presto-main/src/main/java/io/prestosql/operator/PartitionedLookupSource.java/PartitionedLookupSource/positionVisited
class PartitionedLookupSource:        public void positionVisited(int partition, int position)
        {
            if (!written) {
                written = true;
                verify(!finished.get());
                referenceCount.incrementAndGet();
            }
            visitedPositions[partition][position] = true;
        }