Wednesday, 19 October 2011





In this section we discuss some of the advantages of using a DBMS and the capabilities that a good DBMS should possess. The DBA must utilize these capabilities to accomplish a variety of objectives related to the design, administration, and use of a large multi-user database.

Which  Database is best? (MySQL vs MongoDB vs Postgres)

Controlling Redundancy

In traditional software development utilizing file processing, every user group maintains its own files for handling its data-processing applications. In the traditional approach, each group independently keeps files on students. The accounting office also keeps data on registration and related billing information, whereas the registration office keeps track of student courses and grades.
Much of the data is stored twice: once in the files of each user group. Additional user groups may further duplicate some or all of the same data in their own files. This redundancy in storing the same data multiple times leads to several problems. First, there is the need to perform a single logical update-such as entering data on a new student-multiple times: once for each file where student data is recorded. This leads to duplication of effort. Second, storage space is wasted when the same data is stored repeatedly, and this problem may be serious for large databases. Third, files that represent the same data may become inconsistent. This may happen because an update is applied to some of
the files but not to others. Even if an update-such as adding a new student-is applied to all the appropriate files, the data concerning the student may still be inconsistent because the updates are applied independently by each user group. For example, one user group may enter a student's birth date erroneously as JAN-19-1984, whereas the other user groups may enter the correct value of JAN-29-1984.
In the database approach, the views of different user groups are integrated during database design. Ideally, we should have a database design that stores each logical data item-such as a student's name or birth date-in only one placein the database. This ensures consistency, and it saves storage space. However, in practice, it is sometimes necessary to use controlled redundancy for improving the performance of queries.For example, we may store Studentl-Jame and CourseN umber redundantly in a GRADE_REPORT file because whenever we retrieve a GRADE_REPORT record, we want to retrieve the student name and course number along with the grade, student number,
and section identifier. By placing all the data together, we do not have to search
multiple files to collect this data. In such cases, the DBMS should have the capability to control this redundancy so as to prohibit inconsistencies among the files. This may be done by automatically checking that the StudentName-StudentNumber values in any GRADE_REPORT record in Figure 1.5a match one of the Name-StudentNumber values of a STUDENT record (Figure 1.2). Similarly, the SectionIdentifier-CourseNumber values in GRADE_REPORT can be checked against SECTION records. Such checks can be specified to the DBMS during database design and automatically enforced by the DBMS whenever the
GRADE_REPORT file is updated. Figure 1.5b shows a GRADE3EPORT record that is inconsistent with the STUDENT file of Figure 1.2, which may be entered erroneously if the redundancy is not controlled.

Restricting Unauthorized Access

When multiple users share a large database, it is likely that most users will not be authorized to access all information in the database. For example, financial data is often considered confidential, and hence only authorized persons are allowed to access such data. In addition, some users may be permitted only to retrieve data, whereas others are allowed both to retrieve and to update. Hence, the type of access operation-retrieval or update-must also be controlled. Typically, users or user groups are given account numbers protected by passwords, which they can use to gain access to the database. A DBMS should provide a security and authorization subsystem, which the DBA uses to create
accounts and to specify account restrictions. The DBMS should then enforce these restrictions automatically. Notice that we can apply similar controls to the DBMS software. For example, only the DBA's staff may be allowed to use certain privileged software, such as the software for creating new accounts. Similarly, parametric users may be allowed to access the database only through the canned transactions developed for their use. 1.6.3 Providing Persistent Storage for Program Objects Databases can be used to provide persistent storage for program objects and data structures. This is one of the main reasons for object-oriented database systems. Programming languages typically have complex data structures, such as record types in Pascal or class definitions in c++ or Java. The values of program variables are discarded once a program terminates, unless the programmer explicitly stores them in permanent files, which often
involves converting these complex structures into a format suitable for file storage. When the need arises to read this data once more, the programmer must convert from the file format to the program variable structure. Object-oriented database systems are compatible with programming languages such as c++ and Java, and the DBMS software automatically performs any necessary conversions. Hence, a complex object in c++ can be stored permanently in an object-oriented DBMS. Such an object is said to be persistent, since it survives the termination of program execution and can later be directly retrieved by another c+ + program. The persistent storage of program objects and data structures is an important function of database systems. Traditional database systems often suffered from the so called impedance mismatch problem, since the data structures provided by the DBMS were incompatible with the programming language's data structures. Object-oriented database systems typically offer data structure compatibility with one or more object oriented
programming languages.

Providing Storage Structures for Efficient Query
Processing Database systems must provide capabilities for efficiently executing queries and updates. Because the database is typically stored on disk, the DBMS must provide specialized data structures to speed up disk search for the desired records. Auxiliary files called indexes are used for this purpose. Indexes are typically based on tree data structures or hash data structures,
suitably modified for disk search. In order to process the database records needed by a particular query, those records must be copied from disk to memory. Hence, the DBMSoften has a buffering module that maintains parts of the database in main memory buffers. In other cases, the DBMSmay use the operating system to do the buffering of disk data.
The query processing and optimization module of the DBMS is responsible for
choosing an efficient query execution plan for each query based on the existing storage structures. The choice of which indexes to create and maintain is part of physical database design and tuning, which is one of the responsibilities of the DBA staff.

Providing Backup and Recovery

A DBMS must provide facilities for recovering from hardware or software failures. The backup and recovery subsystem of the DBMS is responsible for recovery. For example, if the computer system fails in the middle of a complex update transaction, the recovery subsystem is responsible for making sure that the database is restored to the state it was in before the transaction started executing. Alternatively, the recovery subsystem could ensure that the transaction is resumed from the point at which it was interrupted so that its full effect is recorded in the database.

Providing Multiple User Interfaces

Because many types of users with varying levels of technical knowledge use a database, a DBMS should provide a variety of user interfaces. These include query languages for casual users, programming language interfaces for application programmers, forms and command codes for parametric users, and menu-driven interfaces and natural language interfaces for stand-alone users. Both forms-style interfaces and menu-driven interfaces are commonly known as graphical user interfaces (GU Is). Many specialized languages and environments
exist for specifying GUls. Capabilities for providing Web GUl interfaces to a database- or Web-enabling a database-are also quite common.

Representing Complex Relationships among Data

A database may include numerous varieties of data that are interrelated in many ways. Consider the example shown in Figure 1.2. The record for Brown in the STUDENT file is related to four records in the GRADCREPDRT file. Similarly, each section record is related to one course record as well as to a number of GRADE_REPDRT records-one for each student who completed that section. A DBMS must have the capability to represent a variety of complex relationships among the data as well as to retrieve and update related data easily and efficiently.

Enforcing Integrity ~Constraints

Most database applications have certain integrity constraints that must hold for the data. A DBMSshould provide capabilities for defining and enforcing these constraints. The simplest type of integrity constraint involves specifying a data type for each data item. We may specify that the value of the Class data item within each STUDENT record must be an integer between 1 and 5 and that the value of Name must be a string of no more than 30 alphabetic characters. A more complex type of constraint that frequently occurs involves specifying that a record in one file must be related to records in other files. We can specify that "every section record must be related to a course record." Another type of constraint specifies uniqueness on data item values, such as "every course record must have a unique value for CourseNumber." These constraints are derived from the meaning or semantics of the data and of the miniworld it represents. It is the database designers' responsibility to identify integrity constraints during database design. Some constraints can be specified to the DBMS and automatically enforced. Other constraints may have to be checked by update programs or at the time of data entry. A data item may be entered erroneously and still satisfy the specified integrity constraints. For example, if a student receives a grade of A but a grade of C is entered in the database, the DBMS cannot discover this error automatically, because C is a valid value for the Grade data type. Such data entry errors can only be discovered manually (when
the student receives the grade and complains) and corrected later by updating the database. However, a grade of Z can be rejected automatically by the DBMS, because Z is not a valid value for the Grade data type.

Permitting Inference and Actions Using Rules

Some database systems provide capabilities for defining deduction rules for inference new information from the stored database facts. Such systems are called deductive database systems. For example, there may be complex rules in the miniworld application for determining when a student is on probation. These can be specified declarative as rules, which when compiled and maintained by the DBMS can determine all students on probation. In a traditional DBMS, an explicit procedural prof-,Jmm code would have to be written to support such applications. But if the miniworld rules change, it is generally more convenient to change the declared deduction rules than to re-code procedural programs. More powerful functionality is provided by active database systems, which provide active rules that can automatically initiate actions when certain events and conditions occur.

This section discusses some additional implications of using the database approach that can benefit most organizations. Potential for Enforcing Standards. The database approach permits the DBA to define and enforce standards among database users in a large organization. This facilitates communication and cooperation among various departments, projects, and users within the organization. Standards can be defined for names and formats of data elements,
display formats, report structures, terminology, and so on. The DBA can enforce standards in a centralized database environment more easily than in an environment where each user group has control of its own files and software.
Reduced Application Development Time. A prime selling feature of the database approach is that developing a new application-such as the retrieval of certain data from the database for printing a new report-takes very little time. Designing and implementing a new database from scratch may take more time than writing a single specialized file application. However, once a database is up and running, substantially less time is generally required to create new applications using DBMS facilities. Development time using a DBMS is estimated to be one-sixth to one-fourth of that for a traditional file system.


It may be necessary to change the structure of a database as requirements
change. For example, a new user group may emerge that needs information not currently in the database. In response, it may be necessary to add a file to the database or to extend the data elements in an existing file. Modern DBMSs allow certain types of evolutionary changes to the structure of the database without affecting the stored data and the existing application programs.

Availability of Up-to-Date Information 

A DBMS makes the database available to all users. As soon as one user's update is applied to the database, all other users can immediately see this update. This availability of up-to-date information is essential for many transaction-processing applications, such as reservation systems or banking databases,
and it is made possible by the concurrency control and recovery subsystems of a DBMS. Economies of Scale. The DBMS approach permits consolidation of data and applications, thus reducing the amount of wasteful overlap between activities of data processing personnel in different projects or departments. This enables the whole organization to invest in more powerful processors, storage devices, or communication gear, rather than having each department purchase its own (weaker) equipment. This reduces overall costs of operation and management.


  1. nice one man , you do your best full information , bery useful for me that study database concept ! thank you man.



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