fix: preserve ProjectionExec metadata during serialization

datafusion/physical-plan/src/projection.rs

@@ -652,20 +652,33 @@ impl ExecutionPlan for ProjectionExec {
             metrics: _,
             // Derived plan properties, recomputed on decode.
             cache: _,
-            // Derived metadata comparison, recomputed with the projector.
-            overrides_metadata: _,
+            overrides_metadata,
         } = self;
         let projection_exprs = projector.projection().as_ref();
         let input = ctx.encode_child(input)?;
         let expr = ctx.encode_expressions(projection_exprs.iter().map(|p| &p.expr))?;
         let expr_name = projection_exprs.iter().map(|p| p.alias.clone()).collect();
+        let output_schema = projector.output_schema();
+        // Preserve explicit overrides and field metadata that cannot be
+        // reconstructed from the child. Inherited schema metadata can be.
+        let schema = if *overrides_metadata
+            || output_schema
+                .fields()
+                .iter()
+                .any(|field| !field.metadata().is_empty())
+        {
+            Some(output_schema.as_ref().try_into()?)
+        } else {
+            None
+        };
         Ok(Some(protobuf::PhysicalPlanNode {
             physical_plan_type: Some(
                 protobuf::physical_plan_node::PhysicalPlanType::Projection(Box::new(
                     protobuf::ProjectionExecNode {
                         input: Some(Box::new(input)),
                         expr,
                         expr_name,
+                        schema,
                     },
                 )),
             ),
@@ -701,6 +714,7 @@ impl ProjectionExec {
             input,
             expr,
             expr_name,
+            schema,
         } = &**projection;
         let input =
             ctx.decode_required_child(input.as_deref(), "ProjectionExec", "input")?;
@@ -715,7 +729,15 @@ impl ProjectionExec {
                 })
             })
             .collect::<Result<Vec<ProjectionExpr>>>()?;
-        Ok(Arc::new(ProjectionExec::try_new(exprs, input)?))
+        let projection = match schema {
+            Some(schema) => ProjectionExec::try_new_with_schema_metadata(
+                exprs,
+                input,
+                &Schema::try_from(schema)?,
+            )?,
+            None => ProjectionExec::try_new(exprs, input)?,
+        };
+        Ok(Arc::new(projection))
     }
 }
 

datafusion/proto-models/proto/datafusion.proto

@@ -1526,6 +1526,11 @@ message ProjectionExecNode {
   PhysicalPlanNode input = 1;
   repeated PhysicalExprNode expr = 2;
   repeated string expr_name = 3;
+  // Only field and schema metadata are used; output types are derived from expr.
+  // Present when the projection output has metadata or explicitly overrides
+  // inherited metadata, including an explicit clear. Absent for older plans
+  // and metadata-free projections.
+  datafusion_common.Schema schema = 4;
 }
 
 enum AggregateMode {

datafusion/proto-models/src/generated/pbjson.rs

@@ -24658,6 +24658,9 @@ impl serde::Serialize for ProjectionExecNode {
         if !self.expr_name.is_empty() {
             len += 1;
         }
+        if self.schema.is_some() {
+            len += 1;
+        }
         let mut struct_ser = serializer.serialize_struct("datafusion.ProjectionExecNode", len)?;
         if let Some(v) = self.input.as_ref() {
             struct_ser.serialize_field("input", v)?;
@@ -24668,6 +24671,9 @@ impl serde::Serialize for ProjectionExecNode {
         if !self.expr_name.is_empty() {
             struct_ser.serialize_field("exprName", &self.expr_name)?;
         }
+        if let Some(v) = self.schema.as_ref() {
+            struct_ser.serialize_field("schema", v)?;
+        }
         struct_ser.end()
     }
 }
@@ -24682,13 +24688,15 @@ impl<'de> serde::Deserialize<'de> for ProjectionExecNode {
             "expr",
             "expr_name",
             "exprName",
+            "schema",
         ];
 
         #[allow(clippy::enum_variant_names)]
         enum GeneratedField {
             Input,
             Expr,
             ExprName,
+            Schema,
         }
         impl<'de> serde::Deserialize<'de> for GeneratedField {
             fn deserialize<D>(deserializer: D) -> std::result::Result<GeneratedField, D::Error>
@@ -24713,6 +24721,7 @@ impl<'de> serde::Deserialize<'de> for ProjectionExecNode {
                             "input" => Ok(GeneratedField::Input),
                             "expr" => Ok(GeneratedField::Expr),
                             "exprName" | "expr_name" => Ok(GeneratedField::ExprName),
+                            "schema" => Ok(GeneratedField::Schema),
                             _ => Err(serde::de::Error::unknown_field(value, FIELDS)),
                         }
                     }
@@ -24735,6 +24744,7 @@ impl<'de> serde::Deserialize<'de> for ProjectionExecNode {
                 let mut input__ = None;
                 let mut expr__ = None;
                 let mut expr_name__ = None;
+                let mut schema__ = None;
                 while let Some(k) = map_.next_key()? {
                     match k {
                         GeneratedField::Input => {
@@ -24755,12 +24765,19 @@ impl<'de> serde::Deserialize<'de> for ProjectionExecNode {
                             }
                             expr_name__ = Some(map_.next_value()?);
                         }
+                        GeneratedField::Schema => {
+                            if schema__.is_some() {
+                                return Err(serde::de::Error::duplicate_field("schema"));
+                            }
+                            schema__ = map_.next_value()?;
+                        }
                     }
                 }
                 Ok(ProjectionExecNode {
                     input: input__,
                     expr: expr__.unwrap_or_default(),
                     expr_name: expr_name__.unwrap_or_default(),
+                    schema: schema__,
                 })
             }
         }

datafusion/proto-models/src/generated/prost.rs

@@ -2302,6 +2302,12 @@ pub struct ProjectionExecNode {
     pub expr: ::prost::alloc::vec::Vec<PhysicalExprNode>,
     #[prost(string, repeated, tag = "3")]
     pub expr_name: ::prost::alloc::vec::Vec<::prost::alloc::string::String>,
+    /// Only field and schema metadata are used; output types are derived from expr.
+    /// Present when the projection output has metadata or explicitly overrides
+    /// inherited metadata, including an explicit clear. Absent for older plans
+    /// and metadata-free projections.
+    #[prost(message, optional, tag = "4")]
+    pub schema: ::core::option::Option<super::datafusion_common::Schema>,
 }
 #[derive(Clone, PartialEq, Eq, Hash, ::prost::Message)]
 pub struct PartiallySortedInputOrderMode {