><P
>        Allow queries to retrieve data only from indexes, avoiding heap
        access (Robert Haas, Ibrar Ahmed, Heikki Linnakangas, Tom Lane)
       </P
><P
>        This feature is often called <I
CLASS="FIRSTTERM"
>index-only scans</I
>.
        Heap access can be skipped for heap pages containing only tuples that
        are visible to all sessions, as reported by the visibility map; so
        the benefit applies mainly to mostly-static data.  The visibility map
        was made crash-safe as a n