presto-main/src/main/java/io/prestosql/sql/planner/SystemPartitioningHandle.java/SystemPartitionFunction/createBucketFunction
class SystemPartitionFunction:            @Override
            public BucketFunction createBucketFunction(List<Type> partitionChannelTypes, boolean isHashPrecomputed, int bucketCount)
            {
                if (isHashPrecomputed) {
                    return new HashBucketFunction(new PrecomputedHashGenerator(0), bucketCount);
                }
                int[] hashChannels = new int[partitionChannelTypes.size()];
                for (int i = 0; i < partitionChannelTypes.size(); i++) {
                    hashChannels[i] = i;
                }

                return new HashBucketFunction(new InterpretedHashGenerator(partitionChannelTypes, hashChannels), bucketCount);
            }