What is the difference between an Upsert strategy and a Mod strategy in Delta Lake for incremental data loading?

In Delta Lake, an Upsert (merge) strategy writes new rows and updates existing rows in a single transaction. It uses a merge statement that matches on a key and applies insert or update actions. A Mod strategy, often called a modification or incremental load, typically reads only the changed or new rows (e.g., via a watermark or hash key) and writes them to the target, sometimes using a replace or append operation. The key difference is that Upsert guarantees idempotent writes and handles both insert and update in one step, while Mod may require separate logic to detect changes and may not guarantee atomicity for updates. Use Upsert when you need to maintain a single source of truth with both new and updated records, such as slowly changing dimensions. Use Mod when you can rely on a change‑data‑capture mechanism and only need to append or replace changed rows, which can be more efficient for large streams.