fetch retrieves the content of the result set in the form of a
data frame. If n is -1 then the current implementation fetches
32k rows first and then (if not sufficient) continues with chunks of
512k rows, appending them. If the size of the result set is known in
advance, it is most efficient to set n to that size.
dbClearResult releases the result set.
dbColumnInfo returns basic information about the columns
(fields) in the result set.
dbGetInfo returns an empty list.
dbListResults returns an empty list and warns that JDBC doesn't
track results.