IndexBuildHeapScan and RIDs order

Hi,

I was looking at the code for bitmap index:

http://archives.postgresql.org/pgsql-hackers/2008-10/msg01691.php

and I couldn't understand why during "bmbuild" (the ambuild call for
bitmap indexes) the code checks for not-ordered ItemPointerData(s).

In which cases the ItemPointerData(s) given by IndexBuildHeapScan

are not in order (when allow_sync=false)?



Leonardo