presto-main/src/main/java/io/prestosql/operator/PartitionedLookupSource.java/PartitionedLookupOuterPositionIterator/appendToNext
class PartitionedLookupOuterPositionIterator:        @Override
        public synchronized boolean appendToNext(PageBuilder pageBuilder, int outputChannelOffset)
        {
            while (currentSource < lookupSources.length) {
                while (currentPosition < visitedPositions[currentSource].length) {
                    if (!visitedPositions[currentSource][currentPosition]) {
                        lookupSources[currentSource].appendTo(currentPosition, pageBuilder, outputChannelOffset);
                        currentPosition++;
                        return true;
                    }
                    currentPosition++;
                }
                currentPosition = 0;
                currentSource++;
            }
            return false;
        }