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這篇文章給大家分享的是有關MySQL中索引+explain的使用示例的內容。小編覺得挺實用的，因此分享給大家做個參考。一起跟隨小編過來看看吧。
在mysql中,索引就是數據結構,已經在文件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,但是對我們的數據增刪改效率會降低.
因為一個網站大部分都是查詢,我們主要優化select語句.
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引擎支持(只對英文進行索引,mysql版本5.6也支持),sphinx(中文搜索)
混合索引 多個字段組成的索引.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)
查看表的創建語句
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
主鍵不一定是自增長，但是自增長一定是主鍵。
刪除逐漸之前先要把主鍵索引的自增長去掉。
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
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
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) );
插入數據
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;
有無索引查詢數據對比
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
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或者desc.
explain|desc select * from tablename \G;
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//單表查詢 table: article//查詢的表名 partitions: NULL type: ALL//索引的類型，從好到壞的情況是：system>const>range>index>All possible_keys: NULL//可能使用到的索引 key: NULL//實際使用到的索引 key_len: NULL//索引的長度 ref: NULL rows: 1307580//可能進行掃描表的行數 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//當對主鍵索引進行等值查詢的時候出現const possible_keys: PRIMARY key: PRIMARY//實際使用到的所有primary索引 key_len: 4//索引的長度4 = int占4個字節 ref: const rows: 1//所掃描的行數只有一行 filtered: 100.00 Extra: NULL 1 row in set, 1 warning (0.00 sec) ERROR: No query specified
type項從優到差依次排序:
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:當對索引的值進行范圍查詢的時候會出現,如 where id<100000
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:最差的一種情況,需要避免.
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<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
如果查詢的字段在索引文件存在，那么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。
對于出現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為ref，應該是關聯，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) 速度飛躍 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幾乎還是全表掃描。
加了where就少了一半
where title like ‘%keyword%’ ====>全表掃描
where title like ‘keyword%’ ===>會使用到索引查詢
給title加上鋪索引
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)
因為%沒有出現在like關鍵字查詢的最左邊，所以可以使用到索引查詢
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//范圍查詢 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//全表查詢 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)
針對于limit語句的優化，我們可以在它前面加order by 索引字段
如果order by的字段是索引，會先去索引文件中查找指定行數的數據
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
另外一種針對于limit的優化方法:
索引覆蓋+延時關聯
原理:主要利用索引覆蓋查詢,把覆蓋索引查詢返回的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)
只要查詢的時候出現復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建立復合索引:
//給no_index和title創建一個復合索引 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 //查看創建后的結構 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`), 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) //刪除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) //復合索引使用情況 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)
我們可以定義(程序員)一個sql語句執行的最大執行時間,如果發現某條sql語句的執行時間超過我們所規定的時間界限,那么這條sql就會被記錄下來.
先開啟慢日志查詢
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)
開啟慢日志查詢
mysql> set global slow_query_log=on; Query OK, 0 rows affected (0.00 sec)
再次檢查慢日志配置
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語句的界限時間和慢日志文件的路徑
慢日志的名稱，默認保存在mysql目錄下面的data目錄下面
log-slow-queries = 'man.txt'
設置一個界限時間
long-query-time=5
重啟
通過profile工具分析一條sql語句的時間消耗在哪里
開啟profile
執行一條SQL，(開啟之后執行的所有SQL語句都會被記錄下來
，以查看某條sql語句的具體執行時間耗費哪里)
根據query_id查找到具體的SQL
實例：
//查看profile設置 mysql> show variables like '%profil%'; +------------------------+-------+ | Variable_name | Value | +------------------------+-------+ | have_profiling | YES | | profiling | OFF |//未開啟狀態 | profiling_history_size | 15 | +------------------------+-------+ 3 rows in set (0.00 sec) //開啟操作 mysql> set profiling = on; Query OK, 0 rows affected, 1 warning (0.00 sec) //查看是否開啟成功 mysql> show variables like '%profil%'; +------------------------+-------+ | Variable_name | Value | +------------------------+-------+ | have_profiling | YES | | profiling | ON |//開啟成功 | profiling_history_size | 15 | +------------------------+-------+ 3 rows in set (0.00 sec)
具體查詢
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 |//打開表 | init | 0.000050 | | System lock | 0.000009 | | optimizing | 0.000053 | | statistics | 0.001566 | | preparing | 0.000015 | | executing | 0.000002 | | Sending data | 0.000091 |//磁盤上的發送數據 | 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)
感謝各位的閱讀！關于MySQL中索引+explain的使用示例就分享到這里了，希望以上內容可以對大家有一定的幫助，讓大家可以學到更多知識。如果覺得文章不錯，可以把它分享出去讓更多的人看到吧！
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