加入收藏 | 设为首页 | 会员中心 | 我要投稿 烟台站长网 （https://www.0535zz.com/）- 科技、建站、经验、云计算、5G、大数据,站长网!
当前位置： 首页 > 站长学院 > MySql教程 > 正文

MySQL中索引+explain的引用示例

发布时间：2022-02-24 14:52:56 所属栏目：MySql教程 来源：互联网
导读：给大家分享的是有关MySQL中索引+explain的使用示例的内容。小编觉得挺实用的，因此分享给大家做个参考。一起跟随小编过来看看吧。 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
     给大家分享的是有关MySQL中索引+explain的使用示例的内容。小编觉得挺实用的，因此分享给大家做个参考。一起跟随小编过来看看吧。
 
      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 1
mysql> show create table t_indexG;
*************************** 1. row ***************************
       Table: t_index
Create 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=utf8
1 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: 0
 
mysql> 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: article
Create 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=utf8
1 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>All
possible_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 where
1 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const
possible_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: NULL
1 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=666666
range:当对索引的值进行范围查询的时候会出现,如 where id<100000
index: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: ALL
possible_keys: NULL
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 3
     filtered: 100.00
        Extra: NULL
1 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: const
possible_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: index
possible_keys: NULL
          key: PRIMARY
      key_len: 4
          ref: NULL
         rows: 1307580
     filtered: 100.00
        Extra: Using index
1 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: 0
 
type为ref，应该是关联，但是ref是const
mysql> explain select * from article where no_index=666666G;
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: article
   partitions: NULL
         type: ref
possible_keys: key_no_index
          key: key_no_index
      key_len: 5
          ref: const
         rows: 1
     filtered: 100.00
        Extra: NULL
1 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: index
possible_keys: NULL
          key: PRIMARY
      key_len: 4
          ref: NULL
         rows: 1307580
     filtered: 100.00
        Extra: NULL
1 row in set, 1 warning (0.00 sec)
 
ERROR:
No query specified
 
mysql> explain select * from article where id >0  order by idG;
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: article
   partitions: NULL
         type: range
possible_keys: PRIMARY
          key: PRIMARY
      key_len: 4
          ref: NULL
         rows: 653790
     filtered: 100.00
        Extra: Using where
1 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
 
mysql> alter table article  add key key_index(title);
Query OK, 0 rows affected (2.16 sec)
Records: 0  Duplicates: 0  Warnings: 0
 
mysql> show create table articleG;
*************************** 1. row ***************************
       Table: article
Create 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=utf8
1 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 condition
1 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 where
1 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: NULL
1 row in set, 2 warnings (0.00 sec)
 
ERROR:
No query specified
 
mysql> 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: index
possible_keys: NULL
          key: PRIMARY//使用到了索引
      key_len: 4
          ref: NULL
         rows: 90010
     filtered: 100.00
        Extra: NULL
1 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: article
Create 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=utf8
1 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: 0
 
mysql> alter table article drop key key_no_index;
Query OK, 0 rows affected (0.03 sec)
Records: 0  Duplicates: 0  Warnings: 0
 
mysql> show create table articleG;
*************************** 1. row ***************************
       Table: article
Create 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=utf8
1 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: ref
possible_keys: index_no_index_title
          key: index_no_index_title
      key_len: 97
          ref: const,const
         rows: 1
     filtered: 100.00
        Extra: NULL
1 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: ref
possible_keys: index_no_index_title
          key: index_no_index_title
      key_len: 5
          ref: const
         rows: 1
     filtered: 100.00
        Extra: NULL
1 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)
感谢各位的阅读！关于MySQL中索引+explain的使用示例就分享到这里了，希望以上内容可以对大家有一定的帮助，让大家可以学到更多知识。

（编辑：烟台站长网）

【声明】本站内容均来自网络，其相关言论仅代表作者个人观点，不代表本站立场。若无意侵犯到您的权利，请及时与联系站长删除相关内容!
    相关内容
      推荐文章
      站长推荐
      热点阅读

        【免责声明】本站内容转载自互联网，其发布内容言论不代表本站观点，如果其链接、内容的侵犯您的权益，烦请提交相关链接至邮箱bqsm@foxmail.com我们将及时予以处理。

        建议您使用1920×1080分辨率、谷歌浏览器Google Chrome、Microsoft Edge以获得本站的最佳浏览效果

        Copygight © 2008-2022 https://www.0535zz.com/ All Rights Reserved. 烟台站长网