...
Expressions to apply to each group. If named, results will be
stored in a new column. If unnamed, should return a data frame. You can
use .
to refer to the current group. You can not mix named and
unnamed arguments.
.chunk_size
The size of each chunk to pull into R. If this number is
too big, the process will be slow because R has to allocate and free a lot
of memory. If it's too small, it will be slow, because of the overhead of
talking to the database.