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. AI, ML, and GenAI 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.