Understanding File Structures in DBMS: How Data is Organized on Disk

Explore the different ways data is organized within files in database management systems (DBMS). Learn about the four main types of file organization: sequential, indexed sequential, direct, and hashed. Understand their characteristics, advantages, and disadvantages for various database applications.

DBMS - File Structure

In databases, related data and information are stored in files. A file is a sequence of records, stored in binary format, with records mapped to disk blocks for physical storage. Each disk drive is formatted into several blocks to store these records.

File Organization

File Organization defines how records are arranged within disk blocks. There are four main types of file organization:

Heap File Organization

In Heap File Organization, the operating system allocates memory space for the file without further organizing details. Records can be stored anywhere within this space, and the software manages their placement. Heap files do not support ordering, sequencing, or indexing.

Sequential File Organization

In Sequential File Organization, records are arranged sequentially based on a unique key or search field. Each record contains an attribute that uniquely identifies it. While logically sequential, physical storage may not always reflect this order due to practical limitations.

Hash File Organization

Hash File Organization uses a hash function to determine where records should be placed. The hash function is applied to specific fields, with its output dictating the disk block location for the record.

Clustered File Organization

In Clustered File Organization, related records from one or more tables are stored together within the same disk block. Unlike other methods, records are not ordered based on a primary or search key. This organization is generally unsuitable for large databases.

File Operations

File operations are broadly categorized into update operations and retrieval operations:

Update Operations

Update operations alter data values through insertion, deletion, or modification.

Retrieval Operations

Retrieval operations retrieve data based on optional filtering criteria without modifying it.

In addition to file creation and deletion, other common file operations include:

Open

Files can be opened in read or write mode. Read mode allows for shared data access without alteration, while write mode permits modifications but restricts sharing.

Locate

A file pointer indicates the current read or write position in the file. This pointer can be adjusted to locate data at specific points within the file.

Read

When opened in read mode, the file pointer is positioned at the beginning of the file by default. Users can also specify an initial pointer position when opening the file.

Write

Opening a file in write mode enables data editing. Users can insert, delete, or modify data, and may specify the file pointer location for these actions.

Close

Closing a file releases file locks, saves changes to secondary storage, and frees buffers and file handlers associated with the file.

The organization of data within a file affects how easily records can be located. Different file organizations impact how the file pointer locates desired records, with variations between sequential and clustered arrangements.