public class PrefetchIterator<K>
An iterator that pre-fetches a number of items in order to calculate the size
of the result if possible. This iterator loads at least a number of items,
and then tries to load some more items until the timeout is reached or the
maximum number of entries are read.
Get the size if known. This call might pre-fetch data. The returned value
is unknown if the actual size is larger than the number of pre-fetched
elements, unless the end of the iterator has been reached already.