 analysis, we must use the synchronous approach for
   non-MVCC-compliant snapshots, but an asynchronous scan is workable
   for a query using an MVCC snapshot.
  </P
><P
>   In an <CODE
CLASS="FUNCTION"
>amgetbitmap</CODE
> index scan, the access method does not
   keep an index pin on any of the returned tuples.  Therefore
   it is only safe to use such scans with MVCC-compliant snapshots.
  </P
><P
>   When the <TT
CLASS="STRUCTFIELD"
>ampredlocks</TT
> flag is not set, any scan using that
   index acces