Impala - transform columns to rows without using UNION ALL

I have a table with users and either activity for each week, for example this is the user_activity table:

userid | wk1 | wk2 | wk3
u1     |  1  |  0  |  1
u2     |  0  |  1  |  0
u3     |  1  |  0  |  1

I would like to transform it to:

week   | active
wk1    |   2
wk2    |   1
wk3    |   2

I can achieve this using UNION ALL like this:

SELECT 'wk1' as week,
       SUM( wk1 ) AS active
FROM user_activity
UNION ALL
SELECT 'wk2' as week,
       SUM( wk2 ) AS active
FROM user_activity
UNION ALL
SELECT 'wk3' as week,
       SUM( wk3 ) AS active
FROM user_activity;

Is there a way to achieve the same without using UNION ALL?

Thanks!

Edit: Impala version: 2.6.0

Reason for no UNION ALL: For each SELECT, the entire table will be scanned from HDFS. If I have a huge table, this will result in OOM errors.

Solution

This should solve your performance issue.
The table is being scanned only once.
The records are not being duplicated X3.
The UNION ALL is used here only for a single record.

select  concat('wk',cast(c.i as string))    as week
       ,case c.i 
            when 1 then wk1 
            when 2 then wk2
            when 3 then wk3
        end                 as active

from   (select  sum(wk1) AS wk1
               ,sum(wk2) AS wk2
               ,sum(wk3) AS wk3 

        from    user_activity
        ) t

        cross join (            select 1 as i
                    union all   select 2 
                    union all   select 3
                    ) c
;

+------+--------+
| week | active |
+------+--------+
| wk1  | 2      |
| wk2  | 1      |
| wk3  | 2      |
+------+--------+