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Homework 09

约 614 个字 预计阅读时间 3 分钟

12.1

SSDs can be used as a storage layer between memory and magnetic disks, with some parts of the database (e.g., some relations) stored on SSDs and the rest on magnetic disks. Alternatively, SSDs can be used as a buffer or cache for magnetic disks; frequently used blocks would reside on the SSD layer, while infrequently used blocks would reside on magnetic disks.

a. Which of the two alternatives would you choose if you need to support real-time queries that must be answered within a guaranteed short period of time? Explain why.

b. Which of the two alternatives would you choose if you had a very large customer relation, where only some disk blocks of the relation are accessed frequently, with other blocks rarely accessed.

（a）更倾向于选择第一种方案，因为第一种方案能保证保持一定的读取速率，而如果采用第二种方案，如果碰到 Cache Miss 的情况会造成非常大的时延

（b）更倾向于选择第二种方案，因为只有一些关系会被频繁访问而其他的关系不会被频繁访问，所以可以将频繁访问的关系放在 SSD 上，其他关系放在磁盘上，将 SSD 作为缓存处理

13.5

It is important to be able to quickly find out if a block is present in the buffer, and if so where in the buffer it resides. Given that database buffer sizes are very large, what (in-memory) data structure would you use for this task?

使用 Hash 表来存储，Hash 函数能快速计算出数据块是否在 buffer 当中，且能找到具体的位置

13.9

In the variable-length record representation, a null bitmap is used to indicate if an attribute has the null value.

a. For variable-length fields, if the value is null, what would be stored in the offset and length fields?

b. In some applications, tuples have a very large number of attributes, most of which are null. Can you modify the record representation such that the only overhead for a null attribute is the single bit in the null bitmap?

（a）设置 length field 为 -1，offset field 为任意值

（b）将 null bitmap 放到记录的前面

13.11

List two advantages and two disadvantages of each of the following strategies for storing a relational database:

a. Store each relation in one file.

b. Store multiple relations (perhaps even the entire database) in one file.

（a）优点：

  • 可以更好地将经常访问的关系放在 SSD，不常访问的关系放在磁盘上
  • 每个表对应一个独立文件，​结构简单直观，方便直接定位和管理维护

缺点：

  • 跨表操作效率低
  • 管理复杂度高

（b）优点：

  • 跨表操作效率高
  • 无须跨文件协调，操作更加便捷

缺点：

  • 高并发场景容易造成竞争
  • 单文件体积变大，读写性能下降