Looking for a database(preferrably with an SQL interface) I can run across multiple canisters

Is there anything decently finished?

There’s GitHub - sudograph/sudograph: GraphQL database for the Internet Computer but it looks forgotten and unmaintained
I also found GitHub - duckdb/duckdb-wasm: WebAssembly version of DuckDB which can be compiled to wasm, but I bet it won’t run on icp nor it was tested…

Storing stuff in memory is nice but can get complicated quiet fast, it also would be really nice to have a solid abstraction layer for data storage that is

  • simple to use
  • scalable across multiple canisters
  • can store hundreds of gigabytes of indexed data that is readily available for fetching

Is there anything battle tested in this area or is it yet to be created?

I suggest you also have a look at CanDB. I don’t think it has an SQL interface, but it could fit your use case pretty well