ict.
  </P
><P
>   This solution requires that index scans be <SPAN
CLASS="QUOTE"
>"synchronous"</SPAN
>: we have
   to fetch each heap tuple immediately after scanning the corresponding index
   entry.  This is expensive for a number of reasons.  An
   <SPAN
CLASS="QUOTE"
>"asynchronous"</SPAN
> scan in which we collect many TIDs from the index,
   and only visit the heap tuples sometime later, requires much less index
   locking overhead and can allow a more efficient heap access pattern.
   Per the above