1、學校代碼：10128學 號：200920205048本科畢業設計外文文獻翻譯英文 題目：software database an object-oriented perspective.中文題目:軟件數據庫的面向對象的視角學生姓名:學院:信息工程學院系別:軟件工程系專業:軟件工程班級:指導教師:二o三年六月a historical perspectivefrom the earliest days of computers, storing and manipulating data have been a major application focus- the first general-

2、purpose dbms was designed by charles bachman at general electric in the early 1960s and was called the integrated data store. it formed the basis for the network data model, which was standardized by the conference on data systems languages (codasyl) and strongly influenced database systems through

3、the 1960s- bachman was the first recipient of acm，s turing award (the computer science equivalent of a nobel prize) for work in the database area; he received the award in 1973. in the late 1960s, ibm developed the information management system (ims) dbms, used even today in many major installations

4、. ims formed the basis for an alternative data representation framework called the hierarchical data model. the sabre system for making airline reservations was jointly developed by american airlines and ibm around the same time, and it allowed several people to access the same data through computer

5、 network. interestingly, today the same sabre system is used to power popular web-based travel services such as travelocity!in 1970, edgar codd, at ibm's san jose research laboratory, proposed a new data representation framework called the relational data model. this proved to be a watershed in

6、the development of database systems: it sparked rapid development of several dbmss based on the relational model, along with a rich body of theoretical results that placed the field on a firm foundation. codd won the 1981 turing award for his seminal work. database systems matured as an academic dis

7、cipline, and the popularity of relational dbmss changed the commercial landscape. their benefits were widely recognized, and the use of dbmss for managing corporate data became standard practice.in the 1980s, the relational model consolidated its position as the dominant dbms paradigm, and database

8、systems continued to gain widespread 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, sql-92, was adopted by the american national standards institu

9、te (ansi) and international standards organization (iso). arguably, the most widely used form of concuirent 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 concurr

10、ently 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 lang

11、uages and richer data models, and there has been a big emphasis on supporting complex analysis of data from all parts of an enterprise. several vendors (e.g., ibm's db2, oracle & informix uds) have extended their systems with the ability to store new data types such as images and text, and w

12、ith the ability to ask more complex 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 planni

13、ng(erp) and management resource planning (mrp) packages, which add a substantial layer of applicatioriented features on top of a dbms. widely used packages include systems from baan, oracle, peoplesoft, sap, and siebel. these packages identify a set of common tasks (e.g., inventory management, human

14、 resources planning, financial analysis) encountered by a large number of organizations 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 companies, leading to lower introduction to data

15、base systems overall costs for the companies, compared to the cost of building the application layer from scratch most significantly, perhaps, dbmss have entered the internet age. while the first generation of web sites stored their data exclusively in operating systems files, the use of a dbms to s

16、tore data that is accessed through a web browser is becoming widespread. queries are generated through web-accessible 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 a

17、t making it more suitable for deployment over the internet. database management continues to gain importance as more and more data is brought on-line, and made ever more accessible through computer networking. today the field is being driven by exciting visions such as multimedia databases, interact

18、ive 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 companies to consolidate their decision-making processes and mine their data repositories for useful information about their businesses.

19、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 designlike all other aspects of database design, physical design mu

20、st 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; the workload consists of a mix of queries and updates. users also have certain requirements about how fast certain queries or updates must ru

21、n or how many transactions must be processed per second. the workload description and users5 performance requirements are the basis on which a number of decisions have to be made during physical database design.to create a good physical database design and to tune the system for performance in respo

22、nse to evolving user requirements, the designer needs to understand the workings of a dbms, especially the indexing and query processing techniques supported by the dbms. if the database is expected to be accessed concurrently by many users, or is a distributed database, the task becomes more compli

23、cated, and other features of a dbms come into play.database workloadsthe key to good physical design is arriving at an accurate description of the expected workload. a workload description includes the following elements:1. a list of queries and their frequencies, as a fraction of all queries and up

24、dates.2. a list of updates and their frequencies.3. performance goals for each type of query and update.for each query in the workload, we must identify:which relations are accessed.which attributes are retained (in the select clause).which attributes have selection or join conditions expressed on t

25、hem (in the where clause) and how selective these conditions are likely to be. similarly, for each update in the workload, we must identify:which attributes have selection or join conditions expressed on them (in the where clause) and how selective these conditions are likely to be.the type of updat

26、e (insert, delete, or update) and the updated relation.for update commands, the fields that are modified by the update.remember that queries and updates typically have parameters, for example, a debit or credit operation involves a particular account number. the values of these parameters deteitnine

27、 selectivity of selection and join conditions.updates have a query component that is used to find the target tuples- this component can benefit from a good physical design and the presence of indexes. on the other hand, updates typically require additional work to maintain indexes on the attributes

28、that they modify. thus, while queries can only benefit from the presence of an index, an index may either speed up or slow down a given update. designers should keep this trade-offer in mind when creating indexes.need for database tuningaccurate, detailed workload information may be hard to come by

29、while doing the initial design of the system. consequently, tuning a database after it has been designed and deployed is important一we must refine the initial design in the light of actual usage patterns to obtain the best possible performance.the distinction between database design and database tuni

30、ng is somewhat arbitrary.we could consider the design process to be over once an initial conceptual schema is designed and a set of indexing and clustering decisions is made. any subsequent changes to the conceptual schema or the indexes, say, would then be regarded as a tuning activity.alternativel

31、y, we could consider some refinement of the conceptual schema (and physical design decisions affected by this refinement) to be part of the physical design process.where we draw the line between design and tuning is not very important.overview of database tuningafter the initial phase of database de

32、sign, actual use of the database provides a valuable source of detailed information that can be used to refine the initial design. many of the original assumptions about the expected workload can be replaced by observed usage patterns; in general, some of the initial workload specification will be v

33、alidated, 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; for example,

34、the optimizer may not be using some indexes as intended to produce good plans.continued database tuning is important to get the best possible performance.tuning the conceptual schemain the course of database design, we may realize that our current choice of relation schemas does not enable us meet o

35、ur performance objectives for the given workload with any (feasible) set of physical design choices. if so, we may have to redesign our conceptual schema (and re-examine physical design decisions that are affected by the changes that we make).we may realize that a redesign is necessary during the in

36、itial design process or later, after the system has been in use for a while. once a database has been designed and populated with data, changing the conceptual schema requires a significant effort in terms of mapping the contents of relations that are affected. nonetheless, it may sometimes be neces

37、sary to revise the conceptual schema in light of experience with the system. we now consider the issues involved in conceptual schema (re)design from the point of view of performance.several options must be considered while tuning the conceptual schema:we may decide to settle for a 3nf design instea

38、d of a bcnf design.if there are two ways to decompose a given schema into 3nf or bcnf, our choice should be guided by the workload.sometimes we might decide to further decompose a relation that is already in bcnf.in other situations we might denormalize. that is, we might choose to replace a collect

39、ion of relations obtained by a decomposition from a larger relation with the original (larger) relation, even though it suffers from some redundancy problems. alternatively, we might choose to add some fields to certain relations to speed up some important queries, even if this leads to a redundant

40、storage of some information (and consequently, a schema that is in neither 3nf nor bcnf).this discussion of normalization has concentrated on the technique of decomposition, which amounts to vertical partitioning of a relation. another technique to consider is horizontal partitioning of a relation,

41、which would lead to our having two relations with identical schemas. note that we are not talking about physically partitioning the cuples of a single relation; rather, we want to create two distinct relations (possibly with different constraints and indexes on each).incidentally, when we redesign t

42、he 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 viewsif we notice that a query is running much slower than we expecte

43、d, 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 tuning may be called for if queries on some view run slower than expected.when tuning a query, the first thing to verify is that

44、 the system is using the plan that you expect it to use. it may be that the system is not finding the best plan for a variety of reasons. some common situations that are not handled efficiendy by many optimizers follow:a selection condition involving null values-selection conditions involving arithm

45、etic or string expressions or conditions using the or connective. for example, if we have a condition e.age = 2木d.age in the where clause, the optimizer may correctly utilize an available index on e.age but fail to utilize an available index on d.age. replacing the condition by e.age/2=d.age would r

46、everse the situation.inability to recognize a sophisticated plan such as an index-only scan for an aggregation query involving a group by clause.if the optimizer is not smart enough to and the best plan (using access methods and evaluation strategies supported by the dbms), some systems allow users

47、to guide the choice of a plan by providing hints to the optimizer; for example, users might be able to force the use of a particular index or choose the join order and join method. a user who wishes to guide optimization in this manner should have a thorough understanding of both optimization and th

48、e capabilities of the given dbms.(8)other topicsmobile databasesthe availability of portable computers and wireless communications has created a new breed of nomadic database users. at one level these users are simply accessing a database through a network, which is similar to distributed dbmss. at

49、another level the network as well as data and user characteristics now have several novel properties, which affect basic assumptions in many components of a dbms, including the query engine, transaction manager, and recovery manage匚users are connected through a wireless link whose bandwidth is ten t

50、imes less than ethernet and 100 times less than atm networks. communication costs are therefore significantly higher in proportion to i/o and cpu costs.users9 locations are constantly changing, and mobile computers have a limited battery life. therefore, the true communication costs is connection ti

51、me and battery usage in addition to bytes transferred, and change constantly depending on location. data is frequently replicated to minimize the cost of accessing it from different locations.as a user moves around, data could be accessed from multiple database servers within a single transaction. t

52、he likelihood of losing connections is also much greater than in a traditional network. centralized transaction management may therefore be impractical, especially if some data is resident at the mobile computers. we may in fact have to give up on acid transactions and develop alternative notions of

53、 consistency for user programs. main memory databasesthe price of main memory is now low enough that we can buy enough main memory to hold the entire database for many applications; with 64-bit addressing, modern cpus also have very large address spaces- some commercial systems now have several giga

54、bytes of main memory. this shift prompts a reexamination of some basic dbms design decisions, since disk accesses no longer dominate processing time for a memory-resident database:main memory does not survive system crashes, and so we still have to implement logging and recovery to ensure transactio

55、n atomicity and durability. log records must be written to stable storage at commit time, and this process could become a bottleneck- to minimize this problem, rather than commit each transaction as it completes, we can collect completed transactions and commit them in batches; this is called group

56、commit. recovery algorithms can also be optimized since pages rarely have to be written out to make room for other pages.the implementation of in-memory operations has to be optimized carefully since disk accesses are no longer the limiting factor for performance.a new criterion must be considered w

57、hile optimizing queries, namely the amount of space required to execute a plan. it is important to minimize the space overhead because exceeding available physical memory would lead to swapping pages to disk (through the operating system，s virtual memory mechanisms), greatly slowing down execution.p

58、age-oriented data structures become less important (since pages are no longer the unit of data retrieval), and clustering is not important (since the cost of accessing any region of main memory is uniform).（一）從歷史的角度回顧從數據庫的早期開始，存儲和操縱數據就一直是主要的應用焦點。第一個通用的 dbms是由charles bechman于20世紀60年代早期在通用電器公司設計的，稱為集

59、成數據存儲（integrated data store）.它奠定了網狀數據模型的基礎。網狀數據模型 由數據系統語言協會（c0dasyl）標準化，并在整個20世紀60年代對數據庫系統產生 了巨大的影響。由于bachman在數據庫領域的貢獻，他成為第一個acm圖靈獎（相當 于計算機科學界的諾貝爾獎）的獲得者，并于1973年接受了這一獎勵。20世紀60年代末期，ibm成功開發了信息管理系統（ims）dbmso直至今天，它還 在許多系統中使用。ims奠定了另一個數據表達框架一一層次數據模型的基礎。同時, 美國航空公司和ibm聯合開發出用于飛機訂票的sabre系統，它允許多個用戶通過計 算機網絡存取相同數據。有趣的是，今天sabre系統被用于支持廣為流行的基于web 的旅游服務，如travelocityo1970年，edgar codd在ibm的san jose研究實驗室推出了一種新的，稱為關系 數據模