PostgREST vs MapDB: What are the differences?
PostgREST: Automatic REST API for Any Postgres Database. PostgREST serves a fully RESTful API from any existing PostgreSQL database. It provides a cleaner, more standards-compliant, faster API than you are likely to write from scratch; MapDB: Concurrent Maps, Sets and Queues backed by disk storage or off-heap-memory. MapDB provides Java Maps, Sets, Lists, Queues and other collections backed by off-heap or on-disk storage. It is a hybrid between java collection framework and embedded database engine. It is free and open-source under Apache license.
PostgREST can be classified as a tool in the "Database Tools" category, while MapDB is grouped under "In-Memory Databases".
PostgREST is an open source tool with 12.7K GitHub stars and 595 GitHub forks. Here's a link to PostgREST's open source repository on GitHub.