presto-main/src/main/java/io/prestosql/operator/index/IndexLookupSource.java/IndexLookupSource/getNextJoinPosition
class IndexLookupSource:    @Override
    public long getNextJoinPosition(long currentJoinPosition, int probePosition, Page allProbeChannelsPage)
    {
        long nextPosition = indexedData.getNextJoinPosition(currentJoinPosition);
        checkState(nextPosition != UNLOADED_INDEX_KEY);
        // INVARIANT: currentPosition is -1 or a valid currentPosition greater than or equal to zero
        return nextPosition;
    }