presto-main/src/main/java/io/prestosql/operator/HashAggregationOperator.java/HashAggregationOperator/calculateDefaultOutputHash
class HashAggregationOperator:    private static long calculateDefaultOutputHash(List<Type> groupByChannels, int groupIdChannel, int groupId)
    {
        // Default output has NULLs on all columns except of groupIdChannel
        long result = INITIAL_HASH_VALUE;
        for (int channel = 0; channel < groupByChannels.size(); channel++) {
            if (channel != groupIdChannel) {
                result = CombineHashFunction.getHash(result, NULL_HASH_CODE);
            }
            else {
                result = CombineHashFunction.getHash(result, BigintType.hash(groupId));
            }
        }
        return result;
    }