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数据库索引技术

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Jun 07, 2016 pm 03:30 PM

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1、索引技术的概念索引技术是一种快速文件访问技术，它将一个文件的每个记录在某个或某些域（或称为属性）上的取与该记录的物理地址联系起来，提供了一种根据记录域快速访问文件记录的机制。 2、索引技术的分类 2.1、有序索引有序索引技术利用索引文件实现

1、索引技术的概念

索引技术是一种快速文件访问技术，它将一个文件的每个记录在某个或某些域（或称为属性）上的取值与该记录的物理地址联系起来，提供了一种根据记录域快速访问文件记录的机制。

2、索引技术的分类

2.1、有序索引

有序索引技术利用索引文件实现记录域取值到记录物理地址间的映射关系。这里的记录域是查找码。索引文件由索引记录组成，每个记录记载着一个索引项，索引项记录了某个特定的查找码值和具有该值的数据文件记录的物理地址，查找码也称为排序域。

2.1.1、聚集索引与非聚集索引

对数据文件和它的一个特定的索引文件，如果数据文件中数据记录的排列顺序与索引文件中索引项的排列顺序一致，刚该索引文件称为聚集索引，否则是非聚集索引。

2.1.2、稠密索引与稀疏索引

如果数据文件中的每个查找码值在索引文件中都对应一个索引记录，则该索引称为稠密索引，如果只是一部分查找码值有对应的索引记录，则该索引称为稀疏索引。

2.1.3、主索引与辅助索引

在数据文件包含主码的属性集上建立的索引称为主索引。在数据文件的非主属性上建立的索引称为辅助索引。

2.1.4、单层索引与多层索引

单层索引也称为线性索引，其特点是索引项根据键值在索引文件中的顺序排列，组成一维线性结构，每个索引项直接指向数据文件中的记录。多层索引的典型例子是数据库系统中广泛应该的B+树和B-树索引。

2.2散列技术

散列技术利用一个散列函数实现记录域取值到记录物理地址间的直接映射关系。这里的记录域就是查找码，也称为散列函数的散列域或排序域。

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