|                1225 |
+---------------------+
1 row in set (1.02 sec)
#有重复项不能创建唯一索引
mysql> select k1,count(k1) from t100w group by k1 having count(k1)>1 limit 10;
+------+-----------+
| k1   | count(k1) |
+------+-----------+
| 00   |       258 |
| 01   |       268 |
| 02   |       243 |
| 03   |       258 |
| 04   |       299 |
| 05   |       257 |
| 06   |       275 |
| 07   |       244 |
| 08   |       280 |
| 0a   |       562 |
+------+-----------+
10 rows in set (0.77 sec)
mysql> alter table t100w add unique index ide_k1(k1);
ERROR 1062 (23000): Duplicate entry ‘XE‘ for key ‘ide_k1‘
#创建报错

#正确创建
mysql> alter table t100w add unique index ide_id(id);
Query OK, 0 rows affected (2.14 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc t100w;
+-------+-----------+------+-----+-------------------+-----------------------------+
| Field | Type      | Null | Key | Default           | Extra                       |
+-------+-----------+------+-----+-------------------+-----------------------------+
| id    | int(11)   | YES  | UNI | 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 (0.00 sec)

mysql> show index from t100w;
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| t100w |          0 | ide_id   |            1 | id          | A         |      997335 |     NULL | NULL   | YES  | BTREE      |         |               |
| t100w |          1 | idx_k2   |            1 | k2          | A         |        1222 |     NULL | NULL   | YES  | BTREE      |         |               |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
2 rows in set (0.00 sec)
  1. 创建前缀索引
#前缀索引只能用到字符串类型的数据上
mysql> use world
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Database changed

mysql> desc city;
+-------------+----------+------+-----+---------+----------------+
| Field       | Type     | Null | Key | Default | Extra          |
+-------------+----------+------+-----+---------+----------------+
| ID          | int(11)  | NO   | PRI | NULL    | auto_increment |
| Name        | char(35) | NO   |     |         |                |
| CountryCode | char(3)  | NO   | MUL |         |                |
| District    | char(20) | NO   |     |         |                |
| Population  | int(11)  | NO   |     | 0       |                |
+-------------+----------+------+-----+---------+----------------+
5 rows in set (0.00 sec)

mysql> alter table city add index idx_name(name(5)); #给name列的前5个字符加索引。从左到右（就是前缀索引）
Query OK, 0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc city;
+-------------+----------+------+-----+---------+----------------+
| Field       | Type     | Null | Key | Default | Extra          |
+-------------+----------+------+-----+---------+----------------+
| ID          | int(11)  | NO   | PRI | NULL    | auto_increment |
| Name        | char(35) | NO   | MUL |         |                |
| CountryCode | char(3)  | NO   | MUL |         |                |
| District    | char(20) | NO   |     |         |                |
| Population  | int(11)  | NO   |     | 0       |                |
+-------------+----------+------+-----+---------+----------------+
5 rows in set (0.00 sec)

mysql> show index from city;
+-------+------------+-------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name    | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+-------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| city  |          0 | PRIMARY     |            1 | ID          | A         |        4188 |     NULL | NULL   |      | BTREE      |         |               |
| city  |          1 | CountryCode |            1 | CountryCode | A         |         232 |     NULL | NULL   |      | BTREE      |         |               |
| city  |          1 | idx_name    |            1 | Name        | A         |        3554 |        5 | NULL   |      | BTREE      |         |               |
+-------+------------+-------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
3 rows in set (0.00 sec)
  1. 创建联合索引
mysql> alter table city add index idx_co_po(countrycode,population);
Query OK, 0 rows affected (0.02 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc city;
+-------------+----------+------+-----+---------+----------------+
| Field       | Type     | Null | Key | Default | Extra          |
+-------------+----------+------+-----+---------+----------------+
| ID          | int(11)  | NO   | PRI | NULL    | auto_increment |
| Name        | char(35) | NO   | MUL |         |                |
| CountryCode | char(3)  | NO   | MUL |         |                |
| District    | char(20) | NO   |     |         |                |
| Population  | int(11)  | NO   |     | 0       |                |
+-------------+----------+------+-----+---------+----------------+
5 rows in set (0.00 sec)

mysql> show index from city;
+-------+------------+-------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name    | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+-------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| city  |          0 | PRIMARY     |            1 | ID          | A         |        4188 |     NULL | NULL   |      | BTREE      |         |               |
| city  |          1 | CountryCode |            1 | CountryCode | A         |         232 |     NULL | NULL   |      | BTREE      |         |               |
| city  |          1 | idx_name    |            1 | Name        | A         |        3554 |        5 | NULL   |      | BTREE      |         |               |
| city  |          1 | idx_co_po   |            1 | CountryCode | A         |         232 |     NULL | NULL   |      | BTREE      |         |               |
| city  |          1 | idx_co_po   |            2 | Population  | A         |        4052 |     NULL | NULL   |      | BTREE      |         |               |
+-------+------------+-------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
5 rows in set (0.00 sec)
  1. 删除索引
mysql> alter table city drop index idx_co_po;
Query OK, 0 rows affected (0.00 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> show index from city;
+-------+------------+-------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name    | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+-------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| city  |          0 | PRIMARY     |            1 | ID          | A         |        4188 |     NULL | NULL   |      | BTREE      |         |               |
| city  |          1 | CountryCode |            1 | CountryCode | A         |         232 |     NULL | NULL   |      | BTREE      |         |               |
| city  |          1 | idx_name    |            1 | Name        | A         |        3554 |        5 | NULL   |      | BTREE      |         |               |
+-------+------------+-------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
3 rows in set (0.00 sec)
  1. 执行计划
#作用
上线新的查询语句之前，进行提前预估语句的性能在出现性能问题时，找到合理的解决思路

#获取执行计划
第一种方法：
mysql> desc  select * from test.t100w where k2=‘EF12‘;
+----+-------------+-------+------------+------+---------------+--------+---------+-------+------+----------+-------+
| 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 |  553 |   100.00 | NULL  |
+----+-------------+-------+------------+------+---------------+--------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

#第二种方法
mysql> explain  select * from test.t100w where k2=‘EF12‘;
+----+-------------+-------+------------+------+---------------+--------+---------+-------+------+----------+-------+
| 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 |  553 |   100.00 | NULL  |
+----+-------------+-------+------------+------+---------------+--------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

#说明
table: t100w           表
type: ref              索引的应用级别
possible_keys: idx_k2  可能会使用到的索引  
key: idx_k2			   实际上使用的索引
key_len: 17 		   联合索引覆盖长度
rows: 553		       查询的行数（越少越好）
Extra: NULL			   额外的信息

2.1.2. 执行计划的分析

1.type索引的应用级别 all

1、ALL :   全表扫描，不走索引 ，全表扫描的原因：没建索引，建了索引没走索引
没建索引

全表查询不走索引
mysql> explain select * from test.t100w;
+----+-------------+-------+------------+------+---------------+------+---------+------+--------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows   | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+--------+----------+-------+
|  1 | SIMPLE      | t100w | NULL       | ALL  | NULL          | NULL | NULL    | NULL | 997335 |   100.00 | NULL  |
+----+-------------+-------+------------+------+---------------+------+---------+------+--------+----------+-------+
1 row in set, 1 warning (0.00 sec)

没建索引的列查询不走索引
mysql> desc select * from test.t100w where k1=‘aa‘;
+----+-------------+-------+------------+------+---------------+------+---------+------+--------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows   | filtered | Extra       |
+----+-------------+-------+------------+------+---------------+------+---------+------+--------+----------+-------------+
|  1 | SIMPLE      | t100w | NULL       | ALL  | NULL          | NULL | NULL    | NULL | 997335 |    10.00 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+--------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

不等于的也不走索引,范围查询，不确定值的查询也不走索引（针对辅助索引）。
mysql> desc select * from test.t100w where k2 != ‘aaa‘;
+----+-------------+-------+------------+------+---------------+------+---------+------+--------+----------+-------------+
| 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 | 997335 |    79.44 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+--------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

两个%%号的模糊查询也不走索引（针对辅助索引）
mysql> desc select * from test.t100w where k2 like ‘%xt%‘;
+----+-------------+-------+------------+------+---------------+------+---------+------+--------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows   | filtered | Extra       |
+----+-------------+-------+------------+------+---------------+------+---------+------+--------+----------+-------------+
|  1 | SIMPLE      | t100w | NULL       | ALL  | NULL          | NULL | NULL    | NULL | 997335 |    11.11 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+--------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

单个%号走了索引
mysql> desc select * from test.t100w where k2 like ‘xt%‘;
+----+-------------+-------+------------+-------+---------------+--------+---------+------+------+----------+-----------------------+
| id | select_type | table | partitions | type  | possible_keys | key    | key_len | ref  | rows | filtered | Extra                 |
+----+-------------+-------+------------+-------+---------------+--------+---------+------+------+----------+-----------------------+
|  1 | SIMPLE      | t100w | NULL       | range | idx_k2        | idx_k2 | 17      | NULL |    1 |   100.00 | Using index condition |
+----+-------------+-------+------------+-------+---------------+--------+---------+------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)
  1. Index ：全索引扫描
不加条件的查询，就是是index（全索引扫描），前提你的查询列是有索引的
mysql> desc select k2 from test.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 | 997335 |   100.00 | Using index |
+----+-------------+-------+------------+-------+---------------+--------+---------+------+--------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
  1. range ：索引范围扫描
辅助索引 : > < >= <= like ,   in   or（in or 尽量避免出现）  
主键： != 

范围查询（针对主键）
mysql> explain select id from city where id>3000;
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+--------------------------+
| id | select_type | table | partitions | type  | possible_keys | key     | key_len | ref  | rows | filtered | Extra                    |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+--------------------------+
|  1 | SIMPLE      | city  | NULL       | range | PRIMARY       | PRIMARY | 4       | NULL | 1079 |   100.00 | Using where; Using index |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+--------------------------+
1 row in set, 1 warning (0.00 sec)

mysql> explain select id from city where id != 3000;
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+--------------------------+
| id | select_type | table | partitions | type  | possible_keys | key     | key_len | ref  | rows | filtered | Extra                    |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+--------------------------+
|  1 | SIMPLE      | city  | NULL       | range | PRIMARY       | PRIMARY | 4       | NULL | 3173 |   100.00 | Using where; Using index |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+--------------------------+
1 row in set, 1 warning (0.00 sec)

mysql> explain select id from city where id >= 3000;
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+--------------------------+
| id | select_type | table | partitions | type  | possible_keys | key     | key_len | ref  | rows | filtered | Extra                    |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+--------------------------+
|  1 | SIMPLE      | city  | NULL       | range | PRIMARY       | PRIMARY | 4       | NULL | 1080 |   100.00 | Using where; Using index |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+--------------------------+
1 row in set, 1 warning (0.00 sec)

mysql> desc select * from world.city where countrycode like ‘C%‘;
+----+-------------+-------+------------+-------+---------------+-------------+---------+------+------+----------+-----------------------+
| id | select_type | table | partitions | type  | possible_keys | key         | key_len | ref  | rows | filtered | Extra                 |
+----+-------------+-------+------------+-------+---------------+-------------+---------+------+------+----------+-----------------------+
|  1 | SIMPLE      | city  | NULL       | range | CountryCode   | CountryCode | 3       | NULL |  551 |   100.00 | Using index condition |
+----+-------------+-------+------------+-------+---------------+-------------+---------+------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)

mysql> desc select * from world.city where countrycode in (‘CHN‘,‘USA‘); #生产中不建议使用
+----+-------------+-------+------------+-------+---------------+-------------+---------+------+------+----------+-----------------------+
| 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)

建议改成这样
mysql> desc
    -> select * from world.city where countrycode=‘CHN‘
    -> union all 
    -> select * from world.city where countrycode=‘USA‘;
+----+-------------+-------+------------+------+---------------+-------------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key         | key_len | ref   | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+-------------+---------+-------+------+----------+-------+
|  1 | PRIMARY     | city  | NULL       | ref  | CountryCode   | CountryCode | 3       | const |  363 |   100.00 | NULL  |
|  2 | UNION       | city  | NULL       | ref  | CountryCode   | CountryCode | 3       | const |  274 |   100.00 | NULL  |
+----+-------------+-------+------------+------+---------------+-------------+---------+-------+------+----------+-------+
2 rows in set, 1 warning (0.00 sec)
  1. ref : 辅助索引等值查询
mysql> explain select countrycode from world.city where countrycode=‘CHN‘;
+----+-------------+-------+------------+------+---------------+-------------+---------+-------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key         | key_len | ref   | rows | filtered | Extra       |
+----+-------------+-------+------------+------+---------------+-------------+---------+-------+------+----------+-------------+
|  1 | SIMPLE      | city  | NULL       | ref  | CountryCode   | CountryCode | 3       | const |  363 |   100.00 | Using index |
+----+-------------+-------+------------+------+---------------+-------------+---------+-------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
  1. eq_ref ：在多表连接查询是on的条件列是唯一索引或主键
mysql> desc select a.name,b.name ,b.surfacearea 
    -> 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)
  1. const和system ： 主键或唯一键等值查询
mysql> explain select id 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 | Using index |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
  1. Extra: NULL 额外的信息
Extra:using filesort 出现这种情况可能是你设计索引的索引有问题或者查询方式有问题

mysql> explain select * from city where countrycode=‘CHN‘ order by population;
+----+-------------+-------+------------+------+---------------+-------------+---------+-------+------+----------+---------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key         | key_len | ref   | rows | filtered | Extra                                 |
+----+-------------+-------+------------+------+---------------+-------------+---------+-------+------+----------+---------------------------------------+
|  1 | SIMPLE      | city  | NULL       | ref  | CountryCode   | CountryCode | 3       | const |  363 |   100.00 | Using index condition; Using filesort |
+----+-------------+-------+------------+------+---------------+-------------+---------+-------+------+----------+---------------------------------------+
1 row in set, 1 warning (0.00 sec)

解决方法：添加联合索引
mysql> alter table city add index idx_co_po(countrycode,population);
Query OK, 0 rows affected (0.05 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> explain select * from city where countrycode=‘CHN‘ order by population;
+----+-------------+-------+------------+------+-----------------------+-----------+---------+-------+------+----------+-----------------------+
| id | select_type | table | partitions | type | possible_keys         | key       | key_len | ref   | rows | filtered | Extra                 |
+----+-------------+-------+------------+------+-----------------------+-----------+---------+-------+------+----------+-----------------------+
|  1 | SIMPLE      | city  | NULL       | ref  | CountryCode,idx_co_po | idx_co_po | 3       | const |  363 |   100.00 | Using index condition |
+----+-------------+-------+------------+------+-----------------------+-----------+---------+-------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)
  1. explain(desc)使用场景（面试题）
题目意思:  我们公司业务慢,请你从数据库的角度分析原因
1、mysql出现性能问题,我总结有两种情况:
（1）应急性的慢：突然夯住
应急情况:数据库hang(卡了,资源耗尽)
处理过程:
1、show processlist;  获取到导致数据库hang的语句
2、explain 分析SQL的执行计划,有没有走索引,索引的类型情况
3、建索引,改语句

（2）一段时间慢(持续性的):
1. 记录慢日志slowlog,分析slowlog
2. explain 分析SQL的执行计划,有没有走索引,索引的类型情况
3. 建索引,改语句

mysql mysql索引 聚集索引 索引 sql优化

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