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网站服务器容器,信息流投放,建筑工程机械人才网,武汉网站排名提升mysql 中! 到底走不走索引？ 很多人疑惑! 到底走不走索引， 这里可以肯定的说该操作是可以走索引的，但实际情况中都为啥都不走索引呢？ 首先我们要知道走索引与数据量和数据趋势（cardinality）有很大的关系&…

mysql 中!= 到底走不走索引？

很多人疑惑!= 到底走不走索引，这里可以肯定的说该操作是可以走索引的，但实际情况中都为啥都不走索引呢？首先我们要知道走索引与数据量和数据趋势（cardinality）有很大的关系，如果表本身就上百条记录，那走索引和表扫描区别不大，甚至在存在书签跳转情况下还不如表扫描更有效率，这个时候可能是不走索引的，另外 !=操作后获取的结果集在总结果集中占据的比例也是关键因素，如果返回的结果集过大（大于20%），那么可能也不会走索引，而是选择更有效率的表扫描了。

我们来看看个例子：

CREATE TABLE `b` (`id` int(11) NOT NULL AUTO_INCREMENT,`age` bigint(20) DEFAULT 50,`name` varchar(50) DEFAULT NULL,PRIMARY KEY (`id`),KEY `name` (`name`)
) ENGINE=InnoDBinsert into b (name) values('a'),('b'),('c'),('d'),('e'),('f'),('g'),('h'),('i'); 
--多次执行
insert into b (name) 
select 'k' from b ;select name, count(*) 
from  b 
group by name

explain
select * From b where name !='k';

explain
select * From b where name !='a';

很明显我们从统计中看到 K的数值很大，所以我们!='k'的数据集才8条，在总数据中占比很小，可以很好的走索引，而如果将k替换成 a，则不会走索引，变成了全表扫描

MYSQL系列书籍

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MySQL管理之道，性能调优，高可用与监控（第二版）.pdf: https://url41.ctfile.com/f/49289241-959124249-d59f54?p=2651 (访问密码: 2651)

深入浅出MySQL数据库开发、优化与管理维护第2版.pdf: https://url41.ctfile.com/f/49289241-961464090-68bf10?p=2651 (访问密码: 2651)

高性能MySQL.第3版.Baron Schwartz.pdf: https://url41.ctfile.com/f/49289241-961462308-52cc5d?p=2651 (访问密码: 2651)

MYSQL内核：INNODB存储引擎卷1.pdf: https://url41.ctfile.com/f/49289241-961461357-ee63e3?p=2651 (访问密码: 2651)

MySQL技术内幕InnoDB存储引擎第2版.pdf: https://url41.ctfile.com/f/49289241-959126379-4590a8?p=2651 (访问密码: 2651)

MySQLDBA修炼之道.pdf: https://url41.ctfile.com/f/49289241-961459500-9b201d?p=2651 (访问密码: 2651)

MySQL5.7从入门到精通.pdf: https://url41.ctfile.com/f/49289241-961459329-48cbcf?p=2651 (访问密码: 2651)

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HIVE电子书

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hadoop电子书

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Hadoop & Spark大数据开发实战.pdf: https://url41.ctfile.com/f/49289241-959131032-ba40ea?p=2651 (访问密码: 2651)

Expert Hadoop Administration.pdf: https://url41.ctfile.com/f/49289241-959130468-ba70cd?p=2651 (访问密码: 2651)

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python电子书

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Python编程：从入门到实践.pdf: https://url41.ctfile.com/f/49289241-959128548-ce965d?p=2651 (访问密码: 2651)

Python Projects for Beginners.pdf: https://url41.ctfile.com/f/49289241-959128461-b53321?p=2651 (访问密码: 2651)

kafka电子书

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Apache Kafka Cookbook.pdf: https://url41.ctfile.com/f/49289241-959132547-055c36?p=2651 (访问密码: 2651)

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Advanced Analytics with Spark Patterns for Learning from Data at Scale .pdf: https://url41.ctfile.com/f/49289241-959417997-a5e3f5?p=2651 (访问密码: 2651)

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Big Data Analytics Beyond Hadoop_ Real-Time Applications with Storm, Spark, and More Hadoop Alternatives.pdf: https://url41.ctfile.com/f/49289241-959418042-581fb9?p=2651 (访问密码: 2651)

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