presto-main/src/main/java/io/prestosql/operator/LookupJoinOperator.java/LookupJoinOperator/getPartitionGenerator
class LookupJoinOperator:        private LocalPartitionGenerator getPartitionGenerator()
        {
            if (!partitionGenerator.isPresent()) {
                partitionGenerator = Optional.of(new LocalPartitionGenerator(hashGenerator, lookupSourceFactory.partitions()));
            }
            return partitionGenerator.get();
        }