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 1mysql> show create table t_index\g;*************************** 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 article\g;*************************** 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=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>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=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 user\g;*************************** 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=666666\g;*************************** 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>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: 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=666666\g;*************************** 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 id\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: null1 row in set, 1 warning (0.00 sec)error: no query specifiedmysql> explain select * from article where id >0 order by id\g;*************************** 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
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 article\g;*************************** 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 article\g;*************************** 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 article\g;*************************** 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=77777\g;*************************** 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=77777\g; *************************** 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)
以上就是详解mysql 索引+explain的详细内容。