mysql创建索引的三种方式_mysql里为什么创建组合索引

mysql创建索引的三种方式_mysql里为什么创建组合索引最近困扰自己很久的膝盖积液手术终于做完,在家养伤,逛技术博客看到easyswoole开发组成员仙士可博客有关mysql索引方面的知识,自己打算重温下。 正常业务起步数据表数据量较少,不用考虑使用索引,

mysql索引创建和使用细节(一)

最近困扰自己很久的膝盖积液手术终于做完,在家养伤,逛技术博客看到easyswoole开发组成员仙士可博客有关mysql索引方面的知识,自己打算重温下。

正常业务起步数据表数据量较少,不用考虑使用索引,当后期累积的数据数量非常可观时,使用索引是提升查询的一条途径,其他的像表分区,分库分表等等。

 

【索引创建】

索引的创建需要考虑被创建索引的字段区分度,比如一张表里面有渠道channel,渠道可期种类不超过3种,win系,安卓系,iOS系,而数据表数据量有一百万,平均下来每个渠道各是1/3也就是33万数据,这样的数据量就是否基于channel 索引区别都不会太大。

但是如果基于date字段做索引,如20200114,一年一百万,除以365天,平均下来每天300条数据。这个区分度是相当大。

同样的索引使用 33w数据查询显然效率低于300条数据。

索引可以加快mysql服务查询速度,但不是索引越多越好,因为insert或update的同时存放索引的文件也需要进行更新,会影响数据插入更新的速度,如果对数据实时性有要求的,无疑会受较大影响。

 

【索引失效】

一. 单字段索引:字段是string类型,传入int类型参数。

MySQL [test_db]> show create table test_usersG;
*************************** 1. row ***************************
       Table: test_users
Create Table: CREATE TABLE `test_users` (
  `uid` int(11) unsigned NOT NULL AUTO_INCREMENT,
  `username` char(15) NOT NULL,
  `created_time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,
  `user_id` char(11) NOT NULL DEFAULT "0",
  PRIMARY KEY (`uid`),
  KEY `testindex` (`user_id`)
) ENGINE=InnoDB AUTO_INCREMENT=1306001 DEFAULT CHARSET=utf8mb4
1 row in set (0.04 sec)

ERROR: No query specified

#开启profile
MySQL [test_db]> set profiling=1;
Query OK, 0 rows affected, 1 warning (0.03 sec)
#开始查询 MySQL
[test_db]> select * from test_users where user_id="443587"; Empty set (0.04 sec) MySQL [test_db]> select * from test_users where user_id=97737; Empty set (0.14 sec) #关闭profile MySQL [test_db]> set profiling=0; Query OK, 0 rows affected, 1 warning (0.03 sec) #explain查看一下 MySQL [test_db]> explain select * from test_users where user_id="443587" ; +----+-------------+------------+------------+------+---------------+-----------+---------+-------+------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+------------+------------+------+---------------+-----------+---------+-------+------+----------+-------+ | 1 | SIMPLE | test_users | NULL | ref | testindex | testindex | 44 | const | 1 | 100.00 | NULL | +----+-------------+------------+------------+------+---------------+-----------+---------+-------+------+----------+-------+ 1 row in set, 1 warning (0.05 sec) MySQL [test_db]> explain select * from test_users where user_id=97737; +----+-------------+------------+------------+------+---------------+------+---------+------+--------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+------------+------------+------+---------------+------+---------+------+--------+----------+-------------+ | 1 | SIMPLE | test_users | NULL | ALL | testindex | NULL | NULL | NULL | 306078 | 10.00 | Using where | +----+-------------+------------+------------+------+---------------+------+---------+------+--------+----------+-------------+ 1 row in set, 3 warnings (0.04 sec)
#以上可见当使用user_id匹配int类型时,
key=null,索引失效 #再看profile分析结果,可见加单引号比起不加单引号快上10倍左右 MySQL [test_db]> show profiles; +----------+------------+-------------------------------------------------+ | Query_ID | Duration | Query | +----------+------------+-------------------------------------------------+ | 1 | 0.01234100 | select * from test_users where user_id="443587" | | 2 | 0.10183000 | select * from test_users where user_id=97737 | +----------+------------+-------------------------------------------------+ 2 rows in set, 1 warning (0.04 sec) #再看更详细的分析 MySQL [test_db]> show profile cpu,block io,swaps for query 1; +----------------------+----------+----------+------------+--------------+---------------+-------+ | Status | Duration | CPU_user | CPU_system | Block_ops_in | Block_ops_out | Swaps | +----------------------+----------+----------+------------+--------------+---------------+-------+ | starting | 0.000088 | 0.000000 | 0.000000 | 0 | 0 | 0 | | checking permissions | 0.000006 | 0.000000 | 0.000000 | 0 | 0 | 0 | | Opening tables | 0.000021 | 0.000000 | 0.000000 | 0 | 0 | 0 | | init | 0.003386 | 0.001000 | 0.000000 | 240 | 0 | 0 | | System lock | 0.000027 | 0.000000 | 0.000000 | 0 | 0 | 0 | | optimizing | 0.000011 | 0.000000 | 0.000000 | 0 | 0 | 0 | | statistics | 0.007039 | 0.000000 | 0.000000 | 592 | 0 | 0 | | preparing | 0.000023 | 0.000000 | 0.000000 | 0 | 0 | 0 | | executing | 0.000003 | 0.000000 | 0.000000 | 0 | 0 | 0 | | Sending data | 0.001661 | 0.000000 | 0.000000 | 176 | 0 | 0 | | end | 0.000008 | 0.000000 | 0.000000 | 0 | 0 | 0 | | query end | 0.000011 | 0.000000 | 0.000000 | 0 | 0 | 0 | | closing tables | 0.000012 | 0.000000 | 0.000000 | 0 | 0 | 0 | | freeing items | 0.000044 | 0.000000 | 0.000000 | 0 | 0 | 0 | | cleaning up | 0.000003 | 0.000000 | 0.000000 | 0 | 0 | 0 | +----------------------+----------+----------+------------+--------------+---------------+-------+ 15 rows in set, 1 warning (0.03 sec) MySQL [test_db]> show profile cpu,block io,swaps for query 2; +----------------------+----------+----------+------------+--------------+---------------+-------+ | Status | Duration | CPU_user | CPU_system | Block_ops_in | Block_ops_out | Swaps | +----------------------+----------+----------+------------+--------------+---------------+-------+ | starting | 0.000081 | 0.000000 | 0.000000 | 0 | 0 | 0 | | checking permissions | 0.000006 | 0.000000 | 0.000000 | 0 | 0 | 0 | | Opening tables | 0.000022 | 0.000000 | 0.000000 | 0 | 0 | 0 | | init | 0.002129 | 0.000000 | 0.000000 | 72 | 0 | 0 | | System lock | 0.000010 | 0.000000 | 0.000000 | 0 | 0 | 0 | | optimizing | 0.000009 | 0.000000 | 0.000000 | 0 | 0 | 0 | | statistics | 0.000028 | 0.000000 | 0.000000 | 0 | 0 | 0 | | preparing | 0.000014 | 0.000000 | 0.000000 | 0 | 0 | 0 | | executing | 0.000002 | 0.000000 | 0.000000 | 0 | 0 | 0 | | Sending data | 0.099419 | 0.092986 | 0.000000 | 400 | 0 | 0 | | end | 0.000016 | 0.000000 | 0.000000 | 0 | 0 | 0 | | query end | 0.000012 | 0.000000 | 0.000000 | 0 | 0 | 0 | | closing tables | 0.000026 | 0.001000 | 0.000000 | 0 | 0 | 0 | | freeing items | 0.000054 | 0.000000 | 0.000000 | 0 | 0 | 0 | | cleaning up | 0.000003 | 0.000000 | 0.000000 | 0 | 0 | 0 | +----------------------+----------+----------+------------+--------------+---------------+-------+ 15 rows in set, 1 warning (0.04 sec) #通过对比可以发现主要耗时在sending data,而其他地方相差不大 #mysql官网对sending data对解释 #Sending data:The thread is reading and processing rows for a SELECT statement, and sending data to the client.
#Because operations occurring during this state tend to perform large amounts of disk access (reads), it is often the longest-running state over the lifetime of a given query. #大意即是:线程正在为一个select语句读取和处理行,并且发送数据到客户端。因为这期间操作倾向于大量的磁盘访问(读取),所以这常是整个查询周期中运行时间最长的阶段。 

代码100分

未完待续,下一篇讲int类型,传入string类型参数有什么不一样…

 

 

     

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