fix: Set Substrait output_type on aggregate functions

The Substrait producer exported every aggregate call with
output_type: None, even though the logical plan already knows the
result type. Derive the output field from the logical expression and
write it to AggregateFunction.output_type, mirroring the handling
already used for scalar functions.

Closes #25049.

Author: namanjain24-sudo

datafusion/substrait/src/logical_plan/producer/expr/aggregate_function.rs

@@ -15,10 +15,10 @@
 // specific language governing permissions and limitations
 // under the License.
 
-use crate::logical_plan::producer::SubstraitProducer;
+use crate::logical_plan::producer::{SubstraitProducer, to_substrait_type_from_field};
 use datafusion::common::DFSchemaRef;
-use datafusion::logical_expr::expr;
 use datafusion::logical_expr::expr::AggregateFunctionParams;
+use datafusion::logical_expr::{Expr, ExprSchemable, expr};
 use substrait::proto::aggregate_function::AggregationInvocation;
 use substrait::proto::aggregate_rel::Measure;
 use substrait::proto::function_argument::ArgType;
@@ -54,13 +54,15 @@ pub fn from_aggregate_function(
         });
     }
     let function_anchor = producer.register_function(func.name().to_string());
+    let (_, output_field) = Expr::AggregateFunction(agg_fn.clone()).to_field(schema)?;
+    let output_type = to_substrait_type_from_field(producer, &output_field)?;
     #[expect(deprecated)]
     Ok(Measure {
         measure: Some(AggregateFunction {
             function_reference: function_anchor,
             arguments,
             sorts,
-            output_type: None,
+            output_type: Some(output_type),
             invocation: match distinct {
                 true => AggregationInvocation::Distinct as i32,
                 false => AggregationInvocation::All as i32,
@@ -93,3 +95,51 @@ fn to_substrait_sort_field(
         sort_kind: Some(SortKind::Direction(sort_kind.into())),
     })
 }
+
+#[cfg(test)]
+mod tests {
+    use crate::logical_plan::producer::{
+        DefaultSubstraitProducer, SubstraitProducer, to_substrait_type,
+    };
+    use datafusion::arrow::datatypes::{DataType, Field, Schema};
+    use datafusion::common::{DFSchema, DFSchemaRef};
+    use datafusion::execution::SessionStateBuilder;
+    use datafusion::functions_aggregate::expr_fn::{avg, count, min, sum};
+    use datafusion::logical_expr::Expr;
+    use datafusion::prelude::col;
+
+    #[test]
+    fn aggregate_function_output_type() -> datafusion::common::Result<()> {
+        let state = SessionStateBuilder::default().build();
+        let schema =
+            DFSchemaRef::new(DFSchema::try_from(Schema::new(vec![Field::new(
+                "i",
+                DataType::Int64,
+                false,
+            )]))?);
+        let mut producer = DefaultSubstraitProducer::new(&state);
+
+        // (aggregate, expected output type, expected nullability)
+        let cases = [
+            (count(col("i")), DataType::Int64, false),
+            (sum(col("i")), DataType::Int64, true),
+            (avg(col("i")), DataType::Float64, true),
+            (min(col("i")), DataType::Int64, true),
+        ];
+
+        for (expr, expected_type, expected_nullable) in cases {
+            let Expr::AggregateFunction(agg_fn) = &expr else {
+                panic!("AggregateFunction expected, got {expr}")
+            };
+            let measure = producer.handle_aggregate_function(agg_fn, &schema)?;
+            let expected =
+                to_substrait_type(&mut producer, &expected_type, expected_nullable)?;
+            let output_type = measure
+                .measure
+                .expect("Measure should contain an AggregateFunction")
+                .output_type;
+            assert_eq!(output_type, Some(expected), "output_type for {expr}");
+        }
+        Ok(())
+    }
+}