详解MySQL 索引+explain

mysql视频教程栏目今天着重介绍索引+explain，为需要面试的准备。

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一、索引的介绍

在mysql中,索引就是数据结构,已经在文件中按照索引进行排序好的结构.使用索引可以加快我们的查询速度,但是对我们的数据增删改效率会降低.因为一个网站大部分都是查询,我们主要优化select语句.

二、MySQL中索引的分类

普通索引 key唯一索引 unique key unique key 别名 别名可忽略 别名可忽略主键索引 primary key(字段)全文索引myisam引擎支持(只对英文进行索引,mysql版本5.6也支持),sphinx(中文搜索)混合索引 多个字段组成的索引.如 key key_index(title,email)

三、索引的基本操作

1、给表添加索引

create table t_index(    id int not null auto_increment,    title varchar(30) not null default '',    email varchar(30) not null default '',    primary key(id),    unique key uni_email(email) ,    key key_title(title))engine=innodb charset=utf8;

查看表

desc tablename

mysql> desc t_index;+-------+-------------+------+-----+---------+----------------+| Field | Type        | Null | Key | Default | Extra          |+-------+-------------+------+-----+---------+----------------+| id    | int(11)     | NO   | PRI | NULL    | auto_increment || title | varchar(30) | NO   | MUL |         |                || email | varchar(30) | NO   | UNI |         |                |+-------+-------------+------+-----+---------+----------------+3 rows in set (0.01 sec)

查看表的创建语句

show create table tbalename/G

mysql> show create table t_index/G;ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '/G' at line 1mysql> show create table t_indexG;*************************** 1. row ***************************       Table: t_indexCreate Table: CREATE TABLE `t_index` (  `id` int(11) NOT NULL AUTO_INCREMENT,  `title` varchar(30) NOT NULL DEFAULT '',  `email` varchar(30) NOT NULL DEFAULT '',  PRIMARY KEY (`id`),  UNIQUE KEY `uni_email` (`email`),  KEY `key_title` (`title`)) ENGINE=InnoDB DEFAULT CHARSET=utf81 row in set (0.00 sec)ERROR: No query specified

2、删除索引

删除主键索引

alter table table_name drop primary key;

注意：

mysql> alter table t_index drop primary key;ERROR 1075 (42000): Incorrect table definition; there can be only one auto column and it must be defined as a key

主键不一定是自增长，但是自增长一定是主键。

删除逐渐之前先要把主键索引的自增长去掉。

mysql> alter table t_index modify  id int not null;Query OK, 0 rows affected (0.05 sec)Records: 0  Duplicates: 0  Warnings: 0

再来删除主键

mysql> alter table t_index drop primary key;Query OK, 0 rows affected (0.04 sec)Records: 0  Duplicates: 0  Warnings: 0

删除普通和唯一的索引

alter table table_name drop key ‘索引的别名’

实际操作

mysql> alter table t_index drop key uni_email;Query OK, 0 rows affected (0.03 sec)Records: 0  Duplicates: 0  Warnings: 0

mysql> alter table t_index drop key key_title;Query OK, 0 rows affected (0.02 sec)Records: 0  Duplicates: 0  Warnings: 0

3、添加索引

alter table t_index add key key_title(title);alter table t_index add key uni_email(email);alter table t_index add primary key(id);

4、有无索引对比

create table article(id int not null auto_increment,no_index int,title varchar(30) not null default '',add_time datetime,primary key(id));

插入数据

mysql> insert into article(id,title,add_time) values(null,'ddsd1212123d',now());mysql> insert into article(title,add_time) select title,now() from article;Query OK, 10 rows affected (0.01 sec)Records: 10  Duplicates: 0  Warnings: 0mysql> update article set no_index=id;

有无索引查询数据对比

mysql> select * from article where no_index=1495298;+---------+----------+-----------+---------------------+| id      | no_index | title     | add_time            |+---------+----------+-----------+---------------------+| 1495298 |  1495298 | ddsd1123d | 2019-05-15 23:13:56 |+---------+----------+-----------+---------------------+1 row in set (0.28 sec)

mysql> select * from article where id=1495298;+---------+----------+-----------+---------------------+| id      | no_index | title     | add_time            |+---------+----------+-----------+---------------------+| 1495298 |  1495298 | ddsd1123d | 2019-05-15 23:13:56 |+---------+----------+-----------+---------------------+1 row in set (0.01 sec)

表结构

mysql> show create table articleG;*************************** 1. row ***************************       Table: articleCreate Table: CREATE TABLE `article` (  `id` int(11) NOT NULL AUTO_INCREMENT,  `no_index` int(11) DEFAULT NULL,  `title` varchar(30) NOT NULL DEFAULT '',  `add_time` datetime DEFAULT NULL,  PRIMARY KEY (`id`)) ENGINE=InnoDB AUTO_INCREMENT=1572824 DEFAULT CHARSET=utf81 row in set (0.00 sec)ERROR: No query specified

四、explain分析

使用explain可以对sql语句进行分析到底有没有使用到索引查询,从而更好的优化它.

我们只需要在select语句前面加上一句explain或者desc.

1、语法

explain|desc select * from tablename G;

2、分析

用刚才的两个有无索引对比看看

mysql> mysql> explain select * from article where no_index=1495298G;*************************** 1. row ***************************           id: 1  select_type: SIMPLE//单表查询        table: article//查询的表名   partitions: NULL         type: ALL//索引的类型，从好到坏的情况是：system>const>range>index>Allpossible_keys: NULL//可能使用到的索引          key: NULL//实际使用到的索引      key_len: NULL//索引的长度          ref: NULL         rows: 1307580//可能进行扫描表的行数     filtered: 10.00        Extra: Using where1 row in set, 1 warning (0.00 sec)ERROR: No query specified

mysql> explain select * from article where id=1495298G;*************************** 1. row ***************************           id: 1  select_type: SIMPLE        table: article   partitions: NULL         type: const//当对主键索引进行等值查询的时候出现constpossible_keys: PRIMARY          key: PRIMARY//实际使用到的所有primary索引      key_len: 4//索引的长度4 = int占4个字节          ref: const         rows: 1//所扫描的行数只有一行     filtered: 100.00        Extra: NULL1 row in set, 1 warning (0.00 sec)ERROR: No query specified

3、explain的type项分析

type项从优到差依次排序:

system:一般系统表只有一行记录的时候才会出现const:当对主键值进行等值查询的时候会出现,如where id=666666range:当对索引的值进行范围查询的时候会出现,如 where id<100000index:当我们查询的字段恰好是我们索引文件中的值,就会出现All:最差的一种情况,需要避免.

实际测试

mysql> use mysql;mysql> explain select * from userG;*************************** 1. row ***************************           id: 1  select_type: SIMPLE        table: user   partitions: NULL         type: ALLpossible_keys: NULL          key: NULL      key_len: NULL          ref: NULL         rows: 3     filtered: 100.00        Extra: NULL1 row in set, 1 warning (0.00 sec)

mysql> use test;mysql> explain select * from article where id=666666G;*************************** 1. row ***************************           id: 1  select_type: SIMPLE        table: article   partitions: NULL         type: constpossible_keys: PRIMARY          key: PRIMARY      key_len: 4          ref: const         rows: 1     filtered: 100.00        Extra: NULL

mysql> explain select * from article where id>666666G;mysql> explain select * from article where id<666666G;

mysql> explain select id  from article G;*************************** 1. row ***************************           id: 1  select_type: SIMPLE        table: article   partitions: NULL         type: indexpossible_keys: NULL          key: PRIMARY      key_len: 4          ref: NULL         rows: 1307580     filtered: 100.00        Extra: Using index1 row in set, 1 warning (0.00 sec)ERROR: No query specified

如果查询的字段在索引文件存在，那么就会直接从索引文件中进行查询，我们把这种查询称之为索引覆盖查询。

出现all，我们需要避免，因为进行全面扫描。

对于出现all的，可以给该字段增加普通索引查询

mysql> alter table article add key key_no_index(no_index);Query OK, 0 rows affected (1.92 sec)Records: 0  Duplicates: 0  Warnings: 0type为ref，应该是关联，但是ref是constmysql> explain select * from article where no_index=666666G;*************************** 1. row ***************************           id: 1  select_type: SIMPLE        table: article   partitions: NULL         type: refpossible_keys: key_no_index          key: key_no_index      key_len: 5          ref: const         rows: 1     filtered: 100.00        Extra: NULL1 row in set, 1 warning (0.00 sec)速度飞跃mysql> select * from article where no_index=666666;+--------+----------+-----------+---------------------+| id     | no_index | title     | add_time            |+--------+----------+-----------+---------------------+| 666666 |   666666 | ddsd1123d | 2019-05-15 23:13:55 |+--------+----------+-----------+---------------------+1 row in set (0.00 sec)

4、使用索引的场景

1、 经常出现在where后面的字段,我们需要给他加索引

2、order by 语句使用索引的优化

mysql> explain select * from article order by idG;*************************** 1. row ***************************           id: 1  select_type: SIMPLE        table: article   partitions: NULL         type: indexpossible_keys: NULL          key: PRIMARY      key_len: 4          ref: NULL         rows: 1307580     filtered: 100.00        Extra: NULL1 row in set, 1 warning (0.00 sec)ERROR: No query specifiedmysql> explain select * from article where id >0  order by idG;*************************** 1. row ***************************           id: 1  select_type: SIMPLE        table: article   partitions: NULL         type: rangepossible_keys: PRIMARY          key: PRIMARY      key_len: 4          ref: NULL         rows: 653790     filtered: 100.00        Extra: Using where1 row in set, 1 warning (0.01 sec)ERROR: No query specified

可以看出，即使是使用了索引但是几乎还是全表扫描。

加了where就少了一半

3、针对like的模糊查询索引的优化

where title like ‘%keyword%’ ====>全表扫描

where title like ‘keyword%’ ===>会使用到索引查询

给title加上铺索引

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6 查看详情

mysql> alter table article  add key key_index(title);Query OK, 0 rows affected (2.16 sec)Records: 0  Duplicates: 0  Warnings: 0mysql> show create table articleG;*************************** 1. row ***************************       Table: articleCreate Table: CREATE TABLE `article` (  `id` int(11) NOT NULL AUTO_INCREMENT,  `no_index` int(11) DEFAULT NULL,  `title` varchar(30) NOT NULL DEFAULT '',  `add_time` datetime DEFAULT NULL,  PRIMARY KEY (`id`),  KEY `key_no_index` (`no_index`),  KEY `key_index` (`title`)) ENGINE=InnoDB AUTO_INCREMENT=1507299 DEFAULT CHARSET=utf81 row in set (0.00 sec)

因为%没有出现在like关键字查询的最左边，所以可以使用到索引查询

只要是like左边出现了%，就是全表查询

mysql> explain select * from article where title like 'a%'G;*************************** 1. row ***************************           id: 1  select_type: SIMPLE        table: article   partitions: NULL         type: range//范围查询possible_keys: key_index          key: key_index      key_len: 92//          ref: NULL         rows: 1     filtered: 100.00        Extra: Using index condition1 row in set, 1 warning (0.00 sec)mysql> explain select * from article where title like '%a%'G;*************************** 1. row ***************************           id: 1  select_type: SIMPLE        table: article   partitions: NULL         type: ALL//全表查询possible_keys: NULL          key: NULL      key_len: NULL          ref: NULL         rows: 1307580     filtered: 11.11        Extra: Using where1 row in set, 1 warning (0.00 sec)

4、limit语句的索引使用优化

针对于limit语句的优化，我们可以在它前面加order by 索引字段

如果order by的字段是索引，会先去索引文件中查找指定行数的数据

mysql> explain select sql_no_cache  * from article limit 90000,10 G;*************************** 1. row ***************************           id: 1  select_type: SIMPLE        table: article   partitions: NULL         type: ALL//全表possible_keys: NULL          key: NULL      key_len: NULL          ref: NULL         rows: 1307580     filtered: 100.00        Extra: NULL1 row in set, 2 warnings (0.00 sec)ERROR: No query specifiedmysql> explain select sql_no_cache  * from article order by id  limit 90000,10 G;*************************** 1. row ***************************           id: 1  select_type: SIMPLE        table: article   partitions: NULL         type: indexpossible_keys: NULL          key: PRIMARY//使用到了索引      key_len: 4          ref: NULL         rows: 90010     filtered: 100.00        Extra: NULL1 row in set, 2 warnings (0.00 sec)ERROR: No query specified

另外一种针对于limit的优化方法:

索引覆盖+延时关联

原理:主要利用索引覆盖查询,把覆盖索引查询返回的id作为与我们要查询记录的id进行相关联,

mysql> select sql_no_cache  * from article limit 1000000,10;+---------+----------+----------------+---------------------+| id      | no_index | title          | add_time            |+---------+----------+----------------+---------------------+| 1196579 |  1196579 | ddsd12123123ad | 2019-05-15 23:13:56 || 1196580 |  1196580 | ddsd121231ad   | 2019-05-15 23:13:56 || 1196581 |  1196581 | ddsd1212123d   | 2019-05-15 23:13:56 || 1196582 |  1196582 | ddsd1123123d   | 2019-05-15 23:13:56 || 1196583 |  1196583 | ddsd1123d      | 2019-05-15 23:13:56 || 1196584 |  1196584 | ddsd1123d      | 2019-05-15 23:13:56 || 1196585 |  1196585 | ddsd1123d      | 2019-05-15 23:13:56 || 1196586 |  1196586 | ddsd1123d      | 2019-05-15 23:13:56 || 1196587 |  1196587 | ddsd1123d      | 2019-05-15 23:13:56 || 1196588 |  1196588 | ddsd1123d      | 2019-05-15 23:13:56 |+---------+----------+----------------+---------------------+10 rows in set, 1 warning (0.21 sec)mysql> select t1.* from article as t1 inner join (select id as pid from article  limit 10000,10) as t2 on t1.id=t2.pid;+-------+----------+----------------+---------------------+| id    | no_index | title          | add_time            |+-------+----------+----------------+---------------------+| 13058 |    13058 | ddsd12123123ad | 2019-05-15 23:13:49 || 13059 |    13059 | ddsd121231ad   | 2019-05-15 23:13:49 || 13060 |    13060 | ddsd1212123d   | 2019-05-15 23:13:49 || 13061 |    13061 | ddsd1123123d   | 2019-05-15 23:13:49 || 13062 |    13062 | ddsd1123d      | 2019-05-15 23:13:49 || 13063 |    13063 | ddsd1123d      | 2019-05-15 23:13:49 || 13064 |    13064 | ddsd1123d      | 2019-05-15 23:13:49 || 13065 |    13065 | ddsd1123d      | 2019-05-15 23:13:49 || 13066 |    13066 | ddsd1123d      | 2019-05-15 23:13:49 || 13067 |    13067 | ddsd1123d      | 2019-05-15 23:13:49 |+-------+----------+----------------+---------------------+10 rows in set (0.00 sec)

5、复合(多列)索引的最左原则(面试经常问)

只要查询的时候出现复合索引的最左边的字段才会使用到索引查询

把article表的no_index和title建立复合索引:

//给no_index和title创建一个复合索引mysql> alter table article add key index_no_index_title(no_index,title);Query OK, 0 rows affected (1.18 sec)Records: 0  Duplicates: 0  Warnings: 0//查看创建后的结构mysql> show create table articleG;*************************** 1. row ***************************       Table: articleCreate Table: CREATE TABLE `article` (  `id` int(11) NOT NULL AUTO_INCREMENT,  `no_index` int(11) DEFAULT NULL,  `title` varchar(30) NOT NULL DEFAULT '',  `add_time` datetime DEFAULT NULL,  PRIMARY KEY (`id`),  KEY `key_no_index` (`no_index`),  KEY `key_index` (`title`),  KEY `index_no_index_title` (`no_index`,`title`)) ENGINE=InnoDB AUTO_INCREMENT=1507299 DEFAULT CHARSET=utf81 row in set (0.00 sec)//删除no_index和title的索引mysql> alter table article drop key key_index;Query OK, 0 rows affected (0.05 sec)Records: 0  Duplicates: 0  Warnings: 0mysql> alter table article drop key key_no_index;Query OK, 0 rows affected (0.03 sec)Records: 0  Duplicates: 0  Warnings: 0mysql> show create table articleG;*************************** 1. row ***************************       Table: articleCreate Table: CREATE TABLE `article` (  `id` int(11) NOT NULL AUTO_INCREMENT,  `no_index` int(11) DEFAULT NULL,  `title` varchar(30) NOT NULL DEFAULT '',  `add_time` datetime DEFAULT NULL,  PRIMARY KEY (`id`),  KEY `index_no_index_title` (`no_index`,`title`)) ENGINE=InnoDB AUTO_INCREMENT=1507299 DEFAULT CHARSET=utf81 row in set (0.00 sec)//复合索引使用情况mysql> explain select * from article where title='ddsd1123d' and no_index=77777G;*************************** 1. row ***************************           id: 1  select_type: SIMPLE        table: article   partitions: NULL         type: refpossible_keys: index_no_index_title          key: index_no_index_title      key_len: 97          ref: const,const         rows: 1     filtered: 100.00        Extra: NULL1 row in set, 1 warning (0.00 sec)mysql> explain select * from article where  no_index=77777G; *************************** 1. row ***************************           id: 1  select_type: SIMPLE        table: article   partitions: NULL         type: refpossible_keys: index_no_index_title          key: index_no_index_title      key_len: 5          ref: const         rows: 1     filtered: 100.00        Extra: NULL1 row in set, 1 warning (0.00 sec)

五、慢查询日志

1、介绍

我们可以定义(程序员)一个sql语句执行的最大执行时间,如果发现某条sql语句的执行时间超过我们所规定的时间界限,那么这条sql就会被记录下来.

2、慢查询具体操作

先开启慢日志查询

查看慢日志配置

mysql> show variables like '%slow_query%';+---------------------+--------------------------------------------------+| Variable_name       | Value                                            |+---------------------+--------------------------------------------------+| slow_query_log      | OFF                                              || slow_query_log_file | /usr/local/mysql/data/caredeMacBook-Pro-slow.log |+---------------------+--------------------------------------------------+2 rows in set (0.00 sec)

开启慢日志查询

mysql> set global slow_query_log=on;Query OK, 0 rows affected (0.00 sec)

再次检查慢日志配置

mysql> show variables like '%slow_query%';+---------------------+--------------------------------------------------+| Variable_name       | Value                                            |+---------------------+--------------------------------------------------+| slow_query_log      | ON                                               || slow_query_log_file | /usr/local/mysql/data/caredeMacBook-Pro-slow.log |+---------------------+--------------------------------------------------+2 rows in set (0.00 sec)

去mysql配置文件my.ini中指定sql语句的界限时间和慢日志文件的路径

慢日志的名称，默认保存在mysql目录下面的data目录下面

log-slow-queries = 'man.txt'

设置一个界限时间

long-query-time=5

重启

六、profile工具

1、介绍

通过profile工具分析一条sql语句的时间消耗在哪里

2、具体操作

开启profile

执行一条SQL，(开启之后执行的所有SQL语句都会被记录下来

，以查看某条sql语句的具体执行时间耗费哪里)

根据query_id查找到具体的SQL

实例：

//查看profile设置mysql> show variables like '%profil%';+------------------------+-------+| Variable_name          | Value |+------------------------+-------+| have_profiling         | YES   || profiling              | OFF   |//未开启状态| profiling_history_size | 15    |+------------------------+-------+3 rows in set (0.00 sec)//开启操作mysql> set profiling = on;Query OK, 0 rows affected, 1 warning (0.00 sec)//查看是否开启成功mysql> show variables like '%profil%';+------------------------+-------+| Variable_name          | Value |+------------------------+-------+| have_profiling         | YES   || profiling              | ON    |//开启成功| profiling_history_size | 15    |+------------------------+-------+3 rows in set (0.00 sec)

具体查询

mysql> select * from article where no_index=666666;+--------+----------+-----------+---------------------+| id     | no_index | title     | add_time            |+--------+----------+-----------+---------------------+| 666666 |   666666 | ddsd1123d | 2019-05-15 23:13:55 |+--------+----------+-----------+---------------------+1 row in set (0.02 sec)mysql> show profiles;+----------+------------+---------------------------------------------+| Query_ID | Duration   | Query                                       |+----------+------------+---------------------------------------------+|        1 | 0.00150700 | show variables like '%profil%'              ||        2 | 0.01481100 | select * from article where no_index=666666 |+----------+------------+---------------------------------------------+2 rows in set, 1 warning (0.00 sec)mysql> show profile for query 2;+----------------------+----------+| Status               | Duration |+----------------------+----------+| starting             | 0.000291 || checking permissions | 0.000007 || Opening tables       | 0.012663 |//打开表| init                 | 0.000050 || System lock          | 0.000009 || optimizing           | 0.000053 || statistics           | 0.001566 || preparing            | 0.000015 || executing            | 0.000002 || Sending data         | 0.000091 |//磁盘上的发送数据| end                  | 0.000004 || query end            | 0.000007 || closing tables       | 0.000006 || freeing items        | 0.000037 || cleaning up          | 0.000010 |+----------------------+----------+15 rows in set, 1 warning (0.01 sec)

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