# MySQL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. 索引失效的场景 8. 索引设计原则 9. 总结 ## 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', '17712345678', '美术', 23, 1, '123@qq.com'); insert into tb_user (name, phone, profession, age, status, email) values ('关羽', '17722345678', '物理', 24, 1, '3339@qq.com'); insert into tb_user (name, phone, profession, age, status, email) values ('刘备', '17732345678', '数学', 25, 0, '666@qq.com'); insert into tb_user (name, phone, profession, age, status, email) values ('孙权', '17742345678', '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 | 17712345678 | 美术 | 23 | 1 | 123@qq.com | | 2 | 关羽 | 17722345678 | 物理 | 24 | 1 | 3339@qq.com | | 3 | 刘备 | 17732345678 | 数学 | 25 | 0 | 666@qq.com | | 4 | 孙权 | 17742345678 | 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 | 17712345678 | 美术 | 23 | 1 | 123@qq.com | | 2 | 关羽 | 17722345678 | 物理 | 24 | 1 | 3339@qq.com | | 3 | 刘备 | 17732345678 | 数学 | 25 | 0 | 666@qq.com | | 4 | 孙权 | 17742345678 | 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 = '17712345678'; +----+-------------+---------+------------+-------+----------------+----------------+---------+-------+------+----------+-------+ | 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) ``` 索引使用 创建测试表 ```sql create table tb_sku( id int primary key auto_increment, uuid char(36) ); ``` ## 6、索引使用 ### 6.1、验证索引效率 生成测试数据（1000万条记录） 方式一：使用MySQL的自定义函数 速度较慢，适合较少的数据 ```sql -- 1、定义个生成测试数据的函数 -- 修改语句结束符 delimiter $$ -- 创建函数 create function init_data(total int) returns int begin -- 声明局部变量 declare i int default 0; -- 循环处理 while i < total do -- 生成测试数据 insert into tb_sku (`uuid`) values (uuid()); set i = i + 1; end while; -- 返回值 return i; end -- 结束 $$ -- 修改语句结束符 delimiter ; -- 2、调用函数生成数据(100W) mysql> select init_data(1000000); +--------------------+ | init_data(1000000) | +--------------------+ | 1000000 | +--------------------+ 1 row in set (1 min 16.08 sec) -- 3、删除函数 drop function init_data; ``` 方式二： 使用Python脚本生成，适合生成大量数据 ```python # pip install records mysqlclient import records import uuid db = records.Database('mysql://root:123456@localhost/data?charset=utf8') # 100 * 100000 = 1000万条数据 for i in range(100): data = [{'uuid': str(uuid.uuid4())} for i in range(100000)] db.bulk_query("insert into tb_sku(uuid) values(:uuid)", data) ``` ```sql mysql> select count(*) from tb_sku; +----------+ | count(*) | +----------+ | 10000000 | +----------+ 1 row in set (0.38 sec) mysql> select * from tb_sku limit 1; +----+--------------------------------------+ | id | uuid | +----+--------------------------------------+ | 1 | 166fa508-2911-494d-aed2-2a2ee81a2a64 | +----+--------------------------------------+ 1 row in set (0.00 sec) ``` 验证索引效率 ```sql -- 为建立索引之前，执行如下SQL，查看SQL的耗时 mysql> select * from tb_sku where uuid = '166fa508-2911-494d-aed2-2a2ee81a2a64'; +----+--------------------------------------+ | id | uuid | +----+--------------------------------------+ | 1 | 166fa508-2911-494d-aed2-2a2ee81a2a64 | +----+--------------------------------------+ 1 row in set (2.15 sec) -- 创建索引 mysql> create index idx_sku_uuid on tb_sku (uuid); Query OK, 0 rows affected (21.85 sec) Records: 0 Duplicates: 0 Warnings: 0 -- 再次执行相同的SQL语句，查看SQL耗时 mysql> select * from tb_sku where uuid = '166fa508-2911-494d-aed2-2a2ee81a2a64'; +----+--------------------------------------+ | id | uuid | +----+--------------------------------------+ | 1 | 166fa508-2911-494d-aed2-2a2ee81a2a64 | +----+--------------------------------------+ 1 row in set (0.01 sec) -- 通过主键查询 mysql> select * from tb_sku where id = 1; +----+--------------------------------------+ | id | uuid | +----+--------------------------------------+ | 1 | 166fa508-2911-494d-aed2-2a2ee81a2a64 | +----+--------------------------------------+ 1 row in set (0.00 sec) -- 查看执行计划 mysql> explain select * from tb_sku where uuid = '166fa508-2911-494d-aed2-2a2ee81a2a64'; +----+-------------+--------+------------+------+---------------+--------------+---------+-------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+--------+------------+------+---------------+--------------+---------+-------+------+----------+-------------+ | 1 | SIMPLE | tb_sku | NULL | ref | idx_sku_uuid | idx_sku_uuid | 145 | const | 1 | 100.00 | Using index | +----+-------------+--------+------------+------+---------------+--------------+---------+-------+------+----------+-------------+ 1 row in set, 1 warning (0.00 sec) ``` ### 6.2、最左前缀法则 如果索引了多列（联合索引），要遵守最左前缀法则。 最左前缀法则指的是查询从索引的最左侧列开始，并且不跳过索引中的列。 如果跳过某一列，索引将部分失效（后面的字段索引失效） ```sql -- 查看表数据 select * from tb_user; +----+--------+-------------+------------+------+--------+-------------+ | id | name | phone | profession | age | status | email | +----+--------+-------------+------------+------+--------+-------------+ | 1 | 张飞 | 17712345678 | 美术 | 23 | 1 | 123@qq.com | | 2 | 关羽 | 17722345678 | 物理 | 24 | 1 | 3339@qq.com | | 3 | 刘备 | 17732345678 | 数学 | 25 | 0 | 666@qq.com | | 4 | 孙权 | 17742345678 | 语文 | 20 | 1 | 111@qq.com | +----+--------+-------------+------------+------+--------+-------------+ 4 rows in set (0.01 sec) -- 查看表中的索引 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 (0.06 sec) -- 查询数据 select * from tb_user where profession = '美术' and age = 23 and status = 1; +----+--------+-------------+------------+------+--------+------------+ | id | name | phone | profession | age | status | email | +----+--------+-------------+------------+------+--------+------------+ | 1 | 张飞 | 17712345678 | 美术 | 23 | 1 | 123@qq.com | +----+--------+-------------+------------+------+--------+------------+ 1 row in set (0.00 sec) -- 查看用到的索引:profession、age、status explain select * from tb_user where profession = '美术' and age = 23 and status = 1; +----+-------------+---------+------------+------+--------------------------------+--------------------------------+---------+-------------------+------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+------+--------------------------------+--------------------------------+---------+-------------------+------+----------+-------+ | 1 | SIMPLE | tb_user | NULL | ref | idx_user_profession_age_status | idx_user_profession_age_status | 53 | const,const,const | 1 | 100.00 | NULL | +----+-------------+---------+------------+------+--------------------------------+--------------------------------+---------+-------------------+------+----------+-------+ 1 row in set, 1 warning (0.00 sec) -- 查询profession和age，使用到了索引 explain select * from tb_user where profession = '美术' and age = 23; +----+-------------+---------+------------+------+--------------------------------+--------------------------------+---------+-------------+------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+------+--------------------------------+--------------------------------+---------+-------------+------+----------+-------+ | 1 | SIMPLE | tb_user | NULL | ref | idx_user_profession_age_status | idx_user_profession_age_status | 48 | const,const | 1 | 100.00 | NULL | +----+-------------+---------+------------+------+--------------------------------+--------------------------------+---------+-------------+------+----------+-------+ 1 row in set, 1 warning (0.00 sec) -- 单独查询profession，使用到了索引 explain select * from tb_user where profession = '美术'; +----+-------------+---------+------------+------+--------------------------------+--------------------------------+---------+-------+------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+------+--------------------------------+--------------------------------+---------+-------+------+----------+-------+ | 1 | SIMPLE | tb_user | NULL | ref | idx_user_profession_age_status | idx_user_profession_age_status | 43 | const | 1 | 100.00 | NULL | +----+-------------+---------+------------+------+--------------------------------+--------------------------------+---------+-------+------+----------+-------+ 1 row in set, 1 warning (0.01 sec) -- 查询age和status，没有使用到索引 explain select * from tb_user where age = 23 and status = 1; +----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+ | 1 | SIMPLE | tb_user | NULL | ALL | NULL | NULL | NULL | NULL | 4 | 25.00 | Using where | +----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+ 1 row in set, 1 warning (0.01 sec) -- 单独查询status，没有使用到索引 explain select * from tb_user where status = 1; +----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+ | 1 | SIMPLE | tb_user | NULL | ALL | NULL | NULL | NULL | NULL | 4 | 25.00 | Using where | +----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+ 1 row in set, 1 warning (0.00 sec) -- 跳过中间的age字段，只用到了profession，而status没有用到索引 explain select * from tb_user where profession = '美术' and status = 1; +----+-------------+---------+------------+------+--------------------------------+--------------------------------+---------+-------+------+----------+-----------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+------+--------------------------------+--------------------------------+---------+-------+------+----------+-----------------------+ | 1 | SIMPLE | tb_user | NULL | ref | idx_user_profession_age_status | idx_user_profession_age_status | 43 | const | 1 | 25.00 | Using index condition | +----+-------------+---------+------------+------+--------------------------------+--------------------------------+---------+-------+------+----------+-----------------------+ 1 row in set, 1 warning (0.00 sec) -- 索引字段可以交换顺序，也能使用到索引 explain select * from tb_user where age = 23 and status = 1 and profession = '美术'; +----+-------------+---------+------------+------+--------------------------------+--------------------------------+---------+-------------------+------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+------+--------------------------------+--------------------------------+---------+-------------------+------+----------+-------+ | 1 | SIMPLE | tb_user | NULL | ref | idx_user_profession_age_status | idx_user_profession_age_status | 53 | const,const,const | 1 | 100.00 | NULL | +----+-------------+---------+------------+------+--------------------------------+--------------------------------+---------+-------------------+------+----------+-------+ 1 row in set, 1 warning (0.00 sec) ``` ### 6.3、SQL提示 SQL提示是优化数据库的一个重要手段 简单来说，就是在SQL语句中加入一些认为的提示来达到优化操作的目的 ```sql -- 1、use index 推荐使用索引 explain select * from tb_user use index (idx_user_profession) where profession = '法务经理'; -- 2、ignore index 忽略使用索引 explain select * from tb_user ignore index (idx_user_profession) where profession = '法务经理'; -- 3、force index 强制使用索引 explain select * from tb_user force index (idx_user_profession) where profession = '法务经理'; ``` 测试数据准备 ```sql drop table if exists tb_user; 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(50) comment '专业', age int comment '年龄', status int comment '状态', email varchar(50) comment '邮箱' ); ``` 利用Python脚本生成10万条测试数据 ```python # 安装依赖 pip install faker records mysqlclient from faker import Faker import records # 简体中文：zh_CN faker = Faker(locale="zh_CN") # 指定随机种子，确保每次生成的数据都是一致的 faker.seed(1) # 生成数据插入 insert sql def get_insert_sql(table_name, fields): keys = ', '.join([f'`{key}`'.format(key) for key in fields]) values = ', '.join([f':{key}'.format(key) for key in fields]) return f'INSERT INTO `{table_name}` ({keys}) VALUES ({values})' # 获取数据 def get_row(): return { 'name': faker.name(), 'phone': faker.phone_number(), 'email': faker.email(), 'age': faker.random_int(20, 30), 'status': faker.random_int(0, 1), 'profession': faker.job(), } def main(): db = records.Database('mysql://root:123456@localhost/data?charset=utf8') # 10 * 1000 = 1万条数据 for i in range(10): data = [get_row() for _ in range(1000)] sql = get_insert_sql('tb_user', data[0].keys()) db.bulk_query(sql, data) if __name__ == '__main__': main() ``` 查看生成的测试数据 ```sql mysql> select count(*) from tb_user; +----------+ | count(*) | +----------+ | 10000 | +----------+ select * from tb_user limit 10; +----+-----------+-------------+---------------------------------+------+--------+---------------------+ | id | name | phone | profession | age | status | email | +----+-----------+-------------+---------------------------------+------+--------+---------------------+ | 1 | 费阳 | 13777763170 | 法务经理 | 27 | 1 | wyao@gmail.com | | 2 | 祁海燕 | 13400806360 | 日式厨师 | 23 | 0 | jwan@jin.cn | | 3 | 姬秀英 | 18281241586 | 食品/饮料研发 | 29 | 0 | li97@wang.cn | | 4 | 官桂芳 | 15625851781 | 前台接待/总机/接待生 | 20 | 1 | fpeng@chang.cn | | 5 | 应秀珍 | 13030388368 | 酒店前台 | 20 | 1 | qiang48@hotmail.com | | 6 | 亢婷 | 18207598386 | 药品市场推广主管/专员 | 28 | 1 | ping50@hotmail.com | | 7 | 仰俊 | 13192184011 | 机场代表 | 24 | 0 | wcai@liang.net | | 8 | 匡洁 | 13622482447 | 汽车电工 | 24 | 1 | htang@gmail.com | | 9 | 程建华 | 13748396030 | 市场通路经理/主管 | 28 | 1 | fangguo@yahoo.com | | 10 | 岳荣 | 15080695604 | 培训督导 | 24 | 1 | fanglong@ding.com | +----+-----------+-------------+---------------------------------+------+--------+---------------------+ 10 rows in set (0.00 sec) ``` 创建索引 ```sql -- 创建普通索引 create index idx_user_name on tb_user (name); -- 创建唯一索引 create unique index idx_user_phone on tb_user (phone); -- 创建联合索引 create index idx_user_profession_age_status on tb_user (profession, age, status); -- 创建普通索引 create index idx_user_email on tb_user (email); -- 创建索引 create index idx_user_age on tb_user ( age ); -- 查看索引 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 | 9804 | NULL | NULL | | BTREE | | | YES | NULL | | tb_user | 0 | idx_user_phone | 1 | phone | A | 9804 | NULL | NULL | YES | BTREE | | | YES | NULL | | tb_user | 1 | idx_user_name | 1 | name | A | 9130 | NULL | NULL | YES | BTREE | | | YES | NULL | | tb_user | 1 | idx_user_profession_age_status | 1 | profession | A | 948 | NULL | NULL | YES | BTREE | | | YES | NULL | | tb_user | 1 | idx_user_profession_age_status | 2 | age | A | 6232 | NULL | NULL | YES | BTREE | | | YES | NULL | | tb_user | 1 | idx_user_profession_age_status | 3 | status | A | 7596 | NULL | NULL | YES | BTREE | | | YES | NULL | | tb_user | 1 | idx_user_email | 1 | email | A | 9569 | NULL | NULL | YES | BTREE | | | YES | NULL | | tb_user | 1 | idx_user_age | 1 | age | A | 11 | NULL | NULL | YES | BTREE | | | YES | NULL | +---------+------------+--------------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+ 8 rows in set (0.03 sec) ``` 查看执行计划 ```sql mysql> explain select * from tb_user where profession = '法务经理'; +----+-------------+---------+------------+------+--------------------------------+--------------------------------+---------+-------+------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+------+--------------------------------+--------------------------------+---------+-------+------+----------+-------+ | 1 | SIMPLE | tb_user | NULL | ref | idx_user_profession_age_status | idx_user_profession_age_status | 203 | const | 12 | 100.00 | NULL | +----+-------------+---------+------------+------+--------------------------------+--------------------------------+---------+-------+------+----------+-------+ 1 row in set, 1 warning (0.00 sec) -- 创建一个单列索引 create index idx_user_profession on tb_user (profession); -- 性能分析 explain select * from tb_user where profession = '法务经理'; +----+-------------+---------+------------+------+----------------------------------------------------+--------------------------------+---------+-------+------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+------+----------------------------------------------------+--------------------------------+---------+-------+------+----------+-------+ | 1 | SIMPLE | tb_user | NULL | ref | idx_user_profession_age_status,idx_user_profession | idx_user_profession_age_status | 203 | const | 12 | 100.00 | NULL | +----+-------------+---------+------------+------+----------------------------------------------------+--------------------------------+---------+-------+------+----------+-------+ 1 row in set, 1 warning (0.00 sec) -- 查看执行计划（use index）使用单列索引idx_user_profession explain select * from tb_user use index (idx_user_profession) where profession = '法务经理'; +----+-------------+---------+------------+------+---------------------+---------------------+---------+-------+------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+------+---------------------+---------------------+---------+-------+------+----------+-------+ | 1 | SIMPLE | tb_user | NULL | ref | idx_user_profession | idx_user_profession | 203 | const | 12 | 100.00 | NULL | +----+-------------+---------+------------+------+---------------------+---------------------+---------+-------+------+----------+-------+ 1 row in set, 1 warning (0.01 sec) -- 查看执行计划（ignore index）,忽略单列索引idx_user_profession explain select * from tb_user ignore index (idx_user_profession) where profession = '法务经理'; +----+-------------+---------+------------+------+--------------------------------+--------------------------------+---------+-------+------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+------+--------------------------------+--------------------------------+---------+-------+------+----------+-------+ | 1 | SIMPLE | tb_user | NULL | ref | idx_user_profession_age_status | idx_user_profession_age_status | 203 | const | 12 | 100.00 | NULL | +----+-------------+---------+------------+------+--------------------------------+--------------------------------+---------+-------+------+----------+-------+ 1 row in set, 1 warning (0.01 sec) -- 查看执行计划（force index）强制使用单列索引idx_user_profession explain select * from tb_user force index (idx_user_profession) where profession = '法务经理'; +----+-------------+---------+------------+------+---------------------+---------------------+---------+-------+------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+------+---------------------+---------------------+---------+-------+------+----------+-------+ | 1 | SIMPLE | tb_user | NULL | ref | idx_user_profession | idx_user_profession | 203 | const | 12 | 100.00 | NULL | +----+-------------+---------+------------+------+---------------------+---------------------+---------+-------+------+----------+-------+ 1 row in set, 1 warning (0.00 sec) ``` ### 6.4、覆盖索引 查询使用了索引，并且需要返回的列，在该索引中已经全部能够找到 尽量使用覆盖索引，减少`select * ` extra额外信息： - using index condition ： 查找使用了索引，但是需要回表查询数据（二级索引->聚集索引） - using where; using index: 查找使用了索引，但是需要的数据都在索引列中能找到，所以不需要回表查询数据 ![](img/cover-index.png) 示例 ```sql explain select id, profession from tb_user where profession = '法务经理' and age = 27 and status = 1; explain select id, profession, age, status from tb_user where profession = '法务经理' and age = 27 and status = 1; explain select id, profession, age, status, name from tb_user where profession = '法务经理' and age = 27 and status = 1; explain select * from tb_user where profession = '法务经理' and age = 27 and status = 1; ``` 测试 ```sql -- 查看索引 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 | 9804 | NULL | NULL | | BTREE | | | YES | NULL | | tb_user | 0 | idx_user_phone | 1 | phone | A | 9804 | NULL | NULL | YES | BTREE | | | YES | NULL | | tb_user | 1 | idx_user_name | 1 | name | A | 9130 | NULL | NULL | YES | BTREE | | | YES | NULL | | tb_user | 1 | idx_user_profession_age_status | 1 | profession | A | 948 | NULL | NULL | YES | BTREE | | | YES | NULL | | tb_user | 1 | idx_user_profession_age_status | 2 | age | A | 6232 | NULL | NULL | YES | BTREE | | | YES | NULL | | tb_user | 1 | idx_user_profession_age_status | 3 | status | A | 7596 | NULL | NULL | YES | BTREE | | | YES | NULL | | tb_user | 1 | idx_user_email | 1 | email | A | 9569 | NULL | NULL | YES | BTREE | | | YES | NULL | | tb_user | 1 | idx_user_age | 1 | age | A | 11 | NULL | NULL | YES | BTREE | | | YES | NULL | | tb_user | 1 | idx_user_profession | 1 | profession | A | 948 | NULL | NULL | YES | BTREE | | | YES | NULL | +---------+------------+--------------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+ 9 rows in set (0.06 sec) -- 删除多余的索引 drop index idx_user_age on tb_user; drop index idx_user_profession on tb_user; drop index idx_user_email on tb_user; 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 | 9804 | NULL | NULL | | BTREE | | | YES | NULL | | tb_user | 0 | idx_user_phone | 1 | phone | A | 9804 | NULL | NULL | YES | BTREE | | | YES | NULL | | tb_user | 1 | idx_user_name | 1 | name | A | 9130 | NULL | NULL | YES | BTREE | | | YES | NULL | | tb_user | 1 | idx_user_profession_age_status | 1 | profession | A | 948 | NULL | NULL | YES | BTREE | | | YES | NULL | | tb_user | 1 | idx_user_profession_age_status | 2 | age | A | 6232 | NULL | NULL | YES | BTREE | | | YES | NULL | | tb_user | 1 | idx_user_profession_age_status | 3 | status | A | 7596 | NULL | NULL | YES | BTREE | | | YES | NULL | +---------+------------+--------------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+ 6 rows in set (0.00 sec) -- 查看数据 select * from tb_user limit 10; +----+-----------+-------------+---------------------------------+------+--------+---------------------+ | id | name | phone | profession | age | status | email | +----+-----------+-------------+---------------------------------+------+--------+---------------------+ | 1 | 费阳 | 13777763170 | 法务经理 | 27 | 1 | wyao@gmail.com | | 2 | 祁海燕 | 13400806360 | 日式厨师 | 23 | 0 | jwan@jin.cn | | 3 | 姬秀英 | 18281241586 | 食品/饮料研发 | 29 | 0 | li97@wang.cn | | 4 | 官桂芳 | 15625851781 | 前台接待/总机/接待生 | 20 | 1 | fpeng@chang.cn | | 5 | 应秀珍 | 13030388368 | 酒店前台 | 20 | 1 | qiang48@hotmail.com | | 6 | 亢婷 | 18207598386 | 药品市场推广主管/专员 | 28 | 1 | ping50@hotmail.com | | 7 | 仰俊 | 13192184011 | 机场代表 | 24 | 0 | wcai@liang.net | | 8 | 匡洁 | 13622482447 | 汽车电工 | 24 | 1 | htang@gmail.com | | 9 | 程建华 | 13748396030 | 市场通路经理/主管 | 28 | 1 | fangguo@yahoo.com | | 10 | 岳荣 | 15080695604 | 培训督导 | 24 | 1 | fanglong@ding.com | +----+-----------+-------------+---------------------------------+------+--------+---------------------+ 10 rows in set (0.00 sec) -- 查询数据 select * from tb_user where profession = '法务经理' and age = 27 and status = 1; +----+--------+-------------+--------------+------+--------+----------------+ | id | name | phone | profession | age | status | email | +----+--------+-------------+--------------+------+--------+----------------+ | 1 | 费阳 | 13777763170 | 法务经理 | 27 | 1 | wyao@gmail.com | +----+--------+-------------+--------------+------+--------+----------------+ 1 row in set (0.01 sec) explain select * from tb_user where profession = '法务经理' and age = 27 and status = 1; +----+-------------+---------+------------+------+--------------------------------+--------------------------------+---------+-------------------+------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+------+--------------------------------+--------------------------------+---------+-------------------+------+----------+-------+ | 1 | SIMPLE | tb_user | NULL | ref | idx_user_profession_age_status | idx_user_profession_age_status | 213 | const,const,const | 1 | 100.00 | NULL | +----+-------------+---------+------------+------+--------------------------------+--------------------------------+---------+-------------------+------+----------+-------+ 1 row in set, 1 warning (0.00 sec) -- 使用了索引字段 Using index explain select id, profession from tb_user where profession = '法务经理' and age = 27 and status = 1; +----+-------------+---------+------------+------+--------------------------------+--------------------------------+---------+-------------------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+------+--------------------------------+--------------------------------+---------+-------------------+------+----------+-------------+ | 1 | SIMPLE | tb_user | NULL | ref | idx_user_profession_age_status | idx_user_profession_age_status | 213 | const,const,const | 1 | 100.00 | Using index | +----+-------------+---------+------------+------+--------------------------------+--------------------------------+---------+-------------------+------+----------+-------------+ 1 row in set, 1 warning (0.00 sec) -- 使用了索引字段 Using index explain select id, profession, age, status from tb_user where profession = '法务经理' and age = 27 and status = 1; +----+-------------+---------+------------+------+--------------------------------+--------------------------------+---------+-------------------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+------+--------------------------------+--------------------------------+---------+-------------------+------+----------+-------------+ | 1 | SIMPLE | tb_user | NULL | ref | idx_user_profession_age_status | idx_user_profession_age_status | 213 | const,const,const | 1 | 100.00 | Using index | +----+-------------+---------+------------+------+--------------------------------+--------------------------------+---------+-------------------+------+----------+-------------+ 1 row in set, 1 warning (0.00 sec) explain select id, profession, age, status, name from tb_user where profession = '法务经理' and age = 27 and status = 1; +----+-------------+---------+------------+------+--------------------------------+--------------------------------+---------+-------------------+------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+------+--------------------------------+--------------------------------+---------+-------------------+------+----------+-------+ | 1 | SIMPLE | tb_user | NULL | ref | idx_user_profession_age_status | idx_user_profession_age_status | 213 | const,const,const | 1 | 100.00 | NULL | +----+-------------+---------+------------+------+--------------------------------+--------------------------------+---------+-------------------+------+----------+-------+ 1 row in set, 1 warning (0.00 sec) ``` ### 6.5、思考 问题： 一张表，有四个字段: ``` id, username, password, status ``` 由于数据量较大，需要对以下SQL语句进行优化，该如何进行才是最优方案 ```sql select id, username, password from tb_user where username = 'Tom'; ``` 解答： 需要对`username, password`创建联合索引 联合索引的叶子节点存储了`id`字段，可以`避免回表查询`，从而提高查询效率 ```sql create index idx_user_username_password on tb_user (username, password); ``` ### 6.6、前缀索引 当字段类型为字符串（varchar, text等）时，有时候需要索引很长的字符串， 这会让索引变得很大，查询时，浪费大量的磁盘IO，影响查询效率。 此时可以只将字符串的一部分前缀，建立索引，这样可以打打节约空间，从而提高索引效率。 1、语法 ```sql create index index_name on table_name (column(n)); ``` 2、前缀长度 可以根据索引的选择性来决定，而选择性是指不重复的索引值（基数）和数据表的记录总数的比值 索引选择性越高则查询效率越高 唯一索引的选择性是1，这是最好的索引选择性，性能也是最好的 ```sql -- 求取数据总条数 select count(*) from tb_user; +----------+ | count(*) | +----------+ | 10000 | +----------+ 1 row in set (0.07 sec) -- 求取email不为空的数据条数 select count(email) from tb_user; +--------------+ | count(email) | +--------------+ | 10000 | +--------------+ 1 row in set (0.03 sec) -- 求取email不重复的数据条数 select count(distinct email) from tb_user; +-----------------------+ | count(distinct email) | +-----------------------+ | 9569 | +-----------------------+ 1 row in set (0.02 sec) -- 计算选择性 select count(distinct email) / count(*) from tb_user; +----------------------------------+ | count(distinct email) / count(*) | +----------------------------------+ | 0.9569 | +----------------------------------+ 1 row in set (0.01 sec) -- 截取前缀计算选择性 select count(distinct substring(email, 1, 9)) / count(*) from tb_user; +---------------------------------------------------+ | count(distinct substring(email, 1, 9)) / count(*) | +---------------------------------------------------+ | 0.8633 | +---------------------------------------------------+ 1 row in set (0.02 sec) ``` 建立前缀索引 ```sql -- 查看当前表中存在的索引 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 | 9804 | NULL | NULL | | BTREE | | | YES | NULL | | tb_user | 0 | idx_user_phone | 1 | phone | A | 9804 | NULL | NULL | YES | BTREE | | | YES | NULL | | tb_user | 1 | idx_user_name | 1 | name | A | 9130 | NULL | NULL | YES | BTREE | | | YES | NULL | | tb_user | 1 | idx_user_profession_age_status | 1 | profession | A | 948 | NULL | NULL | YES | BTREE | | | YES | NULL | | tb_user | 1 | idx_user_profession_age_status | 2 | age | A | 6232 | NULL | NULL | YES | BTREE | | | YES | NULL | | tb_user | 1 | idx_user_profession_age_status | 3 | status | A | 7596 | NULL | NULL | YES | BTREE | | | YES | NULL | +---------+------------+--------------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+ 6 rows in set (0.05 sec) -- 创建前缀索引 create index idx_email_5 on tb_user(email(5)); -- 再次查看索引（注意字段：Sub_part） 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 | 9804 | NULL | NULL | | BTREE | | | YES | NULL | | tb_user | 0 | idx_user_phone | 1 | phone | A | 9804 | NULL | NULL | YES | BTREE | | | YES | NULL | | tb_user | 1 | idx_user_name | 1 | name | A | 9130 | NULL | NULL | YES | BTREE | | | YES | NULL | | tb_user | 1 | idx_user_profession_age_status | 1 | profession | A | 948 | NULL | NULL | YES | BTREE | | | YES | NULL | | tb_user | 1 | idx_user_profession_age_status | 2 | age | A | 6232 | NULL | NULL | YES | BTREE | | | YES | NULL | | tb_user | 1 | idx_user_profession_age_status | 3 | status | A | 7596 | NULL | NULL | YES | BTREE | | | YES | NULL | | tb_user | 1 | idx_email_5 | 1 | email | A | 3955 | 5 | NULL | YES | BTREE | | | YES | NULL | +---------+------------+--------------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+ 7 rows in set (0.01 sec) ``` 使用索引 ```sql -- 查询数据 select * from tb_user where email = 'wyao@gmail.com'; +----+--------+-------------+--------------+------+--------+----------------+ | id | name | phone | profession | age | status | email | +----+--------+-------------+--------------+------+--------+----------------+ | 1 | 费阳 | 13777763170 | 法务经理 | 27 | 1 | wyao@gmail.com | +----+--------+-------------+--------------+------+--------+----------------+ 1 row in set (0.01 sec) -- 查看执行计划 explain select * from tb_user where email = 'wyao@gmail.com'; +----+-------------+---------+------------+------+---------------+-------------+---------+-------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+------+---------------+-------------+---------+-------+------+----------+-------------+ | 1 | SIMPLE | tb_user | NULL | ref | idx_email_5 | idx_email_5 | 23 | const | 1 | 100.00 | Using where | +----+-------------+---------+------------+------+---------------+-------------+---------+-------+------+----------+-------------+ 1 row in set, 1 warning (0.01 sec) ``` 查询流程 ![](img/pre-index.png) ### 6.7、单列索引和联合索引 - 单列索引：一个索引只包含单个列 - 联合索引：一个索引包含了多个列（创建索引时，需要考虑字段的先后顺序） 业务场景中，如果存在多个查询条件，考虑针对查询字段建立索引时，建议建立联合索引，而非单列索引 ![](img/union-index.png) 示例 ```sql -- 查看索引 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 | 9804 | NULL | NULL | | BTREE | | | YES | NULL | | tb_user | 0 | idx_user_phone | 1 | phone | A | 9804 | NULL | NULL | YES | BTREE | | | YES | NULL | | tb_user | 1 | idx_user_name | 1 | name | A | 9130 | NULL | NULL | YES | BTREE | | | YES | NULL | | tb_user | 1 | idx_user_profession_age_status | 1 | profession | A | 948 | NULL | NULL | YES | BTREE | | | YES | NULL | | tb_user | 1 | idx_user_profession_age_status | 2 | age | A | 6232 | NULL | NULL | YES | BTREE | | | YES | NULL | | tb_user | 1 | idx_user_profession_age_status | 3 | status | A | 7596 | NULL | NULL | YES | BTREE | | | YES | NULL | | tb_user | 1 | idx_email_5 | 1 | email | A | 3955 | 5 | NULL | YES | BTREE | | | YES | NULL | +---------+------------+--------------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+ 7 rows in set (0.07 sec) -- 查询数据 select id, phone, name from tb_user where phone = '13777763170' and name = '费阳'; +----+-------------+--------+ | id | phone | name | +----+-------------+--------+ | 1 | 13777763170 | 费阳 | +----+-------------+--------+ 1 row in set (0.00 sec) -- 查看执行计划 explain select id, phone, name from tb_user where phone = '13777763170' and name = '费阳'; +----+-------------+---------+------------+-------+------------------------------+----------------+---------+-------+------+----------+-------+ | 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_name | idx_user_phone | 47 | const | 1 | 100.00 | NULL | +----+-------------+---------+------------+-------+------------------------------+----------------+---------+-------+------+----------+-------+ 1 row in set, 1 warning (0.01 sec) -- 创建联合索引 create unique index idx_user_phone_name on tb_user(phone, name); -- 查看执行计划 -- 多条件联合查询时，MySQL优化器会评估哪个字段的索引效率更高，会选择该索引完成本次查询 explain select id, phone, name from tb_user where phone = '13777763170' and name = '费阳'; +----+-------------+---------+------------+-------+--------------------------------------------------+----------------+---------+-------+------+----------+-------+ | 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_name,idx_user_name | idx_user_phone | 47 | const | 1 | 100.00 | NULL | +----+-------------+---------+------------+-------+--------------------------------------------------+----------------+---------+-------+------+----------+-------+ 1 row in set, 1 warning (0.00 sec) -- 查看执行计划（指定使用联合索引） -- Using index 用到覆盖索引，避免回表查询 explain select id, phone, name from tb_user use index(idx_user_phone_name) where phone = '13777763170' and name = '费阳'; +----+-------------+---------+------------+-------+---------------------+---------------------+---------+-------------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+-------+---------------------+---------------------+---------+-------------+------+----------+-------------+ | 1 | SIMPLE | tb_user | NULL | const | idx_user_phone_name | idx_user_phone_name | 130 | const,const | 1 | 100.00 | Using index | +----+-------------+---------+------------+-------+---------------------+---------------------+---------+-------------+------+----------+-------------+ 1 row in set, 1 warning (0.00 sec) ``` ## 7、索引失效的场景 ### 7.1、索引列运算 不要在索引列上进行运算操作，否则索引将失效 ```sql -- 查看数据 mysql> select * from tb_user; +----+--------+-------------+------------+------+--------+-------------+ | id | name | phone | profession | age | status | email | +----+--------+-------------+------------+------+--------+-------------+ | 1 | 张飞 | 17712345678 | 美术 | 23 | 1 | 123@qq.com | | 2 | 关羽 | 17722345678 | 物理 | 24 | 1 | 3339@qq.com | | 3 | 刘备 | 17732345678 | 数学 | 25 | 0 | 666@qq.com | | 4 | 孙权 | 17742345678 | 语文 | 20 | 1 | 111@qq.com | +----+--------+-------------+------------+------+--------+-------------+ 4 rows in set (0.00 sec) -- 查看索引 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 | | 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 (0.01 sec) -- 按照手机号查询数据 mysql> select * from tb_user where phone = '17712345678'; +----+--------+-------------+------------+------+--------+------------+ | id | name | phone | profession | age | status | email | +----+--------+-------------+------------+------+--------+------------+ | 1 | 张飞 | 17712345678 | 美术 | 23 | 1 | 123@qq.com | +----+--------+-------------+------------+------+--------+------------+ 1 row in set (0.00 sec) -- 分析索引使用情况 explain select * from tb_user where phone = '17712345678'; +----+-------------+---------+------------+-------+----------------+----------------+---------+-------+------+----------+-------+ | 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 (0.00 sec) -- 查询手机号后两位 select * from tb_user where substring(phone, 10, 2) = '27'; +----+--------+-------------+------------+------+--------+------------+ | id | name | phone | profession | age | status | email | +----+--------+-------------+------------+------+--------+------------+ | 1 | 张飞 | 17712345678 | 美术 | 23 | 1 | 123@qq.com | +----+--------+-------------+------------+------+--------+------------+ 1 row in set (0.00 sec) -- 查看执行效率 explain select * from tb_user where substring(phone, 10, 2) = '27'; +----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+ | 1 | SIMPLE | tb_user | NULL | ALL | NULL | NULL | NULL | NULL | 4 | 100.00 | Using where | +----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+ 1 row in set, 1 warning (0.01 sec) ``` ### 7.2、字符串不加引号 字符串类型字段使用时，不加引号，索引将失效 ```sql -- 字符串查询可以不加引号 select * from tb_user where phone = 17712345678; +----+--------+-------------+------------+------+--------+------------+ | id | name | phone | profession | age | status | email | +----+--------+-------------+------------+------+--------+------------+ | 1 | 张飞 | 17712345678 | 美术 | 23 | 1 | 123@qq.com | +----+--------+-------------+------------+------+--------+------------+ 1 row in set (0.00 sec) -- 不加引号的字符串查询没有走索引 explain select * from tb_user where phone = 17712345678; +----+-------------+---------+------------+------+----------------+------+---------+------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+------+----------------+------+---------+------+------+----------+-------------+ | 1 | SIMPLE | tb_user | NULL | ALL | idx_user_phone | NULL | NULL | NULL | 4 | 25.00 | Using where | +----+-------------+---------+------------+------+----------------+------+---------+------+------+----------+-------------+ 1 row in set, 3 warnings (0.00 sec) ``` ### 7.3、模糊查询 - 如果仅仅是尾部模糊匹配，索引不会失效； - 如果是头部模糊匹配，索引失效 ```sql -- 尾部模糊查询 select * from tb_user where profession like '美%'; +----+--------+-------------+------------+------+--------+------------+ | id | name | phone | profession | age | status | email | +----+--------+-------------+------------+------+--------+------------+ | 1 | 张飞 | 17712345678 | 美术 | 23 | 1 | 123@qq.com | +----+--------+-------------+------------+------+--------+------------+ 1 row in set (0.00 sec) -- 尾部模糊匹配，索引不会失效； explain select * from tb_user where profession like '美%'; +----+-------------+---------+------------+-------+--------------------------------+--------------------------------+---------+------+------+----------+-----------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+-------+--------------------------------+--------------------------------+---------+------+------+----------+-----------------------+ | 1 | SIMPLE | tb_user | NULL | range | idx_user_profession_age_status | idx_user_profession_age_status | 43 | NULL | 1 | 100.00 | Using index condition | +----+-------------+---------+------------+-------+--------------------------------+--------------------------------+---------+------+------+----------+-----------------------+ 1 row in set, 1 warning (0.00 sec) -- 头部模糊匹配，索引失效 explain select * from tb_user where profession like '%术'; +----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+ | 1 | SIMPLE | tb_user | NULL | ALL | NULL | NULL | NULL | NULL | 4 | 25.00 | Using where | +----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+ 1 row in set, 1 warning (0.01 sec) ``` ### 7.4、or连接的条件 用or分割开的条件，如果or前的条件中有的列有索引，而后面的列中没有索引，那么涉及的索引都不会被用到 ```sql select * from tb_user where id = 1 or age = 23; select * from tb_user where phone = '17742345678' or age = 23; ``` 由于age没有索引，所以即使id，phone有索引，索引也会失效。age也需要建立索引 示例 ```sql -- 查看表数据 select * from tb_user; +----+--------+-------------+------------+------+--------+-------------+ | id | name | phone | profession | age | status | email | +----+--------+-------------+------------+------+--------+-------------+ | 1 | 张飞 | 17712345678 | 美术 | 23 | 1 | 123@qq.com | | 2 | 关羽 | 17722345678 | 物理 | 24 | 1 | 3339@qq.com | | 3 | 刘备 | 17732345678 | 数学 | 25 | 0 | 666@qq.com | | 4 | 孙权 | 17742345678 | 语文 | 20 | 1 | 111@qq.com | +----+--------+-------------+------------+------+--------+-------------+ 4 rows in set (0.01 sec) -- 查看索引 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) -- 查询数据 select * from tb_user where id = 1 or age = 23; +----+--------+-------------+------------+------+--------+------------+ | id | name | phone | profession | age | status | email | +----+--------+-------------+------------+------+--------+------------+ | 1 | 张飞 | 17712345678 | 美术 | 23 | 1 | 123@qq.com | +----+--------+-------------+------------+------+--------+------------+ 1 row in set (0.00 sec) -- 分析 explain select * from tb_user where id = 1 or age = 23; +----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+ | 1 | SIMPLE | tb_user | NULL | ALL | PRIMARY | NULL | NULL | NULL | 4 | 43.75 | Using where | +----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+ 1 row in set, 1 warning (0.00 sec) -- 分析 explain select * from tb_user where phone = '17742345678' or age = 23; +----+-------------+---------+------------+------+----------------+------+---------+------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+------+----------------+------+---------+------+------+----------+-------------+ | 1 | SIMPLE | tb_user | NULL | ALL | idx_user_phone | NULL | NULL | NULL | 4 | 43.75 | Using where | +----+-------------+---------+------------+------+----------------+------+---------+------+------+----------+-------------+ 1 row in set, 1 warning (0.15 sec) -- 创建索引 create index idx_user_age on tb_user ( age ); -- 再次分析性能 explain select * from tb_user where id = 1 or age = 23; +----+-------------+---------+------------+-------------+----------------------+----------------------+---------+------+------+----------+------------------------------------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+-------------+----------------------+----------------------+---------+------+------+----------+------------------------------------------------+ | 1 | SIMPLE | tb_user | NULL | index_merge | PRIMARY,idx_user_age | PRIMARY,idx_user_age | 4,5 | NULL | 2 | 100.00 | Using union(PRIMARY,idx_user_age); Using where | +----+-------------+---------+------------+-------------+----------------------+----------------------+---------+------+------+----------+------------------------------------------------+ 1 row in set, 1 warning (0.00 sec) explain select * from tb_user where phone = '17742345678' or age = 23; +----+-------------+---------+------------+-------------+-----------------------------+-----------------------------+---------+------+------+----------+-------------------------------------------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+-------------+-----------------------------+-----------------------------+---------+------+------+----------+-------------------------------------------------------+ | 1 | SIMPLE | tb_user | NULL | index_merge | idx_user_phone,idx_user_age | idx_user_phone,idx_user_age | 47,5 | NULL | 2 | 100.00 | Using union(idx_user_phone,idx_user_age); Using where | +----+-------------+---------+------------+-------------+-----------------------------+-----------------------------+---------+------+------+----------+-------------------------------------------------------+ 1 row in set, 1 warning (0.00 sec) ``` ### 7.5、数据分布影响 如果MySQL性能评估使用索引比全表更慢，则不使用索引。 ```sql select * from tb_user where phone >= '17742345678'; +----+--------+-------------+------------+------+--------+------------+ | id | name | phone | profession | age | status | email | +----+--------+-------------+------------+------+--------+------------+ | 4 | 孙权 | 17742345678 | 语文 | 20 | 1 | 111@qq.com | +----+--------+-------------+------------+------+--------+------------+ 1 row in set (0.00 sec) explain select * from tb_user where phone >= '17742345678'; +----+-------------+---------+------------+-------+----------------+----------------+---------+------+------+----------+-----------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+-------+----------------+----------------+---------+------+------+----------+-----------------------+ | 1 | SIMPLE | tb_user | NULL | range | idx_user_phone | idx_user_phone | 47 | NULL | 1 | 100.00 | Using index condition | +----+-------------+---------+------------+-------+----------------+----------------+---------+------+------+----------+-----------------------+ 1 row in set, 1 warning (0.00 sec) explain select * from tb_user where phone >= '17712345670'; +----+-------------+---------+------------+-------+----------------+----------------+---------+------+------+----------+-----------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+-------+----------------+----------------+---------+------+------+----------+-----------------------+ | 1 | SIMPLE | tb_user | NULL | range | idx_user_phone | idx_user_phone | 47 | NULL | 4 | 100.00 | Using index condition | +----+-------------+---------+------------+-------+----------------+----------------+---------+------+------+----------+-----------------------+ 1 row in set, 1 warning (0.00 sec) ``` `is null` 和 `is not null` 走不走索引，主要看表中的数据。 ```sql -- is null explain select * from tb_user where profession is null; +----+-------------+---------+------------+------+--------------------------------+--------------------------------+---------+-------+------+----------+-----------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+------+--------------------------------+--------------------------------+---------+-------+------+----------+-----------------------+ | 1 | SIMPLE | tb_user | NULL | ref | idx_user_profession_age_status | idx_user_profession_age_status | 43 | const | 1 | 100.00 | Using index condition | +----+-------------+---------+------------+------+--------------------------------+--------------------------------+---------+-------+------+----------+-----------------------+ 1 row in set, 1 warning (0.01 sec) -- is not null explain select * from tb_user where profession is not null; +----+-------------+---------+------------+-------+--------------------------------+--------------------------------+---------+------+------+----------+-----------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+-------+--------------------------------+--------------------------------+---------+------+------+----------+-----------------------+ | 1 | SIMPLE | tb_user | NULL | range | idx_user_profession_age_status | idx_user_profession_age_status | 43 | NULL | 4 | 100.00 | Using index condition | +----+-------------+---------+------------+-------+--------------------------------+--------------------------------+---------+------+------+----------+-----------------------+ 1 row in set, 1 warning (0.00 sec) -- 将数据更新为null update tb_user set profession = null; -- 再次查看性能 explain select * from tb_user where profession is null; +----+-------------+---------+------------+------+--------------------------------+--------------------------------+---------+-------+------+----------+-----------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+------+--------------------------------+--------------------------------+---------+-------+------+----------+-----------------------+ | 1 | SIMPLE | tb_user | NULL | ref | idx_user_profession_age_status | idx_user_profession_age_status | 43 | const | 4 | 100.00 | Using index condition | +----+-------------+---------+------------+------+--------------------------------+--------------------------------+---------+-------+------+----------+-----------------------+ 1 row in set, 1 warning (0.00 sec) -- is not null explain select * from tb_user where profession is not null; +----+-------------+---------+------------+-------+--------------------------------+--------------------------------+---------+------+------+----------+-----------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+-------+--------------------------------+--------------------------------+---------+------+------+----------+-----------------------+ | 1 | SIMPLE | tb_user | NULL | range | idx_user_profession_age_status | idx_user_profession_age_status | 43 | NULL | 1 | 100.00 | Using index condition | +----+-------------+---------+------------+-------+--------------------------------+--------------------------------+---------+------+------+----------+-----------------------+ 1 row in set, 1 warning (0.00 sec) ``` ### 7.6、范围查询 联合索引中，出现范围查询（>、<），范围查询右侧的列索引失效 ```sql select * from tb_user where profession = '美术' and age > 22 and status = 1; +----+--------+-------------+------------+------+--------+------------+ | id | name | phone | profession | age | status | email | +----+--------+-------------+------------+------+--------+------------+ | 1 | 张飞 | 17712345678 | 美术 | 23 | 1 | 123@qq.com | +----+--------+-------------+------------+------+--------+------------+ 1 row in set (0.00 sec) -- age使用了大于号>，只有profession和age用到了索引 explain select * from tb_user where profession = '美术' and age > 22 and status = 1; +----+-------------+---------+------------+-------+--------------------------------+--------------------------------+---------+------+------+----------+-----------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+-------+--------------------------------+--------------------------------+---------+------+------+----------+-----------------------+ | 1 | SIMPLE | tb_user | NULL | range | idx_user_profession_age_status | idx_user_profession_age_status | 48 | NULL | 1 | 25.00 | Using index condition | +----+-------------+---------+------------+-------+--------------------------------+--------------------------------+---------+------+------+----------+-----------------------+ 1 row in set, 1 warning (0.01 sec) -- 三个字段都用到了索引 explain select * from tb_user where profession = '美术' and age >= 22 and status = 1; +----+-------------+---------+------------+-------+--------------------------------+--------------------------------+---------+------+------+----------+-----------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+-------+--------------------------------+--------------------------------+---------+------+------+----------+-----------------------+ | 1 | SIMPLE | tb_user | NULL | range | idx_user_profession_age_status | idx_user_profession_age_status | 53 | NULL | 1 | 25.00 | Using index condition | +----+-------------+---------+------------+-------+--------------------------------+--------------------------------+---------+------+------+----------+-----------------------+ 1 row in set, 1 warning (0.00 sec) ``` ## 8、索引设计原则 1. 数据量较大(100W)，且查询比较频繁的表建立索引 2. 针对于常作为查询条件（where）、排序（order by）、分组（group by）操作的字段建立索引 3. 尽量选择区分度高的列作为索引，尽量建立唯一索引，区分度越高，使用索引的效率越高 4. 如果是字符串类型，字段的长度较长，可以针对字段的特点，建立前缀索引 5. 尽量使用联合索引，减少单列索引，查询时，联合索引很多时候可以覆盖索引，节省存储空间，避免回表，提高查询效率 6. 要控制索引的数量，索引并不是多多益善，索引越多，维护索引结构的代价也就越大，会影响增删改的效率 7. 如果索引列不能存储null值，请在创建表的时候使用`not null`约束它。当优化器知道每列是否包含null值时，它可以更好地确定哪个索引最有效的用于查询 ## 9、总结 1. 索引概述 - 索引是高效获取数据的数据结构 - 索引是有序的数据结构 3. 索引结构 - B+Tree： - 所有数据都出现在叶子节点 - 叶子节点行成双向链表 - Hash： - Memory存储引擎支持 - 优点：性能较高（计算hash值，直接检索数据） - 缺点：只支持精确匹配，不支持范围查询，不支持索引排序 3. 索引分类 - 常规分类： - 主键索引 primary key - 唯一索引 unique - 常规索引 - 全文索引 - InnoDB: - 聚集索引：叶子节点存放行数据（默认主键索引 、第一个唯一主键、自动生成隐藏的row.id） - 二级索引：叶子节点存放主键 4. 索引语法 ```sql -- 创建索引 create [unique] index on (); -- 查看索引 show index from -- 删除索引 drop index on ``` 5. SQL性能分析 - 执行频次：针对查询为主的sql进行优化 - 慢查询日志 - profile：每条SQL耗时情况 - explain 执行计划：评判SQL语句的性能 6. 索引使用 - 联合索引： - 最左前缀法则：最左边的列必须存在。最左边的列存在，中间跳过某些列，后面的列索引会失效 - 范围查询：范围查询右侧的列会失效，优先使用(>=、<=) - 索引失效： - 索引列进行函数计算 - 隐式类型转换：字符串不加引号 - like模糊匹配：前面加了百分号 - or连接的索引：一侧有索引，一侧没有索引 - 数据分布影响：全表扫描比索引还快 - SQL提示 - 使用索引 `use index` - 忽略索引 `ignore index` - 强制使用索引 `force index` - 覆盖索引 - 查询返回的列在索引结构中都包含了，不用走回表查询 - 回表查询：先走二级索引检索到行数据的id，再根据id到聚集索引查找行数据 - 前缀索引 - 字符串较长, 大文本字段 - 单列/联合索引 - 推荐使用联合索引，避免回表查询 7. 索引设计原则 - 表：数据量大，查询频次较高 - 字段：where/order by/group by之后的字段 - 索引 - 唯一索引 区分度高 - 联合索引 优先使用 - 前缀索引 字符串较长, 大文本字段