**Architectural Logic**: Schema evolution must preserve compatibility and enable safe rollout. **Strategies**: (1) Additive only—add columns with defaults; avoid drops/renames. (2) Schema registry—version schemas; validate on read. (3) Backfill—populate new columns for historical. (4) Multiple versions—support old and new during transition; deprecate. (5) Schema-on-read—struct/variant for flexible. (6) dbt—version models; run migrations. **Why Additive**: Non-additive changes break downstream....
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