** A distributed SQL database optimizes secondary index queries with Index Only Scan, a technique that boosts performance-critical queries by accessing the index directly without making extra hops to the table. This approach is particularly useful for range scans on large tables, where traditional indexes may not be efficient. By adding columns to the index key and including additional columns in the "include" list, developers can create fat indexes that cover multiple queries while minimizing overhead on insert, delete, and update operations. Index Only Scan can significantly improve performance by reducing table access and cache usage, making it an essential optimization technique for distributed SQL databases.