May 2020
Beginner to intermediate
319 pages
7h 3m
Chinese
Content preview from 数据库系统内幕
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B
树的变体
｜
113
缓存无关算法允许我们以两层存储模型对数据结构进行推断，同时具备多级层次结构模
型的优点。这种方法可以没有特定平台的参数，但是仍保证了层次结构的两个级别之间
的传输数量在一个常数内。如果数据结构专门为在任何双层存储层次结构中最优化运行
进行了优化，则它也可以在任何两个
相邻
存储层次结构上最优化运行。这是通过尽可能
地在最高缓存层下进行工作来实现的。
van Emde Boas
布局
一个缓存无关 B 树由一个静态 B 树和一个打包的数组结构组成 [BENDER05]。静态 B
树使用
van Emde Boas
布局构建。它在中间层的边上将树分裂开，然后以类似的方式递
归分裂每个子树，最后得到大小为 sqr(N) 的子树。这种布局的关键思想是，任何递归
树都存储在一个连续内存块中。
图 6-8 是一个 van Emde Boas 布局的例子，其中逻辑上分组在一起的节点紧密地放置在
一起。在顶部，你可以看到逻辑上的布局表示（即节点如何形成树），而在底部，你可以
看到树节点如何在内存和磁盘上布局。
逻辑布局
物理布局
图
6-8
：
van Emde Boas
布局
为了使数据结构动态化（即允许插入、更新和删除），缓存无关树使用
打包数组
（packed
array）数据结构，该数据结构使用连续的内存段来存储元素，但为将来插入的元素保留
间隙。间隙之间的间隔基于
密度阈值
（density threshold）。 图 6-9 展示了一个打包数组结
构，其中元素被间隔开以产生间隙。
图
6-9
：打包数组
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