Cursors are generally better, but they don't give any hint about how much more is in front of the cursor. I don't think there is a O(log n) algorithm to keep track of what position a given item is in a collection (k'th item out of n), which still keeps the other CRUD operations at O(log n). This would be useful to split a large, dynamically changing result set into smaller parts that can be scanned through using a cursor.