DBIsam یعنی چه؟

[nasr]

سلام
آقا DBISam یعنی چه
آیا مخفف چند تا کلمه است
توی Help اش که چیزی ننوشته بود
راستش
یکی پرسید بانک اطلاعات برنامه ات چیه گفتم DBISam گفت DBISam دیگه چیه من هم توش موندم :متفکر: یه کمی توضیح دادم و خلاصه کلش کردم رفت :D

تشکر

[مهدی کرامتی]

DBISAM‌ مخفف دو عبارت DB و ISAM است.
DB مخفف DataBase و ISAM مخفف Indexed Sequential Access Method میباشد.

توضیحات بیشتر از Babylon:

ISAM

Indexed Sequential Access Method - used to locate specific records within a database . An index containing the information needed to access each individual record is kept up to date along with the database.

[Fardin_Lord]

اینم یک مقاله خارجی در مورد ISAM




ISAM (Indexed Sequential Access Method).
ISAM is an access method developed by IBM in the late 1960’s to provide both sequential access and indexed access via a key. ISAM preceeded IBM’s VSAM and B-trees as a way to exploit the random access capability of disks. This capability permitted indexed access for the first time without a sequential scan, which tape based access had supported. Data was accessed via key ranges, the search being directed by index terms that determined ranges of key values. Each index term was composed of a <key,ptr> pair. The key of an index term defined the low key value of keys accessed via the ptr of the index term, the high key value being the key of the next higher index term, much as in a B-tree index. That is, the correct index term for a record was the index term where the key of a record was greater than or equal to the key associated with the term, and less than the key of the immediate higher index term.

ISAM exploited the geometry of the disk in its organization. The root of the index was a node whose index terms identified the disk cylinder on which to continue the search. Each cylinder of an ISAM file had an index on a track of the cylinder whose index terms identified the track upon which to continue the search. Finally, search on the track was done via the disk hardware. IBM disks of this period were organized on the tracks in what was called “key-count-data” format. That is, preceding each block of data was a key that identified the immediately following data, a count that indicated the length of this data, and the data itself. Once the track index had identified the correct track for the data, the disk hardware would search the track for the data associated with the search key. Reorganization of the data in the presence of inserts was handled via overflow buckets. If, for example, there was insufficient room on the track identified by the track index for the data with a given key, this data, with its key, would be added to the overflow area, usually on the same cylinder as the “home” track.

ISAM lost favor among the database community for two reasons. First, managing the overflow area was cumbersome. B-trees and VSAM were much more elegant and efficient. Second, ISAM was specific to IBM disks supporting the key-count-data organization. Over time, disk manufacturers, including IBM, moved to what was called a block oriented organization. This block organization permitted more data to be packed into a disk track.


Back to Dictionary Index

References.
Sakti P. Ghosh and Michael E. Senko, “File Organization: On the Selection of Random Access Index Points for Sequential Files,” Journal of the ACM 16 (October 1969): 569-579.

IBM System/360 Operating System Indexed Sequential Access Method, Form No. Y28-6618, IBM Corp., White Plains, NY. (circa 1968)

Robert E. Wagner, “Indexing Design Considerations,” IBM Systems Journal 12 (December 1973): 351-367.

1/6/03

[مهدی کرامتی]

راستی تا یادم نرفته بگم، اینی که نوشتم معنی تحت‌الفظی کلمه است.

در حقیقت DBISAM اسمی است که از ترکیب موارد فوق بوجود آمده، اما ممکن است نظر سازندگان آن دقیقا این نباشد.

[شفیعی]

سلام
ببخشید DBISAM را از کجا می توان دانلود نمود

[hr110]

توضیحات بیشتر از Babylon
جناب کرامتی از کدامیک از واژه نامه های babylon استفاده میکنید؟؟ :متفکر:

[emad4000]

اگه میشه به صورت مختصر بگید خود ISAM چیه ؟؟ ( به صورت فارسی )
ممنون میشم

[مهدی کرامتی]

به فارسی شرمنده، اما:



ISAM stands for Indexed Sequential Access Method, a method for storing data for fast retrieval. ISAM was originally developed by IBM (http://en.wikipedia.org/wiki/IBM) and today forms the basic data store of almost all databases, both relational (http://en.wikipedia.org/wiki/Relational_database) and otherwise.
In an ISAM system, data is organized into records which are composed of fixed length fields. Records are stored sequentially, originally to speed access on a tape system. A secondary set of hash tables (http://en.wikipedia.org/wiki/Hash_table) known as indexes contain "pointers" into the tables, allowing individual records to be retrieved without having to search the entire data set. This is a departure from the contemporary navigational databases (http://en.wikipedia.org/wiki/Navigational_database), in which the pointers to other data were stored inside the records themselves. The key improvement in ISAM is that the indexes are small and can be searched quickly, allowing the database to then access only the records it needs. Additionally modifications to the data do not require changes to other data, only the table and indexes in question.
Relational databases can be easily built on an ISAM framework with the addition of logic to maintain the validity of the links between the tables. Typically the field being used as the link, the foreign key (http://en.wikipedia.org/wiki/Foreign_key), will be indexed for quick lookup. While this is slower than simply storing the pointer to the related data directly in the records, it also means that changes to the physical layout of the data do not require any updating of the pointers -- the entry will still be valid.
ISAM is very simple to understand and implement, as it primarily consists of direct, sequential access to a database file. It is also very inexpensive. The tradeoff is that each client machine must manage its own connection to each file it accesses. This, in turn, leads to the possibility of conflicting inserts into those files, leading to an inconsistent database state. This is typically solved with the addition of a client-server (http://en.wikipedia.org/wiki/Client-server) framework which marshals client requests and maintains ordering. This is the basic concept behind SQL (http://en.wikipedia.org/wiki/SQL), which is a client layer over the underlying data store.
ISAM was replaced at IBM with a methodology called VSAM (http://en.wikipedia.org/wiki/Virtual_storage_access_method) (Virtual Storage Access Method). Still later, IBM developed DB2 (http://en.wikipedia.org/wiki/DB2), which, as of 2004, IBM promotes as their primary database management system.



مرجع: http://en.wikipedia.org/wiki/ISAM