Indexes

Covering Indexes and Index-Only Scans for Faster Database Reads

An index normally helps a database find rows. A covering index can go further: it contains all columns needed by a query, allowing the database engine to answer some reads without fetching every matching row from the table. That can reduce random I/O for read-heavy workloads, but it also makes indexes larger and writes more expensive. Covering is a workload-specific optimization, not a reason to copy every selected column into every index.