When an index used by the optimizer contains all the data from the underlying table that is required to satisfy the query, it may be possible to completely avoid reading values from the underlying table and retrieve the data directly from the indexes. This is referred to as an index-only retrieval. Index-only retrievals reduce the amount of I/O and cache required to satisfy a query, and improve performance. The optimizer performs an index only retrieval whenever possible.

See also

• FROM clause
• Using predicates in queries
• optimization_goal option [database]
• Reading execution plans