How does HANA merge delta store data into the main store?

A background process called the delta merge periodically takes the accumulated data in the delta store and folds it into the main store, re-applying the main store's heavier compression and columnar optimization to that newly merged data, then clearing the delta store to start accumulating fresh writes again.

flowchart TD
    A[Delta store accumulates writes] --> B{Merge threshold reached?}
    B -->|Yes| C[Delta merge: fold data into main store, re-compress]
    C --> D[Delta store cleared, ready for new writes]
    D --> A

This merge can be triggered automatically (based on delta store size or a time interval) or manually via MERGE DELTA OF <table>. It's a resource-intensive operation (since it involves rebuilding compressed column structures), so it's typically scheduled during lower-activity periods for very large, heavily-written tables, balancing keeping the delta store small (for fast reads) against the cost of running merges too frequently.