Applies R/S-Plus functions to groups of remote DBMS rows without bringing an entire result set all at once. The result set is expected to be sorted by the grouping field.
A list with as many elements as there were groups in the result set.
This generic is meant to handle somewhat gracefully(?) large amounts
of data from the DBMS by bringing into R manageable chunks;
the idea is that the data from individual groups can be handled by R, but
not all the groups at the same time.