What is the difference between logical and physical schema?

A logical schema and a physical schema are two distinct layers in database design that serve different purposes. The logical schema defines the structure of the data from a user or application perspective. It focuses on what data is stored, including tables, columns, relationships, and constraints, without specifying how the data is physically stored. For example, a logical schema might define a customers table with columns like id, name, and email, along with primary keys and foreign keys linking to other tables. This layer is often designed using tools like entity-relationship diagrams (ERDs) and is independent of any database management system (DBMS) or storage details.

The physical schema, in contrast, describes how the data is stored on disk. It includes implementation-specific details like storage engines, indexes, file paths, partitioning strategies, and data types optimized for performance. For instance, the physical schema might specify that the customers table uses a B-tree index on the id column, stores data in a specific filegroup in SQL Server, or uses columnar storage in a system like Apache Parquet. This layer is tightly coupled with the DBMS and hardware, as it addresses efficiency, access speed, and resource utilization. Developers and database administrators often adjust the physical schema to optimize queries or scale the system.

The key difference lies in abstraction: the logical schema is a high-level design that remains stable even if the underlying storage changes, while the physical schema is tied to technical decisions that evolve with performance needs. For example, altering the logical schema (like adding a birthdate column) impacts application code, whereas changing the physical schema (like adding an index) typically doesn’t require rewriting queries. Separating these layers allows developers to modify storage optimizations without disrupting how applications interact with the data, ensuring flexibility and maintainability in database systems.