ule, it is possible for an index reader to
   see an index entry just before it is removed by <TT
CLASS="COMMAND"
>VACUUM</TT
>, and
   then to arrive at the corresponding heap entry after that was removed by
   <TT
CLASS="COMMAND"
>VACUUM</TT
>.
   This creates no serious problems if that item
   number is still unused when the reader reaches it, since an empty
   item slot will be ignored by <CODE
CLASS="FUNCTION"
>heap_fetch()</CODE
>.  But what if a
   third backend has already re-used the item slot for