if there are zero rows to aggregate over, and the API seems both conceptually
and notationally ugly anyway.  We should look for something that improves
on the tsquery-and-text-SELECT version (which is also pretty ugly but at
least it works...), but it seems that will take query infrastructure that
doesn't exist today.  (Hm, I wonder if there's anything in or near SQL2003
window functions that would help?)  Per discussion.