Logical and physical design are two important steps in designing a database. Logical design involves organizing data into a conceptual model, while physical design translates the logical design into a schema optimized for a physical database. There are some key differences between logical and physical design:
Logical Design
Logical design focuses on the data requirements and structural relationships in a database. The main components of logical design include:
– Determining the entities and relationships: Identifying the key entities, attributes, and relationships that need to be represented.
– Creating an entity relationship diagram: Visualizing entities, attributes and relationships in an ERD diagram.
– Choosing keys: Selecting appropriate primary keys and foreign keys to link tables.
– Normalization: Structuring tables to minimize redundancy and dependency.
– Defining integrity constraints: Rules governing column values, uniqueness, primary keys etc.
The goal of logical design is to create an accurate representation of the data requirements, unbiased by physical database considerations. It describes the data in as much detail as possible without regard to how they will be physically implemented.
Physical Design
Physical design focuses on how data will be stored, accessed, and managed in a database management system. Important aspects include:
– Database storage structure: Whether to use a relational structure, object-oriented, flat files etc.
– Table partitioning: Breaking down large tables into smaller partitions.
– Indexes: Adding database indexes to enable fast data access.
– Hardware resources: Estimating server, storage, I/O and network capacity.
– Data clustering: Physically grouping related data together in storage.
– Data access paths: Determining optimal query routes.
The goal of physical design is to translate the logical schema into an optimal physical database focused on performance and efficiency. Unlike logical design, physical design takes into account hardware resources, system workload, indexing, and storage details.
Key Differences
Here are some of the key differences between logical and physical design:
Data Abstraction
Logical design focuses on describing data at a high level using conceptual data models. Physical design is much more concrete, dealing with the implementation of storage and data access.
Independence from Technology
Logical design is independent of specific database technologies. Physical design is tightly coupled to technology like SQL Server, Oracle, MongoDB etc.
Flexibility
Logical design aims to be flexible to changing business needs. Physical design balances flexibility with performance optimization.
Complexity
Logical design models tend to be simpler and easier to understand by non-technical users. Physical design incorporates more implementation details.
Focus Areas
Logical design is focused on data, relationships, entities, and constraints. Physical design deals more with indexes, storage, servers, and hardware capacity.
Order of Execution
Logical design generally comes before and guides physical design. Some physical design decisions may require revisiting logical models.
Audience
Logical design requires more input from business and domain experts. Physical design needs greater participation from DBAs, architects and infrastructure teams.
Adaptability to Change
Logical models are easier to change compared to physical implementation. Extensive physical design changes may require restructuring the database.
Tooling
Logical design uses conceptual modeling tools like ER diagrams. Physical design incorporates database-specific DDL, DCL scripts and diagrams.
Logical Design Process
Here are the key steps in the logical database design process:
Requirements Gathering
Work with business stakeholders to understand data content, transactions, and reporting needs for the system. Document requirements in detail.
Entity Relationship Modeling
Identify key entities, attributes, relationships and cardinality. Create an initial entity relationship diagram. Get feedback from stakeholders.
Normalization
Normalize entity relation diagram to eliminate redundancy and dependencies. Verify using normal forms.
Data Model Review
Review data model with stakeholders. Refine model iteratively based on feedback. Verify completeness and accuracy.
Specify Integrity Rules
Define integrity constraints like primary keys, foreign keys, domain checks, valid values etc.
Document Logical Model
Fully document the logical data model detailing entities, attributes, relationships, keys, and integrity rules.
Model Sign-off
Formal sign-off on logical model. Freeze data model before proceeding to physical design.
Physical Design Process
Here are the key aspects of the physical database design process:
Choose Database Technology
Select DBMS software like Oracle, SQL Server, MySQL etc. based on capabilities, costs and organizational standards.
Map Logical Model to Technology
Map entities, attributes and relationships to database structures supported by chosen DBMS.
Specify Physical Storage
Determine database storage requirements. Allocate storage across tables, files and databases.
Table and Index Design
Design tables and columns to match data types required. Add appropriate indexes to optimize performance.
Partition Tables
Break large tables into smaller partitions to improve manageability. Distribute across storage.
Denormalize Structures
Denormalize where appropriate to optimize read performance. Avoid update anomalies.
Physical Model Review
Perform technical review of model for performance, concurrency, availability, recoverability. Tune based on feedback.
Prototype and Test
Build prototype database. Perform testing to validate design decisions and meet requirements.
Documentation
Document all physical design details including scripts, diagrams, storage layouts, and DDL.
Implementation
Perform final database implementation, testing and migration. Provide support during rollout.
Benefits of Logical Design
Here are some of the benefits provided by proper logical database design:
- Desribes data requirements comprehensively
- Provides flexibility to accommodate new data needs
- Eliminates data redundancy
- Minimizes dependency between modules
- Improves data consistency and integrity
- Simpler to understand for users
- Easier to make changes
- Enables standard data modeling techniques
- Facilitates application development
- Increased ROI through reuse of data
Overall, logical design establishes strong data foundations before physical optimization.
Benefits of Physical Design
Here are some key benefits of proper physical database design:
- Optimizes database performance
- Improves concurrency and throughput
- Enables administrative efficiency
- Maximizes availability and reliability
- Simplifies capacity planning
- Reduces infrastructure costs
- Facilitates backups and recovery
- Provides stability and prevents corruption
- Makes the most of hardware capabilities
- Positions deployment for scalability
Overall, physical design aims to maximize database robustness, efficiency, and capability.
Conclusion
Logical and physical design play complementary roles in database development. Logical design models information requirements and relationships in an implementation-neutral way. Physical design maps the logical model to efficient database structures tuned for performance and scalability. While logical design comes first, physical design is focused on technology implementation. Getting both logical and physical design right is critical for realizing the full value of database systems for an organization.
