if (prefix != null && prefix.length == 0)
prefix = null;
Iterator<Entry<byte[], byte[]>> overlayIterator = overlay.prefixLookup(prefix, snapId, true,
ascending);
InternalDiskIndexIterator diskIndexIterator = null;
if (index != null) {
byte[][] rng = comp.prefixToRange(prefix, ascending);
diskIndexIterator = index.internalRangeLookup(rng[0], rng[1], ascending);
}