Explanation of File Organization | Fourth Semester | BSc.CSIT (TU)

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File OrganizationMeaning of File Organization | BSc.CSIT
Fourth Semester | Second Year | Tribhuvan University (TU)
Subject: System Analysis and Design (SAD)

File Organization
Physical files are defined as a named set of table rows stored in contiguous section of secondary memory. The way the OS arranges table rows in a files is called file organization. With some database technologies, the system designer can choose among different organizations. File organization should provide following properties;

  1. Fast data retrieval
  2. Highest throughput for processing transaction
  3. Efficient use of storage space
  4. Protection from failures or data loss
  5. Minimal need for reorganization
  6. Accommodation of growth
  7. Security from unauthorized use

Often these objectives conflict so, we must select an organization for each file that provides a reasonable balance among above criteria.

Types;

i) Sequential File Organization:-
In sequential file organization, rows in file are stored in sequence according to primary key value. To locate a row, program must normally scan file from beginning until desired row is located. For example; alphabetical list of phone numbers in phone directory. It could be fast if we want to process row sequentially but impractical for random retrieval. Deleting rows can cause waste space or the need to compress file and adding row requires rewriting file.

ii) Indexed File Organization:
In indexed file organization, rows are stored either sequentially or non-sequentially and index is created that allows the software to locate individual rows. For example; card catalog in library. Index can point to unique rows or to potentially more than 1 row.

Disadvantages
• Extra space required to store indexes
• Extra time necessary to access and maintain indexes

Advantages
• Both random and sequential processing are practical
• Can build multiple index structure on same data files
• Software may rapidly find records that have compound condition

When using index, trade-off between important performance for retrieval and degrading for insert, delete, update.

iii) Hashed File Organization
In hashed file organization, location of each row is determined using an algorithm that converts primary key into row address. Rows are located non-sequentially as dictated by hashing algorithm. Sequential data processing is impractical in this file organization but retrieval of random rows are very fast. There may be issues regarding two primary key that translate to same address.

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