数据库管理外文文献

数据库管理外文文献内容摘要：

n. That is, the DBMS is able to structure and tie together the logically related data from several large files. Logical Structures. Identifying these logical relationships is a job of the data administrator. A data definition language is used for this purpose. The DBMS may then employ one of the following logical structuring techniques during storage access, and retrieval operations. List structures. In this logical approach, records are linked together by the use of pointers. A pointer is a data item in one record that identifies the storage location of another logically related record. Records in a customer master file, for example, will contain the name and address of each customer, and each record in this file is identified by an account number. During an accounting period, a customer may buy a number of items on different days. Thus, the pany may maintain an invoice file to reflect these transactions. A list structure could be used in this situation to show the unpaid invoices at any given time. Each record in the customer in the invoice file. This invoice record, in turn, would be linked to later invoices for the customer. The last invoice in the chain would be identified by the use of a special character as a pointer. Hierarchical (tree) structures. In this logical approach, data units are structured in multiple levels that graphically resemble an “upside down” tree with the root at the top and the branches formed below. There’s a superiorsubordinate relationship in a hierarchical (tree) structure. Below the singleroot data ponent are subordinate elements or nodes, each of which, in turn, “own” one or more other elements (or none). Each element or branch in this structure below the root has only a single owner. Thus, a customer owns an invoice, and the invoice has subordinate items. The branches in a tree structure are not connected. Network Structures. Unlike the tree approach, which does not permit the connection of branches, the work structure permits the connection of the nodes in a multidirectional manner. Thus, each node may have several owners and may, in turn, own any number of other data units. Data management software permits the extraction of the needed information from such a structure by beginning with any record in a file. Relational structures. A relational structure is made up of many tables. The data are stored in the form of “relations” in these tables. For example, relation tables could be established to link a college course with the instructor of the course, and with the location of the class. To find the name of the instructor and the location of the English class, the course/instructor relation is searched to get the name (“Fitt”), and the course/location relation is a relatively new database structuring approach that’s expected to be widely implemented in the future. Physical Structures. People visualize or structure data in logical ways for their own purposes. Thus, records R1 and R2 may always be logically linked and processed in sequence in one particular application. However, in a puter system it’s quite possible that these records that are logically contiguous in one application are not physically stored together. Rather, the physical structure of the records in media and hardware may depend not only on the I/O and storage devices and techniques used, but also on the different logical relationships that users may assign to the data found in R1and R2. For example, R1 and R2 may be records of credit customers who have shipments send to the same block in the same city every 2 weeks. From the shipping department manager’s perspective, then, R1 and R2 are sequential entries on a geographically anized shipping report. But in the A/R application, the customers represented by R1 and R2 may be identified, and their accounts may be processed, according to their account numbers which are widely separated. In short, then, the physical location of the stored records in many puterbased information systems is invisible to users. Database Management Features of Oracle Oracle includes many features that make the database easier to manage. We’ve divided the discussion in this section into three categories: Oracle Enterprise Manager, addon packs, backup and recovery. Enterprise Manager As part of every Database Server, Oracle provides the Oracle Enterprise Manager (EM), a database management tool framework with a graphical interface used to manage database users, instances, and features (such as replication) that can provide additional information about the Oracle environment. Prior to the Oracle8i database, the EM software had to be installed on Windows 95/98 or NTbased systems and each repository could be accessed by only a single database manager at a time. Now you can use EM from a browser or load it onto Windows 95/98/2020 or NTbased systems. Multiple database administrators can access the EM repository at the same time. In the EM repository for Oracle9i, the super administrator can define services that should be displayed on other administrators’ consoles, and management regions can be set up. packs Several optional addon packs are available for Oracle, as described in the following sections. In addition to these databasemanagement packs, management packs are available for Oracle Applications and for SAP R/3. (1)standard Management Pack The Standard Management Pack for Oracle provides tools for the management of small Oracle databases (., Oracle Server/Standard Edition). Features include support for performance monitoring of database contention, I/O, load, memory use and。