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这篇文章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。
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.
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)
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_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
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
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);
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 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
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.
explain|desc select * from tablename \G;
用刚才的两个有无索引对比看看
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
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 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<666666\G;
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=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)
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
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
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 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)
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)
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)
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 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)
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.
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
通过profile工具分析一条sql语句的时间消耗在哪里
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。如果觉得文章不错，可以把它分享出去让更多的人看到吧！
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