 something else?
   When using an MVCC-compliant snapshot, there is no problem because
   the new occupant of the slot is certain to be too new to pass the
   snapshot test.  However, with a non-MVCC-compliant snapshot (such as
   <TT
CLASS="LITERAL"
>SnapshotNow</TT
>), it would be possible to accept and return
   a row that does not in fact match the scan keys.  We could defend
   against this scenario by requiring the scan keys to be rechecked
   against the heap row in all cases, but that is too expensi