信息工程—外文文献及翻译(英文译文)

信息工程—外文文献及翻译(英文译文)内容摘要：

rawn. Specifically, it is not clear how much of the typical semantics and facilities of temporal data management can and should be directly incorporated in a database system, and how much should be left to applications and users. In this section, we will provide a list of shortterm research issues that should be examined to shed light on this fundamental question. The focus of research into history data management has been on defining the semantics of time and time interval, and issues related to understanding the semantics of queries and updates against history data stored in an attribute of a record. Typically, in the context of relational databases ,a temporal attribute is defined to hold a sequence of history data for the attribute. A history data consists of a data item and a time interval for which the data item is valid. A query may then be issued to retrieve history data for a specified time interval for the temporal attribute. The mechanism for supporting temporal attributes is to that for supporting setvalued attributes in a database system, such as UniSQL. In the absence of a support for temporal attributes, application developers who need to model and history data have simply simulated temporal attributes by creating attribute for the time interval ,along with the “temporal” attribute. This of course may result in duplication of records in a table, and more plicated search predicates in queries. The one necessary topic of research in history data management is to quantitatively establish the performance (and even productivity) differences between using a database system that directly supports attributes and using a conventional database system that does not support either the setvalued attributes or temporal attributes. Data security, integrity, and independence Data security prevents unauthorized users from viewing or updating the database. Using passwords, users are allowed access to the entire database of the database, called subschemas. For example, an employee database can contain all the data about an individual employee, but one group of users may be authorized to view only payroll data, while others are allowed access to only work history and medical data. Data integrity refers to the accuracy, correctness, or validity of the data in the database. In a database system, data integrity means safeguarding the data against invalid alteration or destruction. In large online database system, data integrity bees a more severe problem and two additional plications arise. The first has to do with many users accessing the database concurrently. For example, if thousands of travel agents book the same seat on the same flight, the first agent’s booking will be lost. In such cases the technique of locking the record or field provides the means for preventing one user from accessing a record while another user is updating the same record. The second plication relates to hardware, software or human error during the course of processing and involves database transaction which is a group of database modifications treated as a single unit. For example, an agent booking an airline reservation involves several database updates (., adding the passenger’s name and address and updating the seatsavailable field), which prise a single transaction. The database transaction is not considered to be pleted until all updates have been pleted。 otherwise, none of the updates will be allowed to take place. An important point about database systems is that the database should exis。