Index Only Scan
Index Only Scan is a database query optimization technique where a query can be answered entirely from an index without needing to access the underlying table data. This occurs when all columns required by the query are present in the index, allowing the database to retrieve results directly from the index structure. It significantly improves performance by reducing I/O operations and avoiding table lookups.
Developers should use Index Only Scan when designing queries and indexes to optimize read-heavy workloads, such as analytical queries or reporting systems. It is particularly valuable in databases like PostgreSQL or MySQL for queries that filter and retrieve columns already indexed, as it minimizes disk access and speeds up response times. Implementing this requires careful index design to include all necessary columns (covering indexes) to enable the scan.