Mysql – Why MySQL doesn’t use indexes in view with union select

I've created a view:

create view dh_hits_new_all_old
as
select * from dh_hits_new_201806
union all
select * from dh_hits_new_201807;

When I select something with a where clause containing a field which has an index on the individual tables, MySQL doesn't use any indexes. See the explain outputs:

mysql> explain select count(*) from dh_hits_new_201806 where id_hits_url = 130442;
+----+-------------+--------------------+------------+------+-----------------------------+-----------------------------+---------+-------+------+----------+-------------+
| id | select_type | table              | partitions | type | possible_keys               | key                         | key_len | ref   | rows | filtered | Extra       |
+----+-------------+--------------------+------------+------+-----------------------------+-----------------------------+---------+-------+------+----------+-------------+
|  1 | SIMPLE      | dh_hits_new_201806 | NULL       | ref  | dh_idx-hits_new-id_hits_url | dh_idx-hits_new-id_hits_url | 4       | const | 4453 |   100.00 | Using index |
+----+-------------+--------------------+------------+------+-----------------------------+-----------------------------+---------+-------+------+----------+-------------+
1 row in set, 1 warning (0,00 sec)

mysql> explain select count(*) from dh_hits_new_201807 where id_hits_url = 130442;
+----+-------------+--------------------+------------+------+-----------------------------+-----------------------------+---------+-------+------+----------+-------------+
| id | select_type | table              | partitions | type | possible_keys               | key                         | key_len | ref   | rows | filtered | Extra       |
+----+-------------+--------------------+------------+------+-----------------------------+-----------------------------+---------+-------+------+----------+-------------+
|  1 | SIMPLE      | dh_hits_new_201807 | NULL       | ref  | dh_idx-hits_new-id_hits_url | dh_idx-hits_new-id_hits_url | 4       | const | 4009 |   100.00 | Using index |
+----+-------------+--------------------+------------+------+-----------------------------+-----------------------------+---------+-------+------+----------+-------------+
1 row in set, 1 warning (0,00 sec)

mysql> explain select count(*) from dh_hits_new_all_old where id_hits_url = 130442;
+----+-------------+--------------------+------------+------+---------------+-------------+---------+-------+-------+----------+-------+
| id | select_type | table              | partitions | type | possible_keys | key         | key_len | ref   | rows  | filtered | Extra |
+----+-------------+--------------------+------------+------+---------------+-------------+---------+-------+-------+----------+-------+
|  1 | PRIMARY     | <derived2>         | NULL       | ref  | <auto_key0>   | <auto_key0> | 4       | const |    10 |   100.00 | NULL  |
|  2 | DERIVED     | dh_hits_new_201806 | NULL       | ALL  | NULL          | NULL        | NULL    | NULL  | 65658 |   100.00 | NULL  |
|  3 | UNION       | dh_hits_new_201807 | NULL       | ALL  | NULL          | NULL        | NULL    | NULL  | 61552 |   100.00 | NULL  |
+----+-------------+--------------------+------------+------+---------------+-------------+---------+-------+-------+----------+-------+
3 rows in set, 1 warning (0,00 sec)

I do not understand why. This behavior is reflected of course also in the execution times:

mysql> select count(*) from dh_hits_new_201806 where id_hits_url = 130442;
+----------+
| count(*) |
+----------+
|     4453 |
+----------+
1 row in set (0,01 sec)

mysql> select count(*) from dh_hits_new_201807 where id_hits_url = 130442;
+----------+
| count(*) |
+----------+
|     4009 |
+----------+
1 row in set (0,00 sec)

mysql> select count(*) from dh_hits_new_all_old where id_hits_url = 130442;
+----------+
| count(*) |
+----------+
|     8462 |
+----------+
1 row in set (2,33 sec)