《DBA Survivor: Become a Rock Star DBA》by Thomas LaRock，Published By Apress.2010
You know that a database is a collection of logically related data elements that may be structured in various ways lo meet the multiple processing and retrieval needs of organizations and individuals. Therersquo;s nothing new about databases—early ones were chiseled in stone, penned on scrolls, and written on index cards. But now databases are commonly recorded on magnetizable media, and computer programs are required to perform the necessary storage and retrieval operations.
Yoursquo;ll see in the following pages that complex data relationships and linkages may be found in all but the simplest databases. The system software package that handles the difficult tasks associated with creating, accessing, and maintaining database records is called a database management system (DBMS) .The programs in a DBMS package establish an interface between the database itself and the users of the database. (These users may be applications programmers, managers and others with information needs, and various OS programs.)
A DBMS can organize, process, and present selected data elements from the database. This capability enables decision makers to search, probe, and query database contents in order to extract answers to nonrecurring and unplanned questions (hat arent available in regular reports. These questions might initially be vague and / or poorly defined, but people can 'browse” through the database until they have the needed information. In short, the DBMS will “manage”the stored data items and assemble the needed items from the common database in response to the queries of those who arenrsquo;t
programmers. In a file-oriented system, users needing special information may communicate their needs to a programmer, who, when time permits, will write one or more programs to extract the data and prepare the information[4].The availability of a DBMS, however, offers users a much faster alternative communications path.
If the DBMS provides a way to interactively and update the database, as well as interrogate it capability allows for managing personal data-Aces however, it does not automatically leave an audit trail of actions and docs not provide the kinds of control a necessary in a multiuser organization. These-controls arc only available when a set of application programs arc customized for each data entry and updating function.
Software for personal computers which perform me of the DBMS functions have been very popular. Personal computers were intended for use by individuals for personal information storage and process- These machines have also been used extensively small enterprises, professionals like doctors, acrylics, engineers, lasers and so on .By the nature of intended usage, database systems on these machines except from several of the requirements of full doge database systems. Since data sharing is not tended, concurrent operations even less so. the fewer can be less complex. Security and integrity maintenance arc de-emphasized or absent. As data limes will be small, performance efficiency is also important. In fact, the only aspect of a database system that is important is data Independence. Data-dependence, as stated earlier, means that applicant programs and user queries need not recognizant physical organization of data on secondary storage. The importance of this aspect, particularly for the personal computer user, is that this greatly simplifies database usage. The user can store, access and manipulate data a( a high level (close to (he application) and be totally shielded from the low level (close to the machine) details of data organization. We will not discuss details of specific PC DBMS software packages here. Let us summarize in the following the strengths and weaknesses of personal computer data-base software systems：
The most obvious positive factor is the user friendliness of the software. A user with no prior computer background would be able to use the system to store personal and professional data, retrieve and perform relayed processing. The user should, of course, satiety himself about the quality of software and the freedom from errors (bugs) so that invest-merits in data arc protected.
For the programmer implementing applications with them, the advantage lies in the support for applications development in terms of input screen generations, output report generation etc. offered by theses stems.
The main negative point concerns absence of data protection features. Unless encrypted, data cane accessed by whoever has access to the machine Data can be destroyed through mistakes or malicious intent. The second weakness of many of the PC-based systems is that of performance. If data volumes grow up to a few thousands of records, performance could be a bottleneck.
For organization where growth in data volumes is expected, availability of. the same or compatible software on large machines should be considered.
This is one of the most common misconceptions about database management systems that are used in personal computers. Thoroughly comprehensive and sophisticated business systems can be developed in dBASE, Paradox and other DBMSs. However, they are created by experienced programmers using the DBMSs own programming language. Thai is not the same as users who create and manage personal files that are not part of the mainstream company system.
Transaction Management of Database
The objective of long-duration transactions is to model long-duration, interactive Database access sessions in application environments. The fundamental assumption about short-duration of transactions that underlies the traditional model of transactions is inappropriate for long-duration transactions. The implementation of the traditional model of transactions may cause intolerably long waits when transactions aleph to acquire locks before accessing data, an
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《DBA Survivor: Become a Rock Star DBA》by Thomas LaRock，Published By Apress.2010
You know that a database is a collection of logically related data elements that may be structured in various ways lo meet the multiple processing and retrieval needs of organizations and individuals. Therersquo;s nothing new about databases—early ones were chiseled in stone, penned on scrolls, and written on index cards. But now databases are commonly recorded on magnetizable media, and computer programs are required to perform the necessary storage and retrieval operations.
Yoursquo;ll see in the following pages that complex data relationships and linkages may be found in all but the simplest databases. The system software package that handles the difficult tasks associated with creating, accessing, and maintaining database records is called a database management system (DBMS) .The programs in a DBMS package establish an interface between the database itself and the users of the database. (These users may be applications programmers, managers and others with information needs, and various OS programs.)
A DBMS can organize, process, and present selected data elements from the database. This capability enables decision makers to search, probe, and query database contents in order to extract answers to nonrecurring and unplanned questions (hat arent available in regular reports. These questions might initially be vague and / or poorly defined, but people can 'browse” through the database until they have the needed information. In short, the DBMS will “manage”the stored data items and assemble the needed items from the common database in response to the queries of those who arenrsquo;t
programmers. In a file-oriented system, users needing special information may communicate their needs to a programmer, who, when time permits, will write one or more programs to extract the data and prepare the information[4].The availability of a DBMS, however, offers users a much faster alternative communications path.
If the DBMS provides a way to interactively and update the database, as well as interrogate it capability allows for managing personal data-Aces however, it does not automatically leave an audit trail of actions and docs not provide the kinds of control a necessary in a multiuser organization. These-controls arc only available when a set of application programs arc customized for each data entry and updating function.
Software for personal computers which perform me of the DBMS functions have been very popular. Personal computers were intended for use by individuals for personal information storage and process- These machines have also been used extensively small enterprises, professionals like doctors, acrylics, engineers, lasers and so on .By the nature of intended usage, database systems on these machines except from several of the requirements of full doge database systems. Since data sharing is not tended, concurrent operations even less so. the fewer can be less complex. Security and integrity maintenance arc de-emphasized or absent. As data limes will be small, performance efficiency is also important. In fact, the only aspect of a database system that is important is data Independence. Data-dependence, as stated earlier, means that applicant programs and user queries need not recognizant physical organization of data on secondary storage. The importance of this aspect, particularly for the personal computer user, is that this greatly simplifies database usage. The user can store, access and manipulate data a( a high level (close to (he application) and be totally shielded from the low level (close to the machine) details of data organization. We will not discuss details of specific PC DBMS software packages here. Let us summarize in the following the strengths and weaknesses of personal computer data-base software systems：
The most obvious positive factor is the user friendliness of the software. A user with no prior computer background would be able to use the system to store personal and professional data, retrieve and perform relayed processing. The user should, of course, satiety himself about the quality of software and the freedom from errors (bugs) so that invest-merits in data arc protected.
For the programmer implementing applications with them, the advantage lies in the support for applications development in terms of input screen generations, output report generation etc. offered by theses stems.
The main negative point concerns absence of data protection features. Unless encrypted, data cane accessed by whoever has access to the machine Data can be destroyed through mistakes or malicious intent. The second weakness of many of the PC-based systems is that of performance. If data volumes grow up to a few thousands of records, performance could be a bottleneck.
For organization where growth in data volumes is expected, availability of. the same or compatible software on large machines should be considered.
This is one of the most common misconceptions about database management systems that are used in personal computers. Thoroughly comprehensive and sophisticated business systems can be developed in dBASE, Paradox and other DBMSs. However, they are created by experienced programmers using the DBMSs own programming language. Thai is not the same as users who create and manage personal files that are not part of the mainstream company system.
Transaction Management of Database
The objective of long-duration transactions is to model long-duration, interactive Database access sessions in application environments. The fundamental assumption about short-duration of transactions that underlies the traditional model of transactions is inappropriate for long-duration transactions. The implementation of the traditional model of transactions may cause intolerably long waits when transactions aleph to acquire locks before accessing data, an
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