presto-iceberg/src/main/java/io/prestosql/plugin/iceberg/IcebergUtil.java/IcebergUtil/getIdentityPartitions
class IcebergUtil:    public static Map<PartitionField, Integer> getIdentityPartitions(PartitionSpec partitionSpec)
    {
        // TODO: expose transform information in Iceberg library
        ImmutableMap.Builder<PartitionField, Integer> columns = ImmutableMap.builder();
        for (int i = 0; i < partitionSpec.fields().size(); i++) {
            PartitionField field = partitionSpec.fields().get(i);
            if (field.transform().toString().equals("identity")) {
                columns.put(field, i);
            }
        }
        return columns.build();
    }