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。 免费推荐： mysql视频教程 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

mysql视频教程栏目今天着重介绍索引+explain，为需要面试的准备。

免费推荐：mysql视频教程

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 specified2、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: 0mysql> alter table t_index drop key key_title;
Query OK, 0 rows affected (0.02 sec)
Records: 0 Duplicates: 0 Warnings: 03、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 specifiedmysql> 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 specified3、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: 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: NULLmysql> 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)

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