Using Advanced Grouping

SELECT country, tier, count(*)
  FROM customers
 GROUP BY GROUPING SETS ((country, tier), (country), (tier), ());

SELECT year, quarter, sum(revenue)
  FROM sales GROUP BY ROLLUP (year, quarter);
-- Equivalent to GROUPING SETS ((year,quarter),(year),())

SELECT region, product, sum(revenue)
  FROM sales GROUP BY CUBE (region, product);
-- All 2^n combinations

SELECT
   coalesce(country,'(all)') AS country,
   coalesce(tier,'(all)')    AS tier,
   GROUPING(country, tier)   AS gid,    -- bitmask
   count(*)
  FROM customers GROUP BY ROLLUP (country, tier);

SELECT country, tier,
       count(*) FILTER (WHERE active),
       count(*) FILTER (WHERE NOT active)
  FROM customers GROUP BY GROUPING SETS ((country, tier),(country),());

SELECT count(*) FROM orders GROUP BY GROUPING SETS (());