D CREATE TABLE approx_distinct_map_test AS SELECT * FROM (VALUES
    (1, MAP {'a': 1, 'b': 2}), (1, MAP {'a': 1, 'b': 2}), (1, MAP {'c': 3}),
    (2, MAP {'d': 4}), (2, NULL),
    (3, NULL), (3, NULL),
    (4, MAP {'e': 5})
  ) AS t(g, m);

D SELECT approx_count_distinct(m) FROM approx_distinct_map_test WHERE g = 1;
┌──────────────────────────┐
│ approx_count_distinct(m) │
│          int64           │
├──────────────────────────┤
│            2             │
└──────────────────────────┘

D SELECT g, approx_count_distinct(m) FROM approx_distinct_map_test GROUP BY g ORDER BY g;
┌───────┬──────────────────────────┐
│   g   │ approx_count_distinct(m) │
│ int32 │          int64           │
├───────┼──────────────────────────┤
│     1 │                        2 │
│     2 │                        1 │
│     3 │                        0 │
│     4 │                        1 │
└───────┴──────────────────────────┘

D SELECT approx_count_distinct(m) FROM approx_distinct_map_test;
┌──────────────────────────┐
│ approx_count_distinct(m) │
│          int64           │
├──────────────────────────┤
│            4             │
└──────────────────────────┘