The DuckDB API I was talking above seem to already meet your use-cases?

- Does this[1] solve the group by wish?

- Depending on what you mean by projection, maybe this[2] or this[3]?

[1] https://duckdb.org/docs/api/python/relational_api#aggregatee...

[2] https://duckdb.org/docs/api/python/relational_api#projectexp...

[3] https://duckdb.org/docs/api/python/expression#column-express...