2011-09-28Index-Only Scan

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. PostgreSQL does not yet support index-only scans, but they will probably come with PostgreSQL 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.