cookie  page_num,
1234      1
1234      2
1234      3
1234      4
4444      1
4444      2

select cookie, collect_set(page_num) as page_set
from mytable
group by cookie

1234  [1,2,3,4]
4444  [1,2]

1234 [2,4,1,3]
4444 [2,1]

SELECT cookie, map_keys(
         collect_max( page_num, row_order, MAX_NUM_PAGES ) )
FROM (
    SELECT cookie, page_num, group_count( cookie ) as row_order
      FROM mutable
      DISTRIBUTE BY cookie
      SORT BY cookie, my_sort_column
) sc
GROUP BY cookie;