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Add support for dictionary for approx_distinct #24646

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2 changes: 2 additions & 0 deletions datafusion/functions-aggregate/src/approx_distinct.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -840,6 +840,7 @@ impl AggregateUDFImpl for ApproxDistinct {
| DataType::Map(_, _)
| DataType::Struct(_)
| DataType::Union(_, _)
| DataType::Dictionary(_, _)

@kumarUjjawal kumarUjjawal Aug 25, 2026

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Would it work to match on the value type instead, so a dictionary is supported exactly when its values would be? Something like DataType::Dictionary(_, value_type)

@mkleen mkleen Aug 25, 2026 •
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Good point. Floats will be unsupported see #23084, So we should exclude them in all container types.

@Jefffrey Jefffrey Aug 26, 2026

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i still feel we can support floats here if we already support it in regular distinct count 🤔

also for reference seems duckdb supports:

memory D select approx_count_distinct(a) from values (1.5::double), (1.5), ('nan'::double), (null), (0) t(a);
┌──────────────────────────┐
│ approx_count_distinct(a) │
│          int64           │
├──────────────────────────┤
│                        3 │
└──────────────────────────┘

@mkleen mkleen Aug 26, 2026

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Ok makes sense. I will look into this.

| DataType::LargeBinary => Box::new(HLLAccumulator::new()),
DataType::Null => {
Box::new(NoopAccumulator::new(ScalarValue::UInt64(Some(0))))
Expand Down Expand Up @@ -919,6 +920,7 @@ fn is_hll_groups_type(data_type: &DataType) -> bool {
| DataType::Map(_, _)
| DataType::Struct(_)
| DataType::Union(_, _)
| DataType::Dictionary(_, _)

@kumarUjjawal kumarUjjawal Aug 25, 2026

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would it make sense to have both dictionary arms on the value type, so Dictionary(_, value_type) is accepted only when is_hll_groups_type(value_type) is true?

@Jefffrey Jefffrey Aug 28, 2026

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this is a good point

@mkleen mkleen Aug 28, 2026

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That's a good idea. Thank you!

)
}

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34 changes: 34 additions & 0 deletions datafusion/sqllogictest/test_files/aggregate.slt
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Expand Up @@ -1793,6 +1793,40 @@ SELECT g, approx_distinct(arrow_cast(arrow_cast(s, 'Binary'), 'FixedSizeBinary(1
4 1


# Dictionary: dictionary-encoded values must hash identically to the plain
# (non-dictionary) values, so the counts below match the Utf8 case above.

# Dictionary non-grouped
query I
SELECT approx_distinct(arrow_cast(s, 'Dictionary(Int32, Utf8)')) FROM approx_distinct_group_test WHERE g = 2;
----
2

# Dictionary grouped
query II
SELECT g, approx_distinct(arrow_cast(s, 'Dictionary(Int32, Utf8)')) FROM approx_distinct_group_test GROUP BY g ORDER BY g;
----
1 2
2 2
3 0
4 1

# Dictionary with a non-string value type (Int32), also exercising a
# larger (Int64) key type
query I
SELECT approx_distinct(arrow_cast(i, 'Dictionary(Int64, Int32)')) FROM approx_distinct_group_test WHERE g = 2;
----
2

query II
SELECT g, approx_distinct(arrow_cast(i, 'Dictionary(Int64, Int32)')) FROM approx_distinct_group_test GROUP BY g ORDER BY g;
----
1 2
2 2
3 0
4 1


# List
statement ok
CREATE TABLE approx_distinct_list_test (g INT, l INT[]) AS VALUES
Expand Down