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FILE ORGANIZATION

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File Structure

In DBMs Data are stored in files

A Database may contain several files Each file contains a number of records

Each record contain various attributes

Each attribute is called a field

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Database Definitions

Entity: Object, Concept or event (subject)

Attribute: a Characteristic of an entity Row or Record: the specific characteristics of

one entity

Table: a collection of records

Database: a collection of tables

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File Organisation

The technique used to represent and store

the records on a file is called the fileorganisation

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TYPES OF FILE ORGANIZATION

Serial Sequential

Indexed Sequential

Direct Access /Random Access

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Serial File Organization

A collection of records

No particular sequence Cannot be used as master

Used as temporary transaction file

Records stored in the order received

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Sequential File Organization

A collection of records

Stored in key sequence Adding/deleting record requires making new

file

Used as master files

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Sequential file

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Advantages

Simple file design

Very efficient when most of the records mustbe processed e.g. Payroll

Very efficient if the data has a natural order

Can be stored on inexpensive devices likemagnetic tape.

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Disadvantages

Entire file must be processed even if a single

record is to be searched. Transactions have to be sorted before

processing

Overall processing is slow

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Direct (Random) File Organization

Records are read directly from or written on

to the file. The records are stored at known address.

Address is calculated by applying amathematical function to the key field.

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Direct (Random) File Organization

A random file would have to be stored on a

direct access backing storage medium e.g.magnetic disc, CD, DVD

Example : Any information retrieval system.

Eg Train timetable system.

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Advantages

Any record can be directly accessed.

Speed of record processing is very fast. Up-to-date file because of online updating.

Concurrent processing is possible.

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Disadvantages

More complex than sequential

Does not fully use memory locations More security and backup problems

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Indexed sequential file

Each record of a file has a key field whichuniquely identifies that record.

An index consists of keys and addresses.

An indexed sequential file is a sequentialfile (i.e. sorted into order of a key field) which

has an index. A full index to a file is one in which there is

an entry for every record.

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Indexed sequential file

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Indexed sequential file

Indexed sequential files are important for

applications where data needs to beaccessed.....

sequentially

randomly using the index.

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Indexed sequential file

An indexed sequential file can only be stored

on a random access devicee.g. magnetic disc, CD.

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Advantages

Provides flexibility for users who need both

type of accesses with the same file Faster than sequential

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Disadvantages

Extra storage space for the index is required