One of the Hydro creators here. Ballista (and the ecosystem around Arrow and Parquet) are much more focused on analytical query processing whereas Hydro is bringing the concepts from the query processing world to the implementation of distributed systems. Our goal isn't to execute a SQL query, but rather to treat your distributed systems code (e.g a microservice implementation) like it is a SQL query. Integration with Arrow and Parquet are definitely planned in our roadmap though!