先决条件:物理和逻辑数据独立
1. 物理数据独立性:
物理数据独立性基本上用于将概念级别与内部/物理级别分开。很容易实现物理数据独立。通过这种类型的独立性,用户能够更改对概念模式有影响的物理存储结构或设备。
物理数据独立性下的更改示例:
- 它是通过使用新的存储设备,如硬盘驱动器或磁带
- 修改数据库中的文件组织技术
- 切换到不同的数据结构。
- 更改访问方法。
- 修改索引。
- 改变压缩技术或散列算法。
- 将数据库的位置从 C 盘更改为 D 盘。
2. 逻辑数据独立性:
逻辑数据独立性用于在不更改以下内容的情况下更改概念方案:
- 外景
- 外部 API 或程序
逻辑数据独立下的更改示例:
- 无需重写现有应用程序即可添加/修改/删除新属性、实体或关系
- 将两条记录合二为一
- 打破现有记录,即将记录分成两个或多个记录
物理和逻辑数据独立性的区别:
Physical Data Independence | Logical Data Independence |
---|---|
It mainly concern about how the data is stored into the system. | It mainly concerned about the structure or the changing data definition. |
It is easy to retrieve. | It is difficult to retrieve because the data is mainly dependent on the logical structure of data. |
As compared to the logical independence it is easy to achieve physical data independence. | As compared to the physical independence it is not easy to achieve logical data independence. |
Any change at the physical level, does not require to change at the application level. | The change in the logical level requires a change at the application level. |
The modifications made at the internal level may or may not be needed to improve the performance of the structure. | The modifications made at the logical level is significant whenever the logical structure of the database is to be changed. |
It is concerned with the internal schema. | It is concerned with the conceptual schema. |
Example: Change in compression techniques, Hashing algorithms and storage devices etc. | Example: Add/Modify or Delete a new attribute. |