When should you denormalize and what are the risks?

When to denormalize: 1) Measured performance bottleneck from JOINs in hot-path queries. 2) Reporting tables that are read-heavy and rarely updated. 3) When the JOIN cost exceeds the storage/sync cost. Methods: add redundant columns (customer_name in orders), materialized views, summary tables, caching layer. Risks: 1) Data inconsistency: redundant data must be kept in sync. Solution: triggers, app