fix: preserve UPPER and LOWER input nullability

datafusion/functions/src/string/lower.rs

@@ -15,14 +15,14 @@
 // specific language governing permissions and limitations
 // under the License.
 
-use arrow::datatypes::DataType;
+use arrow::datatypes::{DataType, Field, FieldRef};
 
 use crate::string::common::to_lower;
 use datafusion_common::Result;
 use datafusion_common::types::logical_string;
 use datafusion_expr::{
-    Coercion, ColumnarValue, Documentation, EncodingPreservation, ScalarFunctionArgs,
-    ScalarUDFImpl, Signature, TypeSignatureClass, Volatility,
+    Coercion, ColumnarValue, Documentation, EncodingPreservation, ReturnFieldArgs,
+    ScalarFunctionArgs, ScalarUDFImpl, Signature, TypeSignatureClass, Volatility,
 };
 use datafusion_macros::user_doc;
 
@@ -80,6 +80,14 @@ impl ScalarUDFImpl for LowerFunc {
         Ok(arg_types[0].clone())
     }
 
+    fn return_field_from_args(&self, args: ReturnFieldArgs) -> Result<FieldRef> {
+        let input = &args.arg_fields[0];
+        Ok(
+            Field::new(self.name(), input.data_type().clone(), input.is_nullable())
+                .into(),
+        )
+    }
+
     fn invoke_with_args(&self, args: ScalarFunctionArgs) -> Result<ColumnarValue> {
         to_lower(&args.args, "lower")
     }
@@ -97,6 +105,21 @@ mod tests {
     use datafusion_common::config::ConfigOptions;
     use std::sync::Arc;
 
+    #[test]
+    fn preserves_input_nullability() -> Result<()> {
+        let func = LowerFunc::new();
+        for nullable in [false, true] {
+            let input = Field::new("input", DataType::Utf8, nullable);
+            let result = func.return_field_from_args(ReturnFieldArgs {
+                arg_fields: &[input.into()],
+                scalar_arguments: &[None],
+            })?;
+            assert_eq!(result.data_type(), &DataType::Utf8);
+            assert_eq!(result.is_nullable(), nullable);
+        }
+        Ok(())
+    }
+
     fn invoke_lower(input: ArrayRef) -> Result<ArrayRef> {
         let func = LowerFunc::new();
         let data_type = input.data_type().clone();

datafusion/functions/src/string/upper.rs

@@ -16,12 +16,12 @@
 // under the License.
 
 use crate::string::common::to_upper;
-use arrow::datatypes::DataType;
+use arrow::datatypes::{DataType, Field, FieldRef};
 use datafusion_common::Result;
 use datafusion_common::types::logical_string;
 use datafusion_expr::{
-    Coercion, ColumnarValue, Documentation, EncodingPreservation, ScalarFunctionArgs,
-    ScalarUDFImpl, Signature, TypeSignatureClass, Volatility,
+    Coercion, ColumnarValue, Documentation, EncodingPreservation, ReturnFieldArgs,
+    ScalarFunctionArgs, ScalarUDFImpl, Signature, TypeSignatureClass, Volatility,
 };
 use datafusion_macros::user_doc;
 
@@ -79,6 +79,14 @@ impl ScalarUDFImpl for UpperFunc {
         Ok(arg_types[0].clone())
     }
 
+    fn return_field_from_args(&self, args: ReturnFieldArgs) -> Result<FieldRef> {
+        let input = &args.arg_fields[0];
+        Ok(
+            Field::new(self.name(), input.data_type().clone(), input.is_nullable())
+                .into(),
+        )
+    }
+
     fn invoke_with_args(&self, args: ScalarFunctionArgs) -> Result<ColumnarValue> {
         to_upper(&args.args, "upper")
     }
@@ -96,6 +104,21 @@ mod tests {
     use datafusion_common::config::ConfigOptions;
     use std::sync::Arc;
 
+    #[test]
+    fn preserves_input_nullability() -> Result<()> {
+        let func = UpperFunc::new();
+        for nullable in [false, true] {
+            let input = Field::new("input", DataType::Utf8, nullable);
+            let result = func.return_field_from_args(ReturnFieldArgs {
+                arg_fields: &[input.into()],
+                scalar_arguments: &[None],
+            })?;
+            assert_eq!(result.data_type(), &DataType::Utf8);
+            assert_eq!(result.is_nullable(), nullable);
+        }
+        Ok(())
+    }
+
     fn invoke_upper(input: ArrayRef) -> Result<ArrayRef> {
         let func = UpperFunc::new();
         let data_type = input.data_type().clone();

datafusion/sqllogictest/test_files/functions.slt

@@ -486,6 +486,30 @@ BAR Dictionary(Int32, Utf8)
 statement ok
 DROP TABLE upper_dictionary_test
 
+# upper and lower preserve input nullability in the planned schema
+query TTT
+DESCRIBE SELECT upper(a) AS ua, lower(a) AS la, upper(b) AS ub
+FROM (VALUES ('Ab', CAST(NULL AS VARCHAR))) AS t(a, b)
+----
+ua Utf8 NO
+la Utf8 NO
+ub Utf8View YES
+
+# An outer join widens a non-nullable input before it reaches upper
+query TTT
+DESCRIBE SELECT upper(t.a) AS ua
+FROM (SELECT 1 AS k) x
+LEFT JOIN (VALUES ('Ab')) AS t(a) ON false
+----
+ua Utf8 YES
+
+query T
+SELECT upper(t.a)
+FROM (SELECT 1 AS k) x
+LEFT JOIN (VALUES ('Ab')) AS t(a) ON false
+----
+NULL
+
 query T
 SELECT btrim('   foo  ')
 ----