presto-main/src/main/java/io/prestosql/operator/OuterLookupSource.java/OuterPositionTracker/getOuterPositionIterator
class OuterPositionTracker:        public synchronized OuterPositionIterator getOuterPositionIterator()
        {
            finished = true;
            return new SharedLookupOuterPositionIterator(lookupSourceSupplier.get(), visitedPositions);
        }