詳解MySQL 索引+explain
- coldplay.xixi轉載
- 2020-11-27 16:57:329416瀏覽
mysql影片教學欄位今天著重介紹索引 explain，為需要面試的準備。
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#一、索引的介紹
- 在mysql中,索引就是資料結構,已經在檔案中按照索引進行排序好的結構.
- 使用索引可以加快我們的查詢速度,但是對我們的資料增刪改效率會降低.
- 因為一個網站大部分都是查詢,我們主要優化select語句.
二、MySQL中索引的分類
- 普通索引 key
- #唯一索引 unique key unique key 別名別名可忽略別名可忽略
- 主鍵索引 primary key(字段)
- 全文索引myisam引擎支援(只對英文進行索引,mysql版本5.6也支援),sphinx(中文搜尋)
- 混合索引 多個欄位組成的索引.如key key_index(title,email)
三、索引的基本運算
1.給表添加索引
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;
查看表格
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)
查看表的建立語句
show create table tbalename/G
##show create table tbalename/G
- 2、刪除索引
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_index\G; *************************** 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
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主鍵不一定是自成長，但自成長一定是主鍵。 刪除逐漸之前先要把主鍵索引的自增長去掉。
- 再來刪除主鍵
mysql> alter table t_index drop primary key; Query OK, 0 rows affected (0.04 sec) Records: 0 Duplicates: 0 Warnings: 0
mysql> alter table t_index modify id int not null; Query OK, 0 rows affected (0.05 sec) Records: 0 Duplicates: 0 Warnings: 0
alter table table_name drop key '索引的別名'
實際操作
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、新增索引
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、有無索引對比
create table article( id int not null auto_increment, no_index int, title varchar(30) not null default '', add_time datetime, primary key(id) );插入資料
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;有無索引查詢資料對比
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)
表結構mysql> show create table article\G;
*************************** 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可以對sql語句進行分析到底有沒有使用到索引查詢,從而更好的優化它.我們只需要在select語句前面加上一句explain或desc.1、語法
explain|desc select * from tablename \G;
2、分析
用剛才的兩個有無索引對比看看mysql> mysql> explain select * from article where no_index=1495298\G;
*************************** 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=1495298\G;
*************************** 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項分析
- type項目從優到差依序排序: ##system
- :一般系統表只有一行記錄的時候才會出現 const
- :當對主鍵值進行等值查詢的時候會出現,如where id=666666 range
- :當對索引的值進行範圍查詢的時候會出現,如where id index
- :當我們查詢的字段恰好是我們索引文件中的值,就會出現 All
#實際測試
mysql> use mysql; mysql> explain select * from user\G; *************************** 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=666666\G; *************************** 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>666666\G; mysql> explain select * from article where id<pre class="brush:php;toolbar:false">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
如果查詢的欄位在索引文件存在，那麼就會直接從索引文件中進行查詢，我們把這種查詢稱之為索引覆蓋查詢。
出現all，我們需要避免，因為進行全面掃描。
對於出現all的，可以給該字段增加普通索引查詢
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=666666\G; *************************** 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、使用索引的場景
#1、 經常出現在where後面的字段,我們需要給他加索引
2、order by 語句使用索引的最佳化mysql> explain select * from article order by id\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: NULL
1 row in set, 1 warning (0.00 sec)
ERROR:
No query specified
mysql> explain select * from article where id >0 order by id\G;
*************************** 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
加了where就少了一半
3、針對like的模糊查詢索引的最佳化
where title like '%keyword%' ====>全表掃描where title like 'keyword%' ===>會使用到索引查詢
#為title加上鋪面索引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 article\G;
*************************** 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)
只要是like左邊出現了%，就是全表查詢
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語句的索引使用最佳化
針對於limit語句的最佳化，我們可以在它前面加order by索引欄位
如果order by的欄位是索引，會先去索引檔案中尋找指定行數的資料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
索引覆寫延時關聯
原理:主要利用索引覆寫查詢,把覆蓋索引查詢傳回的id作為與我們要查詢記錄的id進行相關聯,
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、複合(多列)索引的最左原則(面試經常問)
只要查詢的時候出現複合索引的最左邊的字段才會使用到索引查詢
###把article表的no_index和title建立複合索引:###//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 article\G; *************************** 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 article\G; *************************** 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=77777\G; *************************** 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=77777\G; *************************** 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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