1. Authentication and Authorization: Active Admin provides an easy way to implement authentication and authorization for your admin interface, allowing you to control who can access and perform actions within the system. PostgREST, on the other hand, focuses more on automatically generating RESTful APIs directly from PostgreSQL database schemas, without explicitly providing built-in authentication and authorization features.
2. Querying and Filtering: Active Admin simplifies the process of querying and filtering data in the admin interface by providing pre-built components and functions. PostgREST, in contrast, relies on the capabilities of PostgreSQL for handling complex queries and filtering data directly at the database level, leading to potentially more optimized performance.
3. Client-side Interaction: Active Admin typically involves server-side rendering of the admin interface, with limited options for client-side interactivity. PostgREST, however, allows for building rich client-side applications that can interact with the generated RESTful APIs to perform various actions on the data stored in the PostgreSQL database.
4. Customization Flexibility: Active Admin offers a high degree of customization flexibility through the use of Ruby code and DSL (Domain-Specific Language) configurations. PostgREST, on the other hand, focuses on leveraging the existing PostgreSQL features and structures, which may limit the extent of customization available for certain functionalities in the generated APIs.
5. Data Validation: Active Admin provides built-in support for data validation and form validation, ensuring that the data entered by users meets the specified criteria. In contrast, PostgREST relies on the data validation mechanisms already present in the PostgreSQL database, such as constraints and triggers, to enforce data integrity and consistency.
6. Integration with External Services: Active Admin allows for seamless integration with external services and APIs through custom code implementation. PostgREST, however, primarily focuses on exposing the PostgreSQL database as a RESTful API and may require additional middleware or tools for integrating with external services beyond the database layer.

In Summary, Active Admin and PostgREST differ in terms of authentication, querying capabilities, client-side interaction, customization flexibility, data validation, and integration with external services.