CTAS with partition writes an internal field into generated parquet files

select * from sys.version;
+-----------------+-------------------------------------------+---------------------------------------------------------------------+----------------------------+-----------------+----------------------------+
|     version     |                 commit_id                 |                           commit_message                            |        commit_time         |   build_email   |         build_time         |
+-----------------+-------------------------------------------+---------------------------------------------------------------------+----------------------------+-----------------+----------------------------+
| 1.5.0-SNAPSHOT  | 9a3a5c4ff670a50a49f61f97dd838da59a12f976  | DRILL-4382: Remove dependency on drill-logical from vector package  | 16.02.2016 @ 11:58:48 PST  | jni@apache.org  | 16.02.2016 @ 17:40:44 PST  |
+-----------------+-------------------------------------------+---------------------------------------------------------------------+----------------------------+-----------------

use dfs.tmp;

create table nation_ctas partition by (n_regionkey) as select * from cp.`tpch/nation.parquet`;

select * from dfs.tmp.nation_ctas limit 6;
+--------------+----------------+--------------+-----------------------------------------------------------------------------------------------------------------+----------------------------------------+
| n_nationkey  |     n_name     | n_regionkey  |                                                    n_comment                                                    | P_A_R_T_I_T_I_O_N_C_O_M_P_A_R_A_T_O_R  |
+--------------+----------------+--------------+-----------------------------------------------------------------------------------------------------------------+----------------------------------------+
| 5            | ETHIOPIA       | 0            | ven packages wake quickly. regu                                                                                 | true                                   |
| 15           | MOROCCO        | 0            | rns. blithely bold courts among the closely regular packages use furiously bold platelets?                      | false                                  |
| 14           | KENYA          | 0            |  pending excuses haggle furiously deposits. pending, express pinto beans wake fluffily past t                   | false                                  |
| 0            | ALGERIA        | 0            |  haggle. carefully final deposits detect slyly agai                                                             | false                                  |
| 16           | MOZAMBIQUE     | 0            | s. ironic, unusual asymptotes wake blithely r                                                                   | false                                  |
| 24           | UNITED STATES  | 1            | y final packages. slow foxes cajole quickly. quickly silent platelets breach ironic accounts. unusual pinto be  | true