Shulou(Shulou.com)05/31 Report--

In this issue, the editor will bring you what is the data independence of the database. The article is rich in content and analyzes and describes it from a professional point of view. I hope you can get something after reading this article.

Data independence includes two aspects: physical independence and logical independence.

First of all, the physical independence of data is discussed. Before further explanation, "data independence" should be understood as the physical independence of the data. It should be said that the term "data independence" is not used properly (at least it does not grasp the nature of the problem); however, as it has traditionally been used, the term is still used in this book. The way to understand the meaning of data independence is to find out what is non-data independence. In the old system-before the relational system and before the database system; implemented applications are often data dependent. This also means that in secondary storage, the physical representation of data and related access technologies are considered in the application design, and the knowledge and access technology related to physical representation are directly reflected in the code of the application.

Example:

Suppose an application uses the employee file in the figure, and also assumes that the file is indexed in the employee's first name field. In the old system, the application must have known that there was an index, that the order of records was based on the index, and that the internal structure of the application was designed based on this knowledge. In particular, the exact form of data access and the exception checking program of the application depend to a large extent on the interface details provided by the data management software to the application.

We call the application in this example data dependent, because once the physical representation of the data is changed, it will have a very strong impact on the application. For example, after the example is re-indexed with a hash algorithm, it is impossible to make major changes to the application. Moreover, in this case, the modified part of the application is precisely the part that is closely related to the data management software. The difficulty has nothing to do with the problem that the application initially tries to solve, but is caused by the characteristics of the data management interface. In database systems, applications that depend on data should be avoided as much as possible.

There are at least two reasons for this:

1) different applications look at the same data from different angles. For example, suppose there are two applications An and B before the enterprise establishes a unified database. Each has proprietary files that include customer balances. Assume that An is stored in decimal and B is stored in binary. At this point, it is possible to eliminate redundancy and unify the two files. The condition is that D B M S can and can perform the necessary conversions between the storage format (which may be decimal or binary or otherwise) and the format adopted by each application. For example, if you decide to store data in decimal, each access to B is converted to binary. This is a very small example, and there may be different types of data seen by the application in the database system and physically stored data. Many other possible different scenarios will be considered later in this section.

2) D B A must have the right to change the physical representation and access technology to meet the changing needs without having to change the existing application. For example, new types of data may be added to the database; new standards may be adopted; the priority of the application (and therefore the associated execution requirements) may change; the system may add new storage devices, and so on. If the application is data dependent, these changes will require the program to change accordingly, and the cost of maintenance is tantamount to creating a new application. Similar situations are even not uncommon, such as the typical Y2K problem, which is extremely disadvantageous to make full use of scarce and valuable resources.

In a word, the proposal of data independence is mainly the objective requirement of the database system. Data independence can be defined as an application that does not change due to changes in physical representation and access technologies. Of course, this means that the application should not rely on any particular physical representation and access technology. In chapter 2, the structure of the database system that supports the above basic requirements is described. Before that, let's talk about the specific circumstances in which changes occur, that is, what changes are usually required in D B A, so as to protect the application from this as much as possible.

The above is what the data independence of the database shared by the editor is. If you happen to have similar doubts, you might as well refer to the above analysis to understand. If you want to know more about it, you are welcome to follow the industry information channel.

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