数据库和c语言外文翻译-c语言(编辑修改稿)

数据库和c语言外文翻译-c语言(编辑修改稿)内容摘要：

nt operations even less so, the fewer can be less plex. Security and integrity maintenance are deemphasized or absent. As data limes will be small, performance efficiency is also important. In fact, the only aspect of a database system that is important is data Independence. Datadependence, as stated earlier, means that applicant programs and user queries need not recognizant physical organization of data on secondary storage. The importance of this aspect, particularly for the personal puter user, is that this greatly simplifies database usage. The user can store, access and manipulate data at a high level (close to the application) and be totally shielded from the low level (close to the machine) details of data organization. We will not discuss details of specific PC DBMS software packages here. Let us summarize in the following the strengths and weaknesses of personal puter database software systems： The most obvious positive factor is the user friendliness of the software. A user with no prior puter background would be able to use the system to store personal and professional data, retrieve and perform relayed processing. The user should, of course, satiety himself about the quality of software and the freedom from errors (bugs) so that investmerits in data arc protected. For the programmer implementing applications with them, the advantage lies in the support for applications development in terms of input screen generations, output report generation etc. offered by theses stems. The main negative point concerns absence of data protection features. Unless encrypted, data cane accessed by whoever has access to the machine Data can be destroyed through mistakes or malicious intent. The second weakness of many of the PCbased systems is that of performance. If data volumes grow up to a few thousands of records, performance could be a bottleneck. For organization where growth in data volumes is expected, availability of, the same or patible software on large machines should be considered. This is one of the most mon misconceptions about database management systems that are used in personal puters. Thoroughly prehensive and sophisticated business systems can be developed in dBASE, Paradox and other DBMSs. However, they are created by experienced programmers using the DBMS39。 s own programming language. That is not the same as users who create and manage personal files that are not part of the mainstream pany system. Transaction Management of Database The objective of longduration transactions is to model longduration, interactive Database access sessions in application environments. The fundamental assumption about shortduration 7 of transactions that underlies the traditional model of transactions is inappropriate for longduration transactions. The implementation of the traditional model of transactions may cause intolerably long waits when transactions aleph to acquire locks before accessing data, and may also cause a large amount of work to be lost when transactions are backed out in response to userinitiated aborts or system failure situations. The objective of a transaction model is to provide a rigorous basis for automatically enforcing criterion for database consistency for a set of multiple concurrent read and write accesses to the database in the presence of potential system failure situations. The consistency criterion adopted for traditional transactions is the notion of scrializability. Scrializability is enforced in conventional database systems through the use of locking for automatic concurrency control, and logging for automatic recovery from system failure situations. A “transaction” that doesn’t provide a basis for automatically enforcing database consistency is not really a transaction. To be sure, a longduration transaction need not adopt serializability as its consistency criterion. However, there must be some consistcricy criterion. Version System Management of Database Despite a large number of proposals on version support in the context of puter aided design and software engineering, the absence of a consensus on version semantics has been a key impediment to version support in database systems. Because of the differences between files and databases, it is intuitively clear that the model of versions in database systems cannot be as simple as that adopted in file systems to support software databases, it may be necessary to manage not only versions of single objects (. a software module, document, but also versions of a collection of objects (. a pound document, a user manual, etc. and perhaps even versions of the schema of database (. a table or a class, a collection of tables or classes). Broadly, there are three directions of research and development in versioning. First is the notion of a parameterized versioning, that is, designing and implementing a versioning system whose behavior may be tailored by adjusting system parameters This may be the only viable approach, in view of the fact that there are various plausible choices for virtually every single aspect of second is to revisit these plausible choices for every aspect of versioning, with the view to discarding some of themes either impractical or flawed. The third is the investigation into the semantics and implementation of versioning collections of objects and of versioning the database. There is no consensus of the definition of the term “management information system”. Some writers prefer alternative terminology such as “information processing system”, “information and decision system”, “organizational information system”, or simply “information system” to refer to the puterbased information processing system 8 which supports the operations, management, and decisionmaking functions of a。