presto-main/src/main/java/io/prestosql/operator/OuterLookupSource.java/OuterLookupSource/appendToNext
class OuterLookupSource:        @Override
        public synchronized boolean appendToNext(PageBuilder pageBuilder, int outputChannelOffset)
        {
            while (currentPosition < visitedPositions.length) {
                if (!visitedPositions[currentPosition]) {
                    lookupSource.appendTo(currentPosition, pageBuilder, outputChannelOffset);
                    currentPosition++;
                    return true;
                }
                currentPosition++;
            }
            return false;
        }