SELECT os, browser, sum(page_views) FROM test.ga_tb2
GROUP BY GROUPING SETS (os, browser) -- ここ！
HAVING sum(page_views) > 20
ORDER BY os, browser;

    os     |      browser      |  sum
-----------+-------------------+-------
 Android   |                   |  1036
 iOS       |                   |   703
 Linux     |                   |  1008
 Macintosh |                   |  2730
 Windows   |                   | 34628
           | Android Browser   |   254
           | Chrome            | 17598
           | Firefox           |  7834
           | Internet Explorer | 12265
           | Iron              |   101
           | Opera             |   460
           | Safari            |  1533
           | Safari (in-app)   |    37
(13 rows)