presto-main/src/main/java/io/prestosql/operator/index/UnloadedIndexKeyRecordSet.java/UnloadedIndexKeyRecordCursor/advanceNextPosition
class UnloadedIndexKeyRecordCursor:        @Override
        public boolean advanceNextPosition()
        {
            while (positionIterator == null || !positionIterator.hasNext()) {
                if (!pageAndPositionsIterator.hasNext()) {
                    return false;
                }
                PageAndPositions pageAndPositions = pageAndPositionsIterator.next();
                page = pageAndPositions.getUpdateRequest().getPage();
                checkState(types.size() == page.getChannelCount());
                positionIterator = pageAndPositions.getPositions().iterator();
            }

            position = positionIterator.nextInt();

            return true;
        }