to prefer index scans; raising it will
        make index scans look relatively more expensive.  You can raise
        or lower both values together to change the importance of disk I/O
        costs relative to CPU costs, which are described by the following
        parameters.
       </P
><P
>        Random access to mechanical disk storage is normally much more expensive
        than four-times sequential access.  However, a lower default is used
        (4.0) because the majority of random accesses to d