Implement monotonicity for ScalarUDF

datafusion-examples/examples/advanced_udf.rs

@@ -31,7 +31,9 @@ use arrow::datatypes::Float64Type;
 use datafusion::error::Result;
 use datafusion::prelude::*;
 use datafusion_common::{internal_err, ScalarValue};
-use datafusion_expr::{ColumnarValue, ScalarUDF, ScalarUDFImpl, Signature};
+use datafusion_expr::{
+    ColumnarValue, FuncMonotonicity, ScalarUDF, ScalarUDFImpl, Signature,
+};
 use std::sync::Arc;
 
 /// This example shows how to use the full ScalarUDFImpl API to implement a user
@@ -184,6 +186,10 @@ impl ScalarUDFImpl for PowUdf {
     fn aliases(&self) -> &[String] {
         &self.aliases
     }
+
+    fn monotonicity(&self) -> Result<Option<FuncMonotonicity>> {
+        Ok(Some(vec![Some(true)]))
+    }
 }
 
 /// In this example we register `PowUdf` as a user defined function

datafusion/expr/src/udf.rs

@@ -18,7 +18,8 @@
 //! [`ScalarUDF`]: Scalar User Defined Functions
 
 use crate::{
-    ColumnarValue, Expr, ReturnTypeFunction, ScalarFunctionImplementation, Signature,
+    ColumnarValue, Expr, FuncMonotonicity, ReturnTypeFunction,
+    ScalarFunctionImplementation, Signature,
 };
 use arrow::datatypes::DataType;
 use datafusion_common::Result;
@@ -164,6 +165,13 @@ impl ScalarUDF {
         let captured = self.inner.clone();
         Arc::new(move |args| captured.invoke(args))
     }
+
+    /// This function specifies monotonicity behaviors for User defined scalar functions.
+    ///
+    /// See [`ScalarUDFImpl::monotonicity`] for more details.
+    pub fn monotonicity(&self) -> Result<Option<FuncMonotonicity>> {
+        self.inner.monotonicity()
+    }
 }
 
 impl<F> From<F> for ScalarUDF
@@ -271,6 +279,11 @@ pub trait ScalarUDFImpl: Debug + Send + Sync {
     fn aliases(&self) -> &[String] {
         &[]
     }
+
+    /// This function specifies monotonicity behaviors for User defined scalar functions.
+    fn monotonicity(&self) -> Result<Option<FuncMonotonicity>> {
+        Ok(None)
+    }
 }
 
 /// ScalarUDF that adds an alias to the underlying function. It is better to

datafusion/physical-expr/src/udf.rs

@@ -39,6 +39,79 @@ pub fn create_physical_expr(
         fun.fun(),
         input_phy_exprs.to_vec(),
         fun.return_type(&input_exprs_types)?,
-        None,
+        fun.monotonicity()?,
     )))
 }
+
+#[cfg(test)]
+mod tests {
+    use arrow::datatypes::Schema;
+    use arrow_schema::DataType;
+    use datafusion_common::Result;
+    use datafusion_expr::{
+        ColumnarValue, FuncMonotonicity, ScalarUDF, ScalarUDFImpl, Signature, Volatility,
+    };
+
+    use crate::ScalarFunctionExpr;
+
+    use super::create_physical_expr;
+
+    #[test]
+    fn test_functions() -> Result<()> {
+        #[derive(Debug, Clone)]
+        struct TestScalarUDF {
+            signature: Signature,
+        }
+
+        impl TestScalarUDF {
+            fn new() -> Self {
+                let signature =
+                    Signature::exact(vec![DataType::Float64], Volatility::Immutable);
+
+                Self { signature }
+            }
+        }
+
+        impl ScalarUDFImpl for TestScalarUDF {
+            fn as_any(&self) -> &dyn std::any::Any {
+                self
+            }
+
+            fn name(&self) -> &str {
+                "my_fn"
+            }
+
+            fn signature(&self) -> &Signature {
+                &self.signature
+            }
+
+            fn return_type(&self, _arg_types: &[DataType]) -> Result<DataType> {
+                Ok(DataType::Float64)
+            }
+
+            fn invoke(&self, _args: &[ColumnarValue]) -> Result<ColumnarValue> {
+                unimplemented!("my_fn is not implemented")
+            }
+
+            fn monotonicity(&self) -> Result<Option<FuncMonotonicity>> {
+                Ok(Some(vec![Some(true)]))
+            }
+        }
+
+        // create and register the udf
+        let udf = ScalarUDF::from(TestScalarUDF::new());
+
+        let p_expr = create_physical_expr(&udf, &[], &Schema::empty())?;
+
+        assert_eq!(
+            p_expr
+                .as_any()
+                .downcast_ref::<ScalarFunctionExpr>()
+                .unwrap()
+                .monotonicity(),
+            &Some(vec![Some(true)])
+        );
+
+        Ok(())
+    }
+}