The Index-Only Scan is an index scan without subsequent table access—hence, accessing the index only.
All recent versions of Oracle, SQL Server and MySQL support index-only scans. The PostgreSQL database supports index-only scans since release 9.2.
A query can be executed with an Index-Only Scan if all queried data is available in the index. That means that even the columns that appear in the select clause only, must be included in the index.
The performance advantage of an index-only scans depends on the number of accessed rows and the index clustering factor.
Links
Book-Section: Index-Only Scan: Avoiding Table Access
Glossary: Covering Index — Alternative name for index-only scan
