# 索引 1. 索引概述 2. 索引结构 3. 索引分类 4. 索引语法 5. SQL 性能分析 6. 索引使用 7. 索引设计原则 ## 1. 索引概述 索引（index）帮助 MySQL 高效获取数据的数据结构（有序）。 索引的优缺点 | 优点 | 缺点 | | ------------------------------------------------------------- | -------------------------------------------------------------------- | | 提高数据检索的效率，降低数据库的 IO 成本 | 索引列也要占用空间 | | 通过索引列对数据进行排序，降低数据排序的成本，降低 CPU 的消耗 | 索引大大提高查询效率，同时却降低更新表的速度（insert,update,delete） | ## 2. 索引结构 MySQL 的索引是在存储引擎层实现的，不同的存储引擎有不同的结构 主要包含以下几种 | 索引结构 | 描述 | | ----------- | ---------------------------------------------------------------------------- | | B+Tree 索引 | B+树索引是最常见的索引类型，大部分引擎都支持 | | Hash 索引 | 底层数据结构是用哈希表实现的，只有精确匹配索引列的查询才有效，不支持范围查询 | | R-tree | 空间索引是 MyISAM 引擎的特殊索引类型，主要用于地理空间数据类型，通常很少使用 | | Full-text | 全文索引是一种通过倒排索引，快速匹配文档的方式，类似 Lucene,Solr,ES | 各数据库对不同类型索引的支持情况 | 索引 | innoDB | MyISAM | Memory | | ----------- | ------------ | ------ | ------ | | B+Tree 索引 | 支持 | 支持 | 支持 | | Hash 索引 | 不支持 | 不支持 | 支持 | | R-tree | 不支持 | 支持 | 不支持 | | Full-text | 5.6 之后支持 | 支持 | 不支持 | 算法可视化演示：[https://www.cs.usfca.edu/~galles/visualization/Algorithms.html](https://www.cs.usfca.edu/~galles/visualization/Algorithms.html) ![](img/index.png) ### 2.1、二叉树 二叉树的缺点： 1. 顺序插入时，会行成一个链表，查询性能大大降低 2. 大数据量情况下，层级较深，检索速度慢 ![](img/binary-tree.png) ### 2.2、红黑树 红黑树: 1. 大数据量的情况下，层级较深，检索数独慢 ![](img/red-black-tree.png) ### 2.3、 B Tree B Tree（多路平衡查找树） 以一颗最大度数（max-degree）为5（5阶）的B-Tree为例（每个节点最多存储4个Key,5个指针） 树的度数指的是一个节点的子节点个数 ![](img/B-Tree.png) ### 2.4、 B+Tree 以一颗最大度数（max-degree）为 4（4 阶）的 B+Tree 为例 B Tree 和 B+Tree 的区别 1. 所有的数据都会出现在叶子节点 2. 叶子结点行成一个单向链表 ![](img/B+Tree.png) ### 2.5、MySQL 索引 MySQL 索引数据结构对经典的 B+Tree 进行了优化，在原有 B+Tree 的基础之上，增加了一个指向相邻叶子节点的链表指针，就形成了带有顺序指针的 B+Tree，提高了区间访问的性能 ![](img/mysql-b+tree.png) ### 2.6、Hash 哈希索引就是采用一定的hash算法，将键值换算成新的hash值，映射到对应的槽位上，然后存储在hash表中 如果两个或多个键值，映射到一个相同的槽位上，他们就产生了hash冲突（哈希碰撞），可以通过链表来解决 ![](img/hash.png) Hash索引的特点: 1. hash索引只能用于等值比较（=，in），不支持范围查询（between, >, <） 2. 无法利用索引完成排序操作 3. 查询效率高，通常只需要一次检索就可以，效率通常要高于B+Tree索引 支持Hash索引的存储引擎：Memory InnoDB中具有自适应hash功能 ### 2.7、思考 为什么InnoDB存储引擎选择使用B+Tree索引结构？ 1. 相对于二叉树，层级更少，搜索效率高 2. 相对于B-Tree，无论是叶子节点还是非叶子节点，都会保存数据，这样导致一页中存储的键值减少，指针减少，要保存同样的数据量，只能增加树的高度，导致性能降低 3. 相对于Hash索引，B+Tree支持范围匹配及排序操作 ## 3、索引分类 |分类 | 含义 | 特点 | 关键字| |-|-|-|-| |主键索引 | 针对于表中主键创建的索引|默认自动创建，只能有一个 | primary | |唯一索引 | 避免同一个表中某列数据中的值重复 | 可以有多个 | unique | |常规索引|快读定位特定数据 | 可以有多个| index | |全文索引 |全文索引查找的是文本中的关键词，而不是比较索引中的值 | 可以有多个| fulltext | 根据索引存储形式，又可以分为 |分类 | 含义 | 特点| |-|-|-| | 聚集索引 clustered index | 将数据存储与索引放到了一块，索引结构的叶子节点保存了行数据 | 必须有，而且只有一个 | | 二级索引 secondary index | 将数据与索引分开存储，索引结构的叶子节点关联的是对应的主键 | 可以存在多个 | 聚集索引选取规则： 1. 如果存在主键，主键索引就是聚集索引 2. 如果不存在主键，将使用第一个唯一索引（unique）作为聚集索引 3. 如果表没有主键，或没有合适的唯一索引，则InnoDB会自动生成一个rowid作为隐藏的聚集索引 索引示例 ![](img/cluster-index.png) 查询示例 ![](img/index-return.png) ### 3.1、思考1 以下SQL语句，哪个执行效率高？为什么？ ```sql select * from user where id = 10; select * from user where name = 'Arm'; ``` 备注：id为主键，name字段有索引 直接用id查询效率较高 1. id查询可以使用聚集索引 2. name查询需要使用回表查询，先查询二级索引，查询到id，再查询聚集索引 ### 3.2、思考2 InnoDB主键索引的B+Tree高度为多高？ 假设： 一页中可以存储16k数据，一行数据大小为1k，可以存储16行。InnoDB的指针占用6个字节，主键即使为bigint，占用字节数为8。 树的高度为2： ```js // n为key的数量 (n+1)表示指针数量 n * 8 + ( n + 1) * 6 = 16 * 1024 // 算出n约等于1170 1171 * 16 = 18736 ``` 如果高度为3 ```js 1171 * 1171 * 16 = 21939856 ``` ## 4、索引的语法 创建索引 ```sql create [unique|fulltext] index index_name on table_name (index_column_name,...); ``` 查看索引 ```sql show index form table_name; ``` 删除索引 ```sql drop index index_name on table_name; ``` ### 4.1、案例 需求： 1. name姓名字段，该字段的值可能会重复，该字段创建索引 2. phone手机号字段，是非空，且唯一的，创建唯一索引 3. 为profession、age、status创建联合索引 4. 为email建立合适的索引来提升查询效率 准备测试数据 ```sql create table tb_user( id int primary key auto_increment comment '主键', name varchar(20) comment '姓名', phone varchar(11) comment '手机号', profession varchar(10) comment '专业', age int comment '年龄', status int comment '状态', email varchar(50) comment '邮箱' ); -- 插入测试数据 insert into tb_user (name, phone, profession, age, status, email) values ('张飞', '15210231227', '美术', 23, 1, '123@qq.com'); insert into tb_user (name, phone, profession, age, status, email) values ('关羽', '15210231297', '物理', 24, 1, '3339@qq.com'); insert into tb_user (name, phone, profession, age, status, email) values ('刘备', '15214231297', '数学', 25, 0, '666@qq.com'); insert into tb_user (name, phone, profession, age, status, email) values ('孙权', '15215231297', '语文', 20, 1, '111@qq.com'); -- 查看数据 mysql> select * from tb_user; +----+--------+-------------+------------+------+--------+-------------+ | id | name | phone | profession | age | status | email | +----+--------+-------------+------------+------+--------+-------------+ | 1 | 张飞 | 15210231227 | 美术 | 23 | 1 | 123@qq.com | | 2 | 关羽 | 15210231297 | 物理 | 24 | 1 | 3339@qq.com | | 3 | 刘备 | 15214231297 | 数学 | 25 | 0 | 666@qq.com | | 4 | 孙权 | 15215231297 | 语文 | 20 | 1 | 111@qq.com | +----+--------+-------------+------------+------+--------+-------------+ 4 rows in set (0.00 sec) -- 查看已有索引 mysql> show index from tb_user; +---------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+ | Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | Visible | Expression | +---------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+ | tb_user | 0 | PRIMARY | 1 | id | A | 4 | NULL | NULL | | BTREE | | | YES | NULL | +---------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+ 1 row in set (0.01 sec) -- 或者 mysql> show index from tb_user\G *************************** 1. row *************************** Table: tb_user Non_unique: 0 Key_name: PRIMARY Seq_in_index: 1 Column_name: id Collation: A Cardinality: 4 Sub_part: NULL Packed: NULL Null: Index_type: BTREE Comment: Index_comment: Visible: YES Expression: NULL 1 row in set (0.00 sec) ``` 完成需求 ```sql -- 创建普通索引 create index idx_user_name on tb_user (name); -- 默认创建B+Tree的索引结构 show index from tb_user; +---------+------------+---------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+ | Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | Visible | Expression | +---------+------------+---------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+ | tb_user | 0 | PRIMARY | 1 | id | A | 4 | NULL | NULL | | BTREE | | | YES | NULL | | tb_user | 1 | idx_user_name | 1 | name | A | 4 | NULL | NULL | YES | BTREE | | | YES | NULL | +---------+------------+---------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+ 2 rows in set (0.00 sec) -- 创建唯一索引 create unique index idx_user_phone on tb_user (phone); mysql> show index from tb_user; +---------+------------+----------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+ | Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | Visible | Expression | +---------+------------+----------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+ | tb_user | 0 | PRIMARY | 1 | id | A | 4 | NULL | NULL | | BTREE | | | YES | NULL | | tb_user | 0 | idx_user_phone | 1 | phone | A | 4 | NULL | NULL | YES | BTREE | | | YES | NULL | | tb_user | 1 | idx_user_name | 1 | name | A | 4 | NULL | NULL | YES | BTREE | | | YES | NULL | +---------+------------+----------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+ 3 rows in set (0.01 sec) -- 创建联合索引 create index idx_user_profession_age_status on tb_user (profession, age, status); mysql> show index from tb_user; +---------+------------+--------------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+ | Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | Visible | Expression | +---------+------------+--------------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+ | tb_user | 0 | PRIMARY | 1 | id | A | 4 | NULL | NULL | | BTREE | | | YES | NULL | | tb_user | 0 | idx_user_phone | 1 | phone | A | 4 | NULL | NULL | YES | BTREE | | | YES | NULL | | tb_user | 1 | idx_user_name | 1 | name | A | 4 | NULL | NULL | YES | BTREE | | | YES | NULL | | tb_user | 1 | idx_user_profession_age_status | 1 | profession | A | 4 | NULL | NULL | YES | BTREE | | | YES | NULL | | tb_user | 1 | idx_user_profession_age_status | 2 | age | A | 4 | NULL | NULL | YES | BTREE | | | YES | NULL | | tb_user | 1 | idx_user_profession_age_status | 3 | status | A | 4 | NULL | NULL | YES | BTREE | | | YES | NULL | +---------+------------+--------------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+ 6 rows in set (0.00 sec) -- 创建普通索引 create index idx_user_email on tb_user (email); mysql> show index from tb_user; +---------+------------+--------------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+ | Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | Visible | Expression | +---------+------------+--------------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+ | tb_user | 0 | PRIMARY | 1 | id | A | 4 | NULL | NULL | | BTREE | | | YES | NULL | | tb_user | 0 | idx_user_phone | 1 | phone | A | 4 | NULL | NULL | YES | BTREE | | | YES | NULL | | tb_user | 1 | idx_user_name | 1 | name | A | 4 | NULL | NULL | YES | BTREE | | | YES | NULL | | tb_user | 1 | idx_user_profession_age_status | 1 | profession | A | 4 | NULL | NULL | YES | BTREE | | | YES | NULL | | tb_user | 1 | idx_user_profession_age_status | 2 | age | A | 4 | NULL | NULL | YES | BTREE | | | YES | NULL | | tb_user | 1 | idx_user_profession_age_status | 3 | status | A | 4 | NULL | NULL | YES | BTREE | | | YES | NULL | | tb_user | 1 | idx_user_email | 1 | email | A | 4 | NULL | NULL | YES | BTREE | | | YES | NULL | +---------+------------+--------------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+ 7 rows in set (0.00 sec) ``` ## 5、SQL性能分析 ### 5.1、SQL执行频次 ```sql -- 查看服务器状态信息 show [session|global] status -- 查看当前数据库的CURD（insert、update、delete、select）访问频次 -- 7个下划线 show global status like 'Com_______'; +---------------+-------+ | Variable_name | Value | +---------------+-------+ | Com_binlog | 0 | | Com_commit | 62 | | Com_delete | 4 | | Com_import | 0 | | Com_insert | 32 | | Com_repair | 0 | | Com_revoke | 0 | | Com_select | 1967 | | Com_signal | 0 | | Com_update | 52 | | Com_xa_end | 0 | +---------------+-------+ 11 rows in set (0.01 sec) ``` 重点关注以下4个指标： - Com_delete - Com_insert - Com_select - Com_update ### 5.2、慢查询日志 慢查询日志记录了所有执行时间超过指定参数(long_query_time，单位：秒，默认10秒)的所有SQL语句的日志 MySQL的慢查询日志默认没有开启 ```sql show variables like 'slow_query_log'; +----------------+-------+ | Variable_name | Value | +----------------+-------+ | slow_query_log | OFF | +----------------+-------+ 1 row in set (0.01 sec) ``` 查找`my.cnf`文件路径 ```bash $ mysql --help|grep 'my.cnf' ``` 开启MySQL的慢查询日志，需要在MySQL的配置文件(/etc/my.cnf)中配置如下信息 ```bash # vim /etc/my.cnf # 开启慢查询日志开关 slow_query_log=1 # 设置慢查询时间为2秒，SQL语句执行时间超过2秒，就会视为慢查询，记录慢查询日志 long_query_time=2 ``` 修改参数后，重启MySQL服务 ```bash systemctl restart mysqld ``` mac环境下： 配置文件路径：`/usr/local/etc/my.cnf` 重启服务 ```bash mysql.server restart ``` ```sql show variables like 'slow_query_log'; +----------------+-------+ | Variable_name | Value | +----------------+-------+ | slow_query_log | ON | +----------------+-------+ 1 row in set (0.01 sec) ``` 查看慢查询日志信息 ```sql mysql> show variables like 'slow_query_log_file'; +---------------------+-------------------------------------------------+ | Variable_name | Value | +---------------------+-------------------------------------------------+ | slow_query_log_file | /usr/local/var/mysql/localhost-slow.log | +---------------------+-------------------------------------------------+ 1 row in set (0.00 sec) ``` 查看慢日志 ```bash tail -f /usr/local/var/mysql/localhost-slow.log # Time: 2022-05-06T14:41:31.330012Z # User@Host: root[root] @ localhost [] Id: 8 # Query_time: 0.000408 Lock_time: 0.000184 Rows_sent: 5 Rows_examined: 5 SET timestamp=1651848091; select * from tb_student; ``` 如果看不到效果，可以设置一个较小的慢查询时间 ```bash long_query_time=0.0001 ``` ### 5.3、profile详情 了解时间都耗费在哪里 通过`have_profiling`参数，可以查看当前MySQL是否支持profile操作 ```sql mysql> select @@have_profiling; +------------------+ | @@have_profiling | +------------------+ | YES | +------------------+ 1 row in set, 1 warning (0.00 sec) ``` 默认profiling是关闭的， 通过set语句在session/global级别开启profiling ```sql select @@profiling; +-------------+ | @@profiling | +-------------+ | 0 | +-------------+ 1 row in set, 1 warning (0.00 sec) -- 开启profiling set profiling = 1; ``` 查看执行耗时 ```sql -- 查看每一条SQL的耗时基本操作 show profiles; -- 查看指定query_id的SQL语句各个阶段的耗时情况 show profile for query ; -- 查看指定query_id 的SQL语句CPU的使用情况 show profile cpu for query ; ``` 示例 ```sql mysql> show profiles; +----------+------------+--------------------------+ | Query_ID | Duration | Query | +----------+------------+--------------------------+ | 1 | 0.00282500 | select * from tb_student | +----------+------------+--------------------------+ 1 row in set, 1 warning (0.00 sec) mysql> show profile for query 1; +--------------------------------+----------+ | Status | Duration | +--------------------------------+----------+ | starting | 0.000109 | | Executing hook on transaction | 0.000028 | | starting | 0.000016 | | checking permissions | 0.000012 | | Opening tables | 0.000069 | | init | 0.000036 | | System lock | 0.000027 | | optimizing | 0.000012 | | statistics | 0.000048 | | preparing | 0.000021 | | executing | 0.000006 | | Sending data | 0.000060 | | end | 0.000012 | | query end | 0.000008 | | waiting for handler commit | 0.000015 | | closing tables | 0.000017 | | freeing items | 0.000031 | | logging slow query | 0.002269 | | cleaning up | 0.000029 | +--------------------------------+----------+ 19 rows in set, 1 warning (0.01 sec) mysql> show profile cpu for query 1; +--------------------------------+----------+----------+------------+ | Status | Duration | CPU_user | CPU_system | +--------------------------------+----------+----------+------------+ | starting | 0.000109 | 0.000071 | 0.000012 | | Executing hook on transaction | 0.000028 | 0.000012 | 0.000010 | | starting | 0.000016 | 0.000012 | 0.000004 | | checking permissions | 0.000012 | 0.000009 | 0.000003 | | Opening tables | 0.000069 | 0.000052 | 0.000016 | | init | 0.000036 | 0.000019 | 0.000011 | | System lock | 0.000027 | 0.000020 | 0.000007 | | optimizing | 0.000012 | 0.000009 | 0.000004 | | statistics | 0.000048 | 0.000022 | 0.000024 | | preparing | 0.000021 | 0.000019 | 0.000003 | | executing | 0.000006 | 0.000004 | 0.000002 | | Sending data | 0.000060 | 0.000058 | 0.000002 | | end | 0.000012 | 0.000007 | 0.000004 | | query end | 0.000008 | 0.000006 | 0.000003 | | waiting for handler commit | 0.000015 | 0.000012 | 0.000003 | | closing tables | 0.000017 | 0.000015 | 0.000002 | | freeing items | 0.000031 | 0.000015 | 0.000017 | | logging slow query | 0.002269 | 0.000042 | 0.001699 | | cleaning up | 0.000029 | 0.000014 | 0.000015 | +--------------------------------+----------+----------+------------+ 19 rows in set, 1 warning (0.00 sec) ``` ### 5.4、explain 执行计划 获取MySQL如何执行select语句信息，包括在select语句执行过程中表如何连接和连接的顺序 ```sql explain/desc select 字段列表 from 表名 where 条件; -- eg explain select * from tb_user where id = 1; +----+-------------+---------+------------+-------+---------------+---------+---------+-------+------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+-------+---------------+---------+---------+-------+------+----------+-------+ | 1 | SIMPLE | tb_user | NULL | const | PRIMARY | PRIMARY | 4 | const | 1 | 100.00 | NULL | +----+-------------+---------+------------+-------+---------------+---------+---------+-------+------+----------+-------+ 1 row in set, 1 warning (0.00 sec) ``` 字段含义 id: select 查询的序列号，表示查询中执行select子句或者是操作表的顺序 - id相同，执行顺序从上到下； - id不同，值越大，越先执行。 select_type 表示SElect 类型，常见取值 - simple 简单表，即不使用表连接或者子查询 - primary 主查询，即外层的查询 - union union中的第二个或者后面的查询语句 - subquery select/where之后包含了子查询等 type 表示连接类型 - 性能由好到差：null, system,const(主键、唯一索引),eq_ref,ref(非唯一索引),range,index,all possible_key 可能用到的索引，一个或多个 key 实际使用的索引，如果为null，则没有使用索引 key_len 表示索引中使用的字节数，该值为索引字段最大可能长度，并非实际使用长度，在不损失精确性的前提下，长度越短越好 rows MySQL认为必须要执行查询的行数，在innoDB引擎的表中，是一个估计值，可能并不总是准确的 filtered 表示返回结果的行数，占需读取行数的百分比，值越大越好 extra 额外信息 初始化测试数据 ```sql -- 学生表和课程表行成多对多的关系 create table tb_student( id int primary key auto_increment, name varchar(20) ) comment '学生表'; create table tb_course( id int primary key auto_increment, name varchar(20) ) comment '课程表'; create table tb_student_course( id int primary key auto_increment, student_id int, course_id int ) comment '学生课程关联表'; insert into tb_student (name) values ('张飞'), ('关羽'), ('刘备'), ('曹操'), ('孙权'); insert into tb_course (name) values ('语文'), ('数学'), ('英语'), ('历史'), ('化学'); insert into tb_student_course (student_id, course_id) values (1, 1), (1, 2), (2, 1), (2, 4), (3, 4), (4, 2); mysql> select * from tb_student; +----+--------+ | id | name | +----+--------+ | 1 | 张飞 | | 2 | 关羽 | | 3 | 刘备 | | 4 | 曹操 | | 5 | 孙权 | +----+--------+ 5 rows in set (0.01 sec) mysql> select * from tb_course; +----+--------+ | id | name | +----+--------+ | 1 | 语文 | | 2 | 数学 | | 3 | 英语 | | 4 | 历史 | | 5 | 化学 | +----+--------+ 5 rows in set (0.00 sec) mysql> select * from tb_student_course; +----+------------+-----------+ | id | student_id | course_id | +----+------------+-----------+ | 1 | 1 | 1 | | 2 | 1 | 2 | | 3 | 2 | 1 | | 4 | 2 | 4 | | 5 | 3 | 4 | | 6 | 4 | 2 | +----+------------+-----------+ 6 rows in set (0.00 sec) ``` 查看执行计划 ```sql select s.*, c.* from tb_student s, tb_course c, tb_student_course sc where s.id = sc.student_id and c.id = sc.course_id; +----+--------+----+--------+ | id | name | id | name | +----+--------+----+--------+ | 1 | 张飞 | 1 | 语文 | | 1 | 张飞 | 2 | 数学 | | 2 | 关羽 | 1 | 语文 | | 2 | 关羽 | 4 | 历史 | | 3 | 刘备 | 4 | 历史 | | 4 | 曹操 | 2 | 数学 | +----+--------+----+--------+ 6 rows in set (0.00 sec) -- 查看执行计划 mysql> explain select s.*, c.* from tb_student s, tb_course c, tb_student_course sc -> where s.id = sc.student_id and c.id = sc.course_id; +----+-------------+-------+------------+--------+---------------+---------+---------+-------------------+------+----------+----------------------------------------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+--------+---------------+---------+---------+-------------------+------+----------+----------------------------------------------------+ | 1 | SIMPLE | s | NULL | ALL | PRIMARY | NULL | NULL | NULL | 5 | 100.00 | NULL | | 1 | SIMPLE | sc | NULL | ALL | NULL | NULL | NULL | NULL | 6 | 16.67 | Using where; Using join buffer (Block Nested Loop) | | 1 | SIMPLE | c | NULL | eq_ref | PRIMARY | PRIMARY | 4 | data.sc.course_id | 1 | 100.00 | NULL | +----+-------------+-------+------------+--------+---------------+---------+---------+-------------------+------+----------+----------------------------------------------------+ 3 rows in set, 1 warning (0.00 sec) ``` 查询选修了`语文`课程的学生，通过子查询实现 ```sql select id from tb_course where name = '语文'; +----+ | id | +----+ | 1 | +----+ 1 row in set (0.00 sec) select student_id from tb_student_course where course_id = 1; +------------+ | student_id | +------------+ | 1 | | 2 | +------------+ 2 rows in set (0.00 sec) select * from tb_student where id in (1, 2); +----+--------+ | id | name | +----+--------+ | 1 | 张飞 | | 2 | 关羽 | +----+--------+ 2 rows in set (0.00 sec) -- 组合成子查询语句 select * from tb_student where id in ( select student_id from tb_student_course where course_id = ( select id from tb_course where name = '语文' ) ); +----+--------+ | id | name | +----+--------+ | 1 | 张飞 | | 2 | 关羽 | +----+--------+ 2 rows in set (0.00 sec) -- 查看执行计划 explain select * from tb_student where id in ( select student_id from tb_student_course where course_id = ( select id from tb_course where name = '语文' ) ); +----+-------------+-------------------+------------+------+---------------+------+---------+------+------+----------+----------------------------------------------------------------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------------------+------------+------+---------------+------+---------+------+------+----------+----------------------------------------------------------------------------+ | 1 | PRIMARY | tb_student | NULL | ALL | PRIMARY | NULL | NULL | NULL | 5 | 100.00 | NULL | | 1 | PRIMARY | tb_student_course | NULL | ALL | NULL | NULL | NULL | NULL | 6 | 16.67 | Using where; FirstMatch(tb_student); Using join buffer (Block Nested Loop) | | 3 | SUBQUERY | tb_course | NULL | ALL | NULL | NULL | NULL | NULL | 5 | 20.00 | Using where | +----+-------------+-------------------+------------+------+---------------+------+---------+------+------+----------+----------------------------------------------------------------------------+ 3 rows in set, 1 warning (0.00 sec) ``` 更多示例 ```sql mysql> select 'A'; +---+ | A | +---+ | A | +---+ 1 row in set (0.00 sec) mysql> explain select 'A'; +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+ | 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 tables used | +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+ 1 row in set, 1 warning (0.01 sec) ``` ```sql mysql> select * from tb_user; +----+--------+-------------+------------+------+--------+-------------+ | id | name | phone | profession | age | status | email | +----+--------+-------------+------------+------+--------+-------------+ | 1 | 张飞 | 15210231227 | 美术 | 23 | 1 | 123@qq.com | | 2 | 关羽 | 15210231297 | 物理 | 24 | 1 | 3339@qq.com | | 3 | 刘备 | 15214231297 | 数学 | 25 | 0 | 666@qq.com | | 4 | 孙权 | 15215231297 | 语文 | 20 | 1 | 111@qq.com | +----+--------+-------------+------------+------+--------+-------------+ 4 rows in set (0.00 sec) mysql> show index from tb_user; +---------+------------+--------------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+ | Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | Visible | Expression | +---------+------------+--------------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+ | tb_user | 0 | PRIMARY | 1 | id | A | 4 | NULL | NULL | | BTREE | | | YES | NULL | | tb_user | 0 | idx_user_phone | 1 | phone | A | 4 | NULL | NULL | YES | BTREE | | | YES | NULL | | tb_user | 1 | idx_user_name | 1 | name | A | 4 | NULL | NULL | YES | BTREE | | | YES | NULL | | tb_user | 1 | idx_user_profession_age_status | 1 | profession | A | 4 | NULL | NULL | YES | BTREE | | | YES | NULL | | tb_user | 1 | idx_user_profession_age_status | 2 | age | A | 4 | NULL | NULL | YES | BTREE | | | YES | NULL | | tb_user | 1 | idx_user_profession_age_status | 3 | status | A | 4 | NULL | NULL | YES | BTREE | | | YES | NULL | | tb_user | 1 | idx_user_email | 1 | email | A | 4 | NULL | NULL | YES | BTREE | | | YES | NULL | +---------+------------+--------------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+ 7 rows in set (0.05 sec) mysql> explain select * from tb_user where phone = '15210231227'; +----+-------------+---------+------------+-------+----------------+----------------+---------+-------+------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+-------+----------------+----------------+---------+-------+------+----------+-------+ | 1 | SIMPLE | tb_user | NULL | const | idx_user_phone | idx_user_phone | 47 | const | 1 | 100.00 | NULL | +----+-------------+---------+------------+-------+----------------+----------------+---------+-------+------+----------+-------+ 1 row in set, 1 warning (0.01 sec) mysql> explain select * from tb_user where name = '张飞'; +----+-------------+---------+------------+------+---------------+---------------+---------+-------+------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+------+---------------+---------------+---------+-------+------+----------+-------+ | 1 | SIMPLE | tb_user | NULL | ref | idx_user_name | idx_user_name | 83 | const | 1 | 100.00 | NULL | +----+-------------+---------+------------+------+---------------+---------------+---------+-------+------+----------+-------+ 1 row in set, 1 warning (0.01 sec) mysql> explain select count(*) from tb_user; +----+-------------+---------+------------+-------+---------------+----------------+---------+------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+-------+---------------+----------------+---------+------+------+----------+-------------+ | 1 | SIMPLE | tb_user | NULL | index | NULL | idx_user_phone | 47 | NULL | 4 | 100.00 | Using index | +----+-------------+---------+------------+-------+---------------+----------------+---------+------+------+----------+-------------+ 1 row in set, 1 warning (0.00 sec) ```