Question

What is a fragment of a relation? What are the main types of fragments? Why is fragmentation a useful concept in distributed database design?

Short answer

A fragment of a relation is a subset of a database, which can be stored separately. The main types of fragments are horizontal, vertical, and hybrid. Fragmentation improves efficiency, speed, reliability, and provides local autonomy in a distributed database system.

Step-by-step solution

Definition of Fragmentation

Fragmentation is the process of dividing a database into several pieces or fragments. Each fragment is stored in a separate location, but the system allows the fragments to work together to form a single logical database.

Main Types of Fragments

The main types of fragments in a database are horizontal fragmentation, vertical fragmentation, and hybrid fragmentation. Horizontal fragmentation divides the database into subsets of tuples. Vertical fragmentation divides the database by attribute or column, thus creating fragments that only contain some of the attributes. Hybrid fragmentation uses a combination of both horizontal and vertical fragmentation.

Advantages of Fragmentation

Fragmentation is a useful concept in distributed database design due to its numerous benefits. One of these benefits includes improved efficiency and speed because queries can be processed in parallel on different fragments. It also allows for increased reliability and availability - even if one fragment fails, the rest can continue to operate. Additionally, it accommodates for local autonomy, letting users maintain control over data related to them.

Key concepts

Distributed Database Design

Distributed database design is a complex process that involves the systematic planning and creation of databases spread across different locations, interconnected by a network. This design strategy aims to handle data in a way that optimizes availability, reliability, and performance across various geographical sites.

A crucial aspect of distributed database design is ensuring data integrity and consistency despite its dispersion. Data is strategically placed and replicated to enhance access speed and reduce network traffic, which is essential for businesses that operate on a global scale. Local and global transactions are handled in a manner that maintains the ACID properties (Atomicity, Consistency, Isolation, Durability) of the database operations.

Distributed databases can improve organizational flexibility by allowing for data storage closer to the point of use, enhancing user response times and satisfaction. However, this approach to database design also introduces challenges in synchronization, transaction management, and security that must be carefully addressed during the system architecture phase.

Horizontal Fragmentation

Horizontal fragmentation, an integral type of fragmentation in databases, entails dividing a table or relation into rows or tuples based on specific criteria. This method creates subsets of a table where each row of the original table appears in at least one subset.

For instance, in a university database, student records might be horizontally fragmented by academic department. This means that student data for the computer science department could be stored separately from the data of the art department.

Benefits of Horizontal Fragmentation:

• Performance Improvement: Queries searching for specific rows can be directed to the relevant fragment instead of scanning the entire table.
• Enhanced Security: Sensitive information can be isolated in separate fragments and safeguarded accordingly.
• Localized Data Management: Data closer to the users that require it reduces access times and improves overall efficiency.

Vertical Fragmentation

Vertical fragmentation complements horizontal fragmentation by splitting a table or relation into columns instead of rows. This process creates new tables that contain one or more columns from the original table, each carrying its own primary key to ensure data integrity.

In practical terms, if a customer database table initially contains columns for customer ID, name, address, and order history, vertical fragmentation could result in one table with customer ID and name, and another with customer ID and order history. Such an approach can align the database structure with application requirements and access patterns.

Why Use Vertical Fragmentation:

• Access Optimization: Applications that only require certain attributes of data can interact with leaner, more targeted tables.
• Reduction in I/O: Reading fewer columns for each query can significantly reduce input/output operations.
• Improved Security and Control: As with horizontal fragmentation, sensitive columns can be stored and protected separately from the rest of the dataset.

Vertical fragmentation necessitates careful attention to the inclusion of primary keys in each fragment to maintain referential integrity and allow for proper reassembly of data when needed.

Hybrid Fragmentation

Hybrid fragmentation represents the synthesis of both horizontal and vertical fragmentation techniques. It enables the splitting of a database’s relation into smaller, more manageable segments using a two-dimensional approach – by rows and columns simultaneously.

This dual stratagem can be particularly beneficial when there's a need to optimize for complex queries that address a subset of rows and require specific columns. For example, in a national sales database, a hybrid fragment could consist of sales records (rows) for a particular region that only includes columns for product ID and sales figures.

Implementing Hybrid Fragmentation:

• Determining Criteria: Decisions about how to fragment must consider the most routine and critical queries to ensure that each fragment efficiently serves its purpose.
• Reconstruction Ability: Despite fragmentation, there must be a mechanism to recombine the data accurately when needed, which often involves maintaining certain key attributes within each fragment.
• Optimization of Resources: By targeting specific data points with hybrid fragmentation, storage, and computation resources, utilization can be significantly optimized.

Hybrid fragmentation, while offering many advantages, also involves greater complexity in terms of design and maintenance. Therefore, it should be applied judiciously, keeping in mind the unique requirements of the database and its users.