This interface is used during Reduce phrase to process tuples
in batch mode. It is used by POPackage when all of the UDFs can be
called in accumulative mode. Tuples are not pulled all at once,
instead, each time, only a specified number of tuples are pulled out
of iterator and put in an buffer. Then this buffer is wrapped into
a bag to be passed to the operators in reduce plan.
The purpose of doing this is to reduce memory usage and avoid spilling.