SELECT os, browser, sum(page_views) FROM test.ga_tb2
GROUP BY os, browser
HAVING sum(page_views) > 20 -- 見やすさのため主要なものに限定. 以下のクエリも同じ
ORDER BY os, browser;

    os     |      browser      |  sum
-----------+-------------------+-------
 Android   | Android Browser   |   254
 Android   | Chrome            |   472
 Android   | Firefox           |   232
 Android   | Opera             |    73
 iOS       | Chrome            |    41
 iOS       | Safari            |   625
 iOS       | Safari (in-app)   |    37
 Linux     | Chrome            |   301
 Linux     | Firefox           |   692
 Macintosh | Chrome            |  1392
 Macintosh | Firefox           |   409
 Macintosh | Safari            |   897
 Windows   | Chrome            | 15377
 Windows   | Firefox           |  6492
 Windows   | Internet Explorer | 12264
 Windows   | Iron              |    84
 Windows   | Opera             |   359
(17 rows)