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
树的实现
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69
4.6
压缩
存储原始的、未压缩的数据会导致显著的开销，因此许多数据库提供了压缩数据的方法
以节省存储空间。这里显然是要在访问速度和压缩率之间进行权衡：较大的压缩率可以
提升数据存储空间利用率，允许你在一次访问中获取更多的数据，但这可能需要更多的
内存和 CPU 周期来进行压缩和解压缩。
我们可以在不同的粒度级别上进行压缩。尽管压缩整个文件可以产生更好的压缩率，但
由于整个文件必须在更新时被重新压缩，所以它的应用有限。更细粒度的压缩通常更适
合较大的数据集。压缩整个索引文件既不切实际也难以高效实现：为了寻址一个特定页，
必须访问整个文件或其包含压缩元数据的段（以便定位一个已压缩的段），然后将其解压
缩并使其可用。
另一种选择是按页压缩数据。它很适合我们所讨论的情形，因为到目前为止我们讨论的
算法都使用固定大小的页。页可以独立地进行压缩和解压缩，从而允许你将压缩与页的
加载和刷盘耦合起来。然而，这样一来，被压缩的页只能占用磁盘块的一部分。并且，由
于传输通常以磁盘块为单位进行，所以可能需要换入额外的字节 [RAY95]。在图 4-10 中，
你可以看到压缩页（a）占用的空间小于磁盘块，当我们加载该页时，我们还会换入属于另
一页的额外字节。对于跨越多个磁盘块的页，如图 4-10b，我们必须读取一个附加的块。
磁盘块
a） b）
图
4-10
：压缩和块填充
另一种方法是仅压缩数据，要么按行（压缩整条数据记录），要么按列（对每一列进行压
缩）。在这种情况下，页的管理和压缩是解耦的。
在撰写本书时参考的大多数开源数据库都使用了可插拔的压缩方法 ...
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