May 2020
Beginner to intermediate
319 pages
7h 3m
Chinese
Content preview from 数据库系统内幕
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简介与概述
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19
因为诸如磁盘之类的持久性介质上的数据通常是按块访问的（换句话说，磁盘访问的最
小单位是块），所以单个块可能将包含某行中所有列的数据。这对于我们希望访问整个用
户记录的情况非常有用，但这样的存储布局会使访问多个用户记录某个字段的查询（例
如，只获取电话号码的查询）开销更大，因为其他字段的数据在这个过程中也会被读入。
1.3.2
面向列的数据布局
面向列的数据库垂直地
将数据进行分区（即通过列进行分区），而不是将其按行存储。在
这种数据存储布局中，同一列的值被连续地存储在磁盘上（而不是像前面的示例那样将
行连续地存储）。例如，如果我们要存储股票市场的历史价格，那么股票价格这一列的
数据便会被存储在一起。将不同列的值存储在不同的文件或文件段中，可以按列进行有
效的查询，因为它们可以一次性地被读取出来，而不是先对整行进行读取后再丢弃掉不
需要的列。
面向列的存储非常适合计算聚合的分析型工作负载，例如查找趋势、计算平均值等。如
果逻辑记录具有多个字段，但是其中某些字段（在本例中为股票价格）具有不同的重要
性并且该字段所存储的数据经常被一起使用，那么我们一般使用复杂聚合来处理这样的
情况。
从逻辑角度看，表示股票市场价格的数据仍旧可以表示为表的形式：
| ID | Symbol | Date | Price |
| 1 | DOW | 08 Aug 2018 | 24,314.65 |
| 2 | DOW | 09 Aug 2018 | 24,136.16 |
| 3 | S&P | 08 Aug 2018 ...
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