【正文】 spread use. The SQL query language for relational databases, developed as part of IBM’s System R project, is now the standard query language. SQL was standardized in the late 1980s, and the current standard, SQL92, was adopted by the American National Standards Institute (ANSI) and International Standards Organization (ISO). Arguably, the most widely used form of 2 concurrent programming is the concurrent execution of database programs (called transactions). Users write programs as if they are to be run by themselves, and the responsibility for running them concurrently is given to the DBMS. James Gray won the 1999 Turing award for his contributions to the field of transaction management in a DBMS. In the late 1980s and the 1990s, advances have been made in many areas of database systems. Considerable research has been carried out into more powerful query languages and richer data models, and there has been a big emphasis on supporting plex analysis of data from all parts of an enterprise. Several vendors (., IBM’s DB2, Oracle 8, Informix UDS) have extended their systems with the ability to store new data types such as images and text, and with the ability to ask more plex queries. Specialized systems have been developed by numerous vendors for creating data warehouses, consolidating data from several databases, and for carrying out specialized analysis. An interesting phenomenon is the emergence of several enterprise resource planning(ERP) and management resource planning (MRP) packages, which add a substantial layer of applicationoriented features on top of a DBMS. Widely used packages include systems from Baan, Oracle, PeopleSoft, SAP, and Siebel. These packages identify a set of mon tasks (., inventory management, human resources planning, financial analysis) encountered by a large number of anizations and provide a general application layer to carry out these tasks. The data is stored in a relational DBMS, and the application layer can be customized to different panies, leading to lower Introduction to Database Systems overall costs for the panies, pared to the cost of building the application layer from scratch. Most significantly, perhaps, DBMSs have entered the Inter Age. While the first generation of Web sites stored their data exclusively in operating systems files, the use of a DBMS to store data that is accessed through a Web browser is being widespread. Queries are generated through Webaccessible forms and answers are formatted using a markup language such as HTML, in order to be easily displayed in a browser. All the database vendors are adding features to their DBMS aimed at making it more suitable for deployment over the Inter. Database management continues to gain importance as more and more data is brought online, and made ever more accessible through puter working. Today the field is being driven by exciting visions such as 3 multimedia databases, interactive video, digital libraries, a host of scientific projects such as the human genome mapping effort and NASA’s Earth Observation System project, and the desire of panies to consolidate their decisionmaking processes and mine their data repositories for useful information about their businesses. Commercially, database manage ment systems represent one of the largest and most vigorous market segments. Thusthes tudy of database systems could prove to be richly rewarding in more ways than one! INTRODUCTION TO PHYSICAL DATABASE DESIGN Like all other aspects of database design, physical design must be guided by the nature of the data and its intended use. In particular, it is important to understand the typical workload that the database must support。 in general, some of the initial workload specification will be validated, and some of it will turn out to be wrong. Initial guesses about the size of data can be replaced with actual statistics from the system catalogs (although this information will keep changing as the system evolves). Careful monitoring of queries can reveal unexpected problems。 rather, we want to create two distinct relations (possibly with different constraints and indexes on each). Incidentally, when we redesign the conceptual schema, especially if we are tuning an existing database schema, it is worth considering whether we should create views to mask these changes from users for whom the original schema is more natural. TUNING QUERIES AND VIEWS If we notice that a query is running much slower than we expected, we have to examine the query carefully to end the problem. Some rewriting of the query, perhaps in conjunction with some index tuning, can often ?x the problem. Similar