May 2020
Beginner to intermediate
319 pages
7h 3m
Chinese
Content preview from 数据库系统内幕
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136
｜
第
7
章
7.3.5
压缩
我们已经在 B 树的上下文中讨论过压缩（参见 4.6 节）。类似的思想也适用于 LSM 树。
这里的主要区别在于 LSM 树表是不可变的，并且通常是在一次请求中写入的。当按页
压缩数据时，压缩后的页不是页对齐的，因为它们的大小比未压缩的小。
为了能够处理被压缩的页，我们需要在写入它们的内容时跟踪它们的地址边界。我们可
以用零来填充压缩的页，使它们与页大小对齐，但这样我们就失去了压缩的好处。
为了使被压缩的页可寻址，我们需要一个间接层来存储偏移量和压缩页的大小。图 7-10
展示了压缩块和未压缩块之间的映射。压缩后的页
一定
比原始的小，否则压缩它们就没
有意义了。
偏移表
压缩的页
（
磁盘
）
未压缩的页
（
虚拟
）
图
7-10
：读取压缩块（虚线表示从映射表到磁盘上压缩页偏移量的指针。未压缩的页一般位于
页缓存中）
在压缩和刷盘期间，压缩的页被顺序地追加，并且压缩信息（原始未压缩的页偏移量和
实际压缩的页偏移量）存储在单独的文件段中。在读取过程中，将压缩后的页偏移量及
其大小查询出来，之后在内存中解压缩并物化。
7.4
无序
LSM
存储
到目前为止讨论过的大多数存储结构都是
有序
地存储数据的。可变和不可变的 B 树页、
FD 树中的有序段、LSM 树中的 SSTable 都是按键顺序存储数据记录的。这些结构保留
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