presto-main/src/main/java/io/prestosql/operator/PartitionedLookupSource.java/PartitionedLookupSource/getJoinPositionCount
class PartitionedLookupSource:    @Override
    public long getJoinPositionCount()
    {
        return Arrays.stream(lookupSources)
                .mapToLong(LookupSource::getJoinPositionCount)
                .sum();
    }