UDAF and group by

[Koert Kuipers]

If i run my own UDAF with group by, can i be sure that a single UDAF
instance initialized once will process all members in a group? Or should i
code so as to take into account the situation where even within a group
multiple UDAFs could run, and i would have to deal with terminatePartial()
and merge() even within a group?
My problem is that my results within a group are not easily merged, but
between groups they are.