For large data volumes a data integration with an ETL tool to directly connect to the PostgreSQL database will be the most efficient.
The frePPLe database schema is very transparent and straightforward to allow this type of integration.
Keep in mind however:
Your ETL-interface will need to capture and handle all types of invalid data. With all other integration methods the frePPLe API layer catches and reports such errors, but not in this integration mode.
The database schema can change between releases. We provide no guarantuee that your ETL-interface will be future-proof.