a tbl

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.

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.