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HomeDatabaseMysql Tutorialmysql 索引分类以及用途分析

MySQL索引分为普通索引、唯一性索引、全文索引、单列索引、多列索引等等。这里将为大家介绍着几种索引各自的用途。

一、 MySQL: 索引以B树格式保存

  Memory存储引擎可以选择Hash或BTree索引,Hash索引只能用于=或的等式比较。

  1、普通索引:create index on Tablename(列的列表)

  alter table TableName add index (列的列表)

  create table TableName([...], index [IndexName] (列的列表)

  2、唯一性索引:create unique index

  alter ... add unique

  主键:一种唯一性索引,必须指定为primary key

  3、全文索引:从3.23.23版开始支持全文索引和全文检索,FULLTEXT,

  可以在char、varchar或text类型的列上创建。

  4、单列索引、多列索引:

  多个单列索引与单个多列索引的查询效果不同,因为:

  执行查询时,MySQL只能使用一个索引,会从多个索引中选择一个限制最为严格的索引。

  5、最左前缀(Leftmost Prefixing):多列索引,例如:fname_lname_age索引,以下的搜索条件MySQL都将使用

  fname_lname_age索引:firstname,lastname,age;firstname,lastname;firstname,其他情况将不使用。

二、根据sql查询语句确定创建哪种类型的索引,如何优化查询

  选择索引列:

  a.性能优化过程中,选择在哪个列上创建索引是最重要的步骤之一。可以考虑使用索引的主要有

  两种类型的列:在where子句中出现的列,在join子句中出现的列。

  b.考虑列中值的分布,索引的列的基数越大,索引的效果越好。

  c.使用短索引,如果对字符串列进行索引,应该指定一个前缀长度,可节省大量索引空间,提升查询速度。

  d.利用最左前缀

  e.不要过度索引,只保持所需的索引。每个额外的索引都要占用额外的磁盘空间,并降低写操作的性能。

  在修改表的内容时,索引必须进行更新,有时可能需要重构,因此,索引越多,所花的时间越长。

  MySQL只对一下操作符才使用索引:,>=,between,in,

  以及某些时候的like(不以通配符%或_开头的情形)。

mysql 索引分类

在数据库表中,对字段建立索引可以大大提高查询速度。通过善用这些索引,可以令 MySQL的查询和运行更加高效。索引是快速搜索的关键。MySQL索引的建立对于MySQL的高效运行是很重要的。下面介绍几种常见的MySQL索引类型。
1、普通型索引
这是最基本的索引类型,而且它没有唯一性之类的限制。普通索引可以通过以下几种方式创建:
(1)创建索引,例如CREATE INDEX 索引的名字 ON tablename (列名1,列名2,...);
(2)修改表,例如ALTER TABLE tablename ADD INDEX 索引的名字 (列名1,列名2,...);
(3)创建表的时候指定索引,例如CREATE TABLE tablename ( [...], INDEX 索引的名字 (列名1,列名
2,...) );
2、唯一索引
这种索引和前面的“普通索引”基本相同,但有一个区别:索引列的所有值都只能出现一次,即必须唯一。唯一性索引可以用以下几种方式创建:
(1)创建索引,例如CREATE UNIQUE INDEX 索引的名字 ON tablename (列的列表);
(2)修改表,例如ALTER TABLE tablename ADD UNIQUE 索引的名字 (列的列表);
(3)创建表的时候指定索引,例如CREATE TABLE tablename ( [...], UNIQUE 索引的名字 (列的列
表) );
3、主键
主键是一种唯一性索引,但它必须指定为“PRIMARY KEY”。如果你曾经用过AUTO_INCREMENT类型的列,你可能已经熟悉主键之类的概念了。主键一般在创建表的时候指定,例如“CREATE TABLE tablename ( [...], PRIMARY KEY (列的列表) ); ”。但是,我们也可以通过修改表的方式加入主键,例如“ALTER TABLE tablename ADD PRIMARY KEY (列的列表); ”。每个表只能有一个主键。 (主键相当于聚合索引,是查找最快的索引)
4、单列索引和多列索引
索引可以是单列索引,也可以是多列索引。
(1)单列索引就是常用的一个列字段的索引,常见的索引。
(2)多列索引就是含有多个列字段的索引
alter table student add index sy(name,age,score);
索引sy就为多列索引,多列索引在以下几中情况下才能有效:
select * from student where name='jia' and age>='12' //where条件中含有索引的首列字段和
第二个字段
select * from student where name='jia' //where条件中只含有首列字段
select * from student where name='jia' and score段
总结:多列索引只有在where条件中含有索引中的首列字段时才有效
5、选择索引列
应该怎样选择索引列,首先要看查询条件,一般将查询条件中的列作为索引
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