Backfills and Reprocessing

A backfill recomputes historical data after a bug fix, a new column, or a logic change, and it is where fragile pipelines break. The safe pattern is partition-by-partition reprocessing with idempotent writes so reruns do not double-count, on isolated compute so production stays healthy, and validated against the old table before you swap. Applied-AI interviews probe it because backfilling years of data without corrupting live tables or melting the warehouse separates engineers who have run production from those who have not.