Accumulating Snapshots: Lifecycle Tracking

Accumulating snapshots are the least understood fact table type and the one that creates the most interview separation. The grain is one row per lifecycle instance (one order, one loan application, one support ticket). The row is created at the first milestone and updated as subsequent milestones are reached. This is the only fact table type where rows are updated. The Schema the Interviewer Expects The Update Pattern: Why This Is the Only Updatable Fact Type The Idempotency Guard You Must Mention The WHERE shipped_date_sk IS NULL clause makes this update idempotent. If the pipeline runs twice, the second execution finds no rows to update. Mentioning this idempotency guard unprompted is a strong interview signal. The Follow-Up Trap The Comparison Table That Closes the Question