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Merged
kumarUjjawal merged 7 commits into from
Apr 30, 2026
+620 −376
Merged

Use shared statistics merge for union stats #21430

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3018d63
Use shared statistics merge for union stats
kumarUjjawal Apr 7, 2026
4a97bdb
test: restore shared NDV coverage and reset testing submodule
kumarUjjawal Apr 7, 2026
e08e95f
test: add full shared NDV edge-case table
kumarUjjawal Apr 7, 2026
a964076
fix(stats): preserve NDV fallback semantics in shared merge
kumarUjjawal Apr 20, 2026
891da8f
explicit NDV fallback mode
kumarUjjawal Apr 23, 2026
4e3515e
Merge branch 'main' into feat/consolidated_satistics
kumarUjjawal Apr 28, 2026
62f1101
Merge branch 'main' into feat/consolidated_satistics
kumarUjjawal Apr 30, 2026
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300 changes: 297 additions & 3 deletions datafusion/common/src/stats.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -372,6 +372,27 @@ pub struct Statistics {
pub column_statistics: Vec<ColumnStatistics>,
}

/// Fallback to use when NDV overlap can not be estimated from column bounds.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
pub enum NdvFallback {
/// Use the larger input NDV. This is the conservative default for
/// related fragments such as files from the same table.
#[default]
Max,
/// Sum the input NDVs. This is a conservative upper bound for
/// independent inputs such as `UNION ALL`.
Sum,
}

impl NdvFallback {
fn merge(self, left: usize, right: usize) -> usize {
match self {
Self::Max => usize::max(left, right),
Self::Sum => left.saturating_add(right),
}
}
}

impl Default for Statistics {
/// Returns a new [`Statistics`] instance with all fields set to unknown
/// and no columns.
Expand Down Expand Up @@ -630,6 +651,9 @@ impl Statistics {
/// The method assumes that all statistics are for the same schema.
/// If not, maybe you can call `SchemaMapper::map_column_statistics` to make them consistent.
///
/// This method uses [`NdvFallback::Max`] when `distinct_count` overlap
/// can not be estimated from column bounds.
///
/// Returns an error if the statistics do not match the specified schemas.
///
/// # Example
Expand Down Expand Up @@ -670,6 +694,19 @@ impl Statistics {
/// Precision::Exact(ScalarValue::Int64(Some(1500))));
/// ```
pub fn try_merge_iter<'a, I>(items: I, schema: &Schema) -> Result<Statistics>
where
I: IntoIterator<Item = &'a Statistics>,
{
Self::try_merge_iter_with_ndv_fallback(items, schema, NdvFallback::Max)
}

/// Same as [`Statistics::try_merge_iter`], but lets callers choose the
/// fallback used when `distinct_count` overlap can not be estimated.
pub fn try_merge_iter_with_ndv_fallback<'a, I>(
items: I,
schema: &Schema,
ndv_fallback: NdvFallback,
) -> Result<Statistics>
where
I: IntoIterator<Item = &'a Statistics>,
{
Expand Down Expand Up @@ -717,7 +754,7 @@ impl Statistics {
) {
(Some(&l), Some(&r)) => Precision::Inexact(
estimate_ndv_with_overlap(col_stats, item_cs, l, r)
.unwrap_or_else(|| usize::max(l, r)),
.unwrap_or_else(|| ndv_fallback.merge(l, r)),
),
_ => Precision::Absent,
};
Expand Down Expand Up @@ -1465,6 +1502,44 @@ mod tests {
}
}

fn make_single_i64_ndv_stats(
distinct_count: Precision<usize>,
min_value: Option<i64>,
max_value: Option<i64>,
) -> Statistics {
let to_precision = |value| Precision::Exact(ScalarValue::Int64(Some(value)));

Statistics::default()
.with_num_rows(Precision::Exact(10))
.add_column_statistics(
ColumnStatistics::new_unknown()
.with_distinct_count(distinct_count)
.with_min_value(
min_value.map(to_precision).unwrap_or(Precision::Absent),
)
.with_max_value(
max_value.map(to_precision).unwrap_or(Precision::Absent),
),
)
}

fn merge_single_i64_ndv_distinct_count(
left: Statistics,
right: Statistics,
ndv_fallback: NdvFallback,
) -> Precision<usize> {
let schema = Schema::new(vec![Field::new("a", DataType::Int64, true)]);

Statistics::try_merge_iter_with_ndv_fallback(
[&left, &right],
&schema,
ndv_fallback,
)
.unwrap()
.column_statistics[0]
.distinct_count
}

#[test]
fn test_try_merge() {
// Create a schema with two columns
Expand Down Expand Up @@ -1815,7 +1890,7 @@ mod tests {

let schema = Schema::new(vec![Field::new("a", DataType::Int32, true)]);
let merged = Statistics::try_merge_iter([&stats1, &stats2], &schema).unwrap();
// No min/max -> fallback to max(5, 8)
// No min/max -> default fallback is max
assert_eq!(
merged.column_statistics[0].distinct_count,
Precision::Inexact(8)
Expand Down Expand Up @@ -1851,13 +1926,55 @@ mod tests {

let schema = Schema::new(vec![Field::new("a", DataType::Utf8, true)]);
let merged = Statistics::try_merge_iter([&stats1, &stats2], &schema).unwrap();
// distance() unsupported for strings -> fallback to max
// distance() unsupported for strings -> default fallback is max
assert_eq!(
merged.column_statistics[0].distinct_count,
Precision::Inexact(8)
);
}

#[test]
fn test_try_merge_ndv_non_numeric_types_sum_fallback() {
let stats1 = Statistics::default()
.with_num_rows(Precision::Exact(10))
.add_column_statistics(
ColumnStatistics::new_unknown()
.with_min_value(Precision::Exact(ScalarValue::Utf8(Some(
"aaa".to_string(),
))))
.with_max_value(Precision::Exact(ScalarValue::Utf8(Some(
"zzz".to_string(),
))))
.with_distinct_count(Precision::Exact(5)),
);
let stats2 = Statistics::default()
.with_num_rows(Precision::Exact(10))
.add_column_statistics(
ColumnStatistics::new_unknown()
.with_min_value(Precision::Exact(ScalarValue::Utf8(Some(
"bbb".to_string(),
))))
.with_max_value(Precision::Exact(ScalarValue::Utf8(Some(
"yyy".to_string(),
))))
.with_distinct_count(Precision::Exact(8)),
);

let schema = Schema::new(vec![Field::new("a", DataType::Utf8, true)]);
let merged = Statistics::try_merge_iter_with_ndv_fallback(
[&stats1, &stats2],
&schema,
NdvFallback::Sum,
)
.unwrap();

// distance() unsupported for strings -> sum fallback is caller-selected
assert_eq!(
merged.column_statistics[0].distinct_count,
Precision::Inexact(13)
);
}

#[test]
fn test_try_merge_ndv_constant_columns() {
// Same constant: [5,5]+[5,5] -> max
Expand Down Expand Up @@ -1910,6 +2027,183 @@ mod tests {
);
}

#[test]
fn test_try_merge_ndv_original_union_edge_cases() {
struct NdvTestCase {
name: &'static str,
left_ndv: Precision<usize>,
left_min: Option<i64>,
left_max: Option<i64>,
right_ndv: Precision<usize>,
right_min: Option<i64>,
right_max: Option<i64>,
expected: Precision<usize>,
}

let cases = vec![
NdvTestCase {
name: "disjoint ranges",
left_ndv: Precision::Exact(5),
left_min: Some(0),
left_max: Some(10),
right_ndv: Precision::Exact(3),
right_min: Some(20),
right_max: Some(30),
expected: Precision::Inexact(8),
},
NdvTestCase {
name: "identical ranges",
left_ndv: Precision::Exact(10),
left_min: Some(0),
left_max: Some(100),
right_ndv: Precision::Exact(8),
right_min: Some(0),
right_max: Some(100),
expected: Precision::Inexact(10),
},
NdvTestCase {
name: "partial overlap",
left_ndv: Precision::Exact(100),
left_min: Some(0),
left_max: Some(100),
right_ndv: Precision::Exact(50),
right_min: Some(50),
right_max: Some(150),
expected: Precision::Inexact(125),
},
NdvTestCase {
name: "right contained in left",
left_ndv: Precision::Exact(100),
left_min: Some(0),
left_max: Some(100),
right_ndv: Precision::Exact(50),
right_min: Some(25),
right_max: Some(75),
expected: Precision::Inexact(100),
},
NdvTestCase {
name: "same constant value",
left_ndv: Precision::Exact(1),
left_min: Some(5),
left_max: Some(5),
right_ndv: Precision::Exact(1),
right_min: Some(5),
right_max: Some(5),
expected: Precision::Inexact(1),
},
NdvTestCase {
name: "different constant values",
left_ndv: Precision::Exact(1),
left_min: Some(5),
left_max: Some(5),
right_ndv: Precision::Exact(1),
right_min: Some(10),
right_max: Some(10),
expected: Precision::Inexact(2),
},
NdvTestCase {
name: "left constant within right range",
left_ndv: Precision::Exact(1),
left_min: Some(5),
left_max: Some(5),
right_ndv: Precision::Exact(10),
right_min: Some(0),
right_max: Some(10),
expected: Precision::Inexact(10),
},
NdvTestCase {
name: "left constant outside right range",
left_ndv: Precision::Exact(1),
left_min: Some(20),
left_max: Some(20),
right_ndv: Precision::Exact(10),
right_min: Some(0),
right_max: Some(10),
expected: Precision::Inexact(11),
},
NdvTestCase {
name: "right constant within left range",
left_ndv: Precision::Exact(10),
left_min: Some(0),
left_max: Some(10),
right_ndv: Precision::Exact(1),
right_min: Some(5),
right_max: Some(5),
expected: Precision::Inexact(10),
},
NdvTestCase {
name: "right constant outside left range",
left_ndv: Precision::Exact(10),
left_min: Some(0),
left_max: Some(10),
right_ndv: Precision::Exact(1),
right_min: Some(20),
right_max: Some(20),
expected: Precision::Inexact(11),
},
NdvTestCase {
name: "missing bounds exact plus exact",
left_ndv: Precision::Exact(10),
left_min: None,
left_max: None,
right_ndv: Precision::Exact(5),
right_min: None,
right_max: None,
expected: Precision::Inexact(15),
},
NdvTestCase {
name: "missing bounds exact plus inexact",
left_ndv: Precision::Exact(10),
left_min: None,
left_max: None,
right_ndv: Precision::Inexact(5),
right_min: None,
right_max: None,
expected: Precision::Inexact(15),
},
NdvTestCase {
name: "missing bounds inexact plus inexact",
left_ndv: Precision::Inexact(7),
left_min: None,
left_max: None,
right_ndv: Precision::Inexact(3),
right_min: None,
right_max: None,
expected: Precision::Inexact(10),
},
NdvTestCase {
name: "exact plus absent",
left_ndv: Precision::Exact(10),
left_min: None,
left_max: None,
right_ndv: Precision::Absent,
right_min: None,
right_max: None,
expected: Precision::Absent,
},
NdvTestCase {
name: "inexact plus absent",
left_ndv: Precision::Inexact(4),
left_min: None,
left_max: None,
right_ndv: Precision::Absent,
right_min: None,
right_max: None,
expected: Precision::Absent,
},
];

for case in cases {
let actual = merge_single_i64_ndv_distinct_count(
make_single_i64_ndv_stats(case.left_ndv, case.left_min, case.left_max),
make_single_i64_ndv_stats(case.right_ndv, case.right_min, case.right_max),
NdvFallback::Sum,
);

assert_eq!(actual, case.expected, "case {} failed", case.name);
}
}

#[test]
fn test_with_fetch_basic_preservation() {
// Test that column statistics and byte size are preserved (as inexact) when applying fetch
Expand Down
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