[bugfix] ScalarFunctionExpr does not preserve the nullable flag on roundtrip

datafusion/proto/proto/datafusion.proto

@@ -838,6 +838,7 @@ message PhysicalScalarUdfNode {
   repeated PhysicalExprNode args = 2;
   optional bytes fun_definition = 3;
   datafusion_common.ArrowType return_type = 4;
+  bool nullable = 5;
 }
 
 message PhysicalAggregateExprNode {

datafusion/proto/src/physical_plan/from_proto.rs

@@ -358,12 +358,15 @@ pub fn parse_physical_expr(
 
             let args = parse_physical_exprs(&e.args, registry, input_schema, codec)?;
 
-            Arc::new(ScalarFunctionExpr::new(
-                e.name.as_str(),
-                scalar_fun_def,
-                args,
-                convert_required!(e.return_type)?,
-            ))
+            Arc::new(
+                ScalarFunctionExpr::new(
+                    e.name.as_str(),
+                    scalar_fun_def,
+                    args,
+                    convert_required!(e.return_type)?,
+                )
+                .with_nullable(e.nullable),
+            )
         }
         ExprType::LikeExpr(like_expr) => Arc::new(LikeExpr::new(
             like_expr.negated,

datafusion/proto/src/physical_plan/to_proto.rs

@@ -347,6 +347,7 @@ pub fn serialize_physical_expr(
                     args: serialize_physical_exprs(expr.args(), codec)?,
                     fun_definition: (!buf.is_empty()).then_some(buf),
                     return_type: Some(expr.return_type().try_into()?),
+                    nullable: expr.nullable(),
                 },
             )),
         })

datafusion/proto/tests/cases/roundtrip_physical_plan.rs

@@ -39,6 +39,7 @@ use crate::cases::{
 use datafusion::arrow::array::ArrayRef;
 use datafusion::arrow::compute::kernels::sort::SortOptions;
 use datafusion::arrow::datatypes::{DataType, Field, IntervalUnit, Schema};
+use datafusion::datasource::empty::EmptyTable;
 use datafusion::datasource::file_format::csv::CsvSink;
 use datafusion::datasource::file_format::json::JsonSink;
 use datafusion::datasource::file_format::parquet::ParquetSink;
@@ -83,7 +84,7 @@ use datafusion::physical_plan::windows::{
     WindowAggExec,
 };
 use datafusion::physical_plan::{
-    ExecutionPlan, InputOrderMode, Partitioning, PhysicalExpr, Statistics,
+    displayable, ExecutionPlan, InputOrderMode, Partitioning, PhysicalExpr, Statistics,
 };
 use datafusion::prelude::SessionContext;
 use datafusion::scalar::ScalarValue;
@@ -106,6 +107,7 @@ use datafusion_proto::physical_plan::{
     AsExecutionPlan, DefaultPhysicalExtensionCodec, PhysicalExtensionCodec,
 };
 use datafusion_proto::protobuf;
+use datafusion_proto::protobuf::PhysicalPlanNode;
 
 /// Perform a serde roundtrip and assert that the string representation of the before and after plans
 /// are identical. Note that this often isn't sufficient to guarantee that no information is
@@ -1525,3 +1527,42 @@ fn roundtrip_unnest() -> Result<()> {
     );
     roundtrip_test(Arc::new(unnest))
 }
+
+#[tokio::test]
+async fn roundtrip_coalesce() -> Result<()> {
+    let ctx = SessionContext::new();
+    ctx.register_table(
+        "t",
+        Arc::new(EmptyTable::new(Arc::new(Schema::new(Fields::from([
+            Arc::new(Field::new("f", DataType::Int64, false)),
+        ]))))),
+    )?;
+    let df = ctx.sql("select coalesce(f) as f from t").await?;
+    let plan = df.create_physical_plan().await?;
+
+    let node = PhysicalPlanNode::try_from_physical_plan(
+        plan.clone(),
+        &DefaultPhysicalExtensionCodec {},
+    )?;
+    let node = PhysicalPlanNode::decode(node.encode_to_vec().as_slice())
+        .map_err(|e| DataFusionError::External(Box::new(e)))?;
+    let restored = node.try_into_physical_plan(
+        &ctx,
+        ctx.runtime_env().as_ref(),
+        &DefaultPhysicalExtensionCodec {},
+    )?;
+
+    assert_eq!(
+        plan.schema(),
+        restored.schema(),
+        "Schema mismatch for plans:\n>> initial:\n{}>> final: \n{}",
+        displayable(plan.as_ref())
+            .set_show_schema(true)
+            .indent(true),
+        displayable(restored.as_ref())
+            .set_show_schema(true)
+            .indent(true),
+    );
+
+    Ok(())
+}