Add skewness(x) in Aggregation function

README.md

@@ -84,4 +84,5 @@ SELECT min_by(x, y) FROM VALUES (1, 10), (2, 5), (3, 15), (4, 8) as tab(x, y);
 - [x] `mode(expression) -> scalar` - Returns the most frequent (mode) value from a column of data.
 - [x] `max_by(expression1, expression2) -> scalar` - Returns the value of `expression1` associated with the maximum value of `expression2`.
 - [x] `min_by(expression1, expression2) -> scalar` - Returns the value of `expression1` associated with the minimum value of `expression2`.
+- [x] `skewness(expression) -> scalar` - Computes the skewness value for `expression`.
 - [x] `kurtois_pop(expression) -> scalar` - Computes the excess kurtosis (Fisher’s definition) without bias correction.

src/lib.rs

@@ -29,18 +29,21 @@ pub mod common;
 pub mod kurtosis_pop;
 pub mod max_min_by;
 pub mod mode;
+pub mod skewness;
 pub mod expr_extra_fn {
     pub use super::kurtosis_pop::kurtosis_pop;
     pub use super::max_min_by::max_by;
     pub use super::max_min_by::min_by;
     pub use super::mode::mode;
+    pub use super::skewness::skewness;
 }
 
 pub fn all_extra_aggregate_functions() -> Vec<Arc<AggregateUDF>> {
     vec![
         mode_udaf(),
         max_min_by::max_by_udaf(),
         max_min_by::min_by_udaf(),
+        skewness::skewness_udaf(),
         kurtosis_pop::kurtosis_pop_udaf(),
     ]
 }

src/skewness.rs

@@ -0,0 +1,161 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+use arrow::array::{ArrayRef, AsArray};
+use arrow::datatypes::{Float64Type, UInt64Type};
+use datafusion::arrow::datatypes::{DataType, Field};
+use datafusion::common::ScalarValue;
+use datafusion::logical_expr::{function::AccumulatorArgs, function::StateFieldsArgs};
+use datafusion::logical_expr::{Accumulator, AggregateUDFImpl, Signature, Volatility};
+use std::any::Any;
+use std::ops::{Div, Mul, Sub};
+
+make_udaf_expr_and_func!(SkewnessFunc, skewness, x, "Computes the skewness value.", skewness_udaf);
+
+#[derive(Debug)]
+pub struct SkewnessFunc {
+    name: String,
+    signature: Signature,
+}
+
+impl Default for SkewnessFunc {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl SkewnessFunc {
+    pub fn new() -> Self {
+        Self {
+            name: "skewness".to_string(),
+            signature: Signature::coercible(vec![DataType::Float64], Volatility::Immutable),
+        }
+    }
+}
+
+impl AggregateUDFImpl for SkewnessFunc {
+    fn as_any(&self) -> &dyn Any {
+        self
+    }
+    fn name(&self) -> &str {
+        &self.name
+    }
+
+    fn signature(&self) -> &Signature {
+        &self.signature
+    }
+
+    fn return_type(&self, _arg_types: &[DataType]) -> datafusion::common::Result<DataType> {
+        Ok(DataType::Float64)
+    }
+
+    fn accumulator(&self, _acc_args: AccumulatorArgs) -> datafusion::common::Result<Box<dyn Accumulator>> {
+        Ok(Box::new(SkewnessAccumulator::new()))
+    }
+
+    fn state_fields(&self, _args: StateFieldsArgs) -> datafusion::common::Result<Vec<Field>> {
+        Ok(vec![
+            Field::new("count", DataType::UInt64, true),
+            Field::new("sum", DataType::Float64, true),
+            Field::new("sum_sqr", DataType::Float64, true),
+            Field::new("sum_cub", DataType::Float64, true),
+        ])
+    }
+}
+
+/// Accumulator for calculating the skewness
+/// This implementation follows the DuckDB implementation:
+/// <https://github.com/duckdb/duckdb/blob/main/src/core_functions/aggregate/distributive/skew.cpp>
+#[derive(Debug)]
+pub struct SkewnessAccumulator {
+    count: u64,
+    sum: f64,
+    sum_sqr: f64,
+    sum_cub: f64,
+}
+
+impl SkewnessAccumulator {
+    fn new() -> Self {
+        Self {
+            count: 0,
+            sum: 0f64,
+            sum_sqr: 0f64,
+            sum_cub: 0f64,
+        }
+    }
+}
+
+impl Accumulator for SkewnessAccumulator {
+    fn update_batch(&mut self, values: &[ArrayRef]) -> datafusion::common::Result<()> {
+        let array = values[0].as_primitive::<Float64Type>();
+        for val in array.iter().flatten() {
+            self.count += 1;
+            self.sum += val;
+            self.sum_sqr += val.powi(2);
+            self.sum_cub += val.powi(3);
+        }
+        Ok(())
+    }
+    fn evaluate(&mut self) -> datafusion::common::Result<ScalarValue> {
+        if self.count <= 2 {
+            return Ok(ScalarValue::Float64(None));
+        }
+        let count = self.count as f64;
+        let t1 = 1f64 / count;
+        let p = (t1 * (self.sum_sqr - self.sum * self.sum * t1)).powi(3).max(0f64);
+        let div = p.sqrt();
+        if div == 0f64 {
+            return Ok(ScalarValue::Float64(None));
+        }
+        let t2 = count.mul(count.sub(1f64)).sqrt().div(count.sub(2f64));
+        let res =
+            t2 * t1 * (self.sum_cub - 3f64 * self.sum_sqr * self.sum * t1 + 2f64 * self.sum.powi(3) * t1 * t1) / div;
+        Ok(ScalarValue::Float64(Some(res)))
+    }
+
+    fn size(&self) -> usize {
+        std::mem::size_of_val(self)
+    }
+
+    fn state(&mut self) -> datafusion::common::Result<Vec<ScalarValue>> {
+        Ok(vec![
+            ScalarValue::from(self.count),
+            ScalarValue::from(self.sum),
+            ScalarValue::from(self.sum_sqr),
+            ScalarValue::from(self.sum_cub),
+        ])
+    }
+
+    fn merge_batch(&mut self, states: &[ArrayRef]) -> datafusion::common::Result<()> {
+        let counts = states[0].as_primitive::<UInt64Type>();
+        let sums = states[1].as_primitive::<Float64Type>();
+        let sum_sqrs = states[2].as_primitive::<Float64Type>();
+        let sum_cubs = states[3].as_primitive::<Float64Type>();
+
+        for i in 0..counts.len() {
+            let c = counts.value(i);
+            if c == 0 {
+                continue;
+            }
+            self.count += c;
+            self.sum += sums.value(i);
+            self.sum_sqr += sum_sqrs.value(i);
+            self.sum_cub += sum_cubs.value(i);
+        }
+        Ok(())
+    }
+}

tests/main.rs

@@ -310,3 +310,57 @@ async fn test_kurtosis_pop() {
 - +--------------------+
 "###);
 }
+
+#[tokio::test]
+async fn test_skewness() {
+    let mut execution = TestExecution::new().await.unwrap().with_setup(TEST_TABLE).await;
+
+    // Test with int64
+    let actual = execution
+        .run_and_format("SELECT skewness(int64_col) FROM test_table")
+        .await;
+
+    insta::assert_yaml_snapshot!(actual, @r###"
+        - +--------------------------------+
+        - "| skewness(test_table.int64_col) |"
+        - +--------------------------------+
+        - "| -0.8573214099741201            |"
+        - +--------------------------------+
+    "###);
+
+    // Test with float64
+    let actual = execution
+        .run_and_format("SELECT skewness(float64_col) FROM test_table")
+        .await;
+
+    insta::assert_yaml_snapshot!(actual, @r###"
+    - +----------------------------------+
+    - "| skewness(test_table.float64_col) |"
+    - +----------------------------------+
+    - "| -0.8573214099741201              |"
+    - +----------------------------------+
+"###);
+
+    // Test with single value
+    let actual = execution.run_and_format("SELECT skewness(1.0)").await;
+
+    insta::assert_yaml_snapshot!(actual, @r###"
+        - +----------------------+
+        - "| skewness(Float64(1)) |"
+        - +----------------------+
+        - "|                      |"
+        - +----------------------+
+    "###);
+
+    let actual = execution
+        .run_and_format("SELECT skewness(col) FROM VALUES (1.0), (2.0) as tab(col)")
+        .await;
+
+    insta::assert_yaml_snapshot!(actual, @r###"
+        - +-------------------+
+        - "| skewness(tab.col) |"
+        - +-------------------+
+        - "|                   |"
+        - +-------------------+
+    "###);
+}