1. Data Hosting: Amazon RDS for PostgreSQL provides a fully managed relational database service, offering features like backups, monitoring, and scaling, while PostgREST is a standalone tool that acts as a REST API server for any PostgreSQL database, allowing developers to directly interact with the database.
2. Elasticity and Scalability: Amazon RDS for PostgreSQL allows for easy scaling of compute and storage resources based on the workload, while PostgREST may require additional configurations and optimizations for scalability depending on the database infrastructure.
3. Management Complexity: Amazon RDS for PostgreSQL simplifies database management tasks like patching, backups, and high availability setup, whereas with PostgREST, developers need to handle these tasks manually or through additional automation tools.
4. Query Optimization: Amazon RDS for PostgreSQL offers query optimization features like Performance Insights and automatic tuning, enhancing database performance, whereas PostgREST relies on the underlying PostgreSQL database performance capabilities without additional optimization tools.
5. Security Features: Amazon RDS for PostgreSQL provides security features like encryption at rest and in transit, IAM integration, and VPC isolation for enhanced data protection, while PostgREST may require additional security configurations to ensure data privacy and integrity.
6. Cost Structure: Amazon RDS for PostgreSQL follows a pay-as-you-go pricing model based on database instance size and allocated storage, while PostgREST is an open-source tool with no direct costs associated, but may require investments in infrastructure and maintenance.

In Summary, Amazon RDS for PostgreSQL offers a fully managed relational database service with features like scalability and security, while PostgREST serves as a standalone tool for building REST APIs directly on a PostgreSQL database, requiring manual management and optimization.