I want exactly that but a) for postgres and b) written in javascript/node so I can use it for in-process access while running test suites.
I know they speak mysql as a wire protocol, just would be cool to avoid another process/port to manage.
Basically, it's a custom version of PostgreSQL that supports git + Docker-like operations (versioning, sharing, branching, delta compression, Dockerfiles for datasets etc). It's not in-memory but it runs in Docker and has some tooling built around it to make it simple to spin up and manage a throwaway DB for integration test purposes.
Here's an example of it in action in use cases like machine learning [1] where we can switch between different versions (test/train) of the dataset without having to change the client application.
[0] https://github.com/splitgraph/splitgraph
[1] https://github.com/splitgraph/splitgraph/blob/master/example...