presto-main/src/main/java/io/prestosql/operator/PartitionedLookupSource.java/PartitionedLookupSource/getJoinPosition
class PartitionedLookupSource:    @Override
    public long getJoinPosition(int position, Page hashChannelsPage, Page allChannelsPage, long rawHash)
    {
        int partition = partitionGenerator.getPartition(rawHash);
        LookupSource lookupSource = lookupSources[partition];
        long joinPosition = lookupSource.getJoinPosition(position, hashChannelsPage, allChannelsPage, rawHash);
        if (joinPosition < 0) {
            return joinPosition;
        }
        return encodePartitionedJoinPosition(partition, toIntExact(joinPosition));
    }