ss method to apply these tests itself, which
   means that it must reach into the heap to check the commit status of
   any row that is shown to have a duplicate key according to the index
   contents.  This is without a doubt ugly and non-modular, but it saves
   redundant work: if we did a separate probe then the index lookup for
   a conflicting row would be essentially repeated while finding the place to
   insert the new row's index entry.  What's more, there is no obvious way
   to avoid race conditio