Add support for serializing null scalar values (#3303)

datafusion/proto/src/from_proto.rs

@@ -627,9 +627,7 @@ impl TryFrom<&protobuf::PrimitiveScalarType> for ScalarValue {
         use protobuf::PrimitiveScalarType;
 
         Ok(match scalar {
-            PrimitiveScalarType::Null => {
-                return Err(proto_error("Untyped null is an invalid scalar value"));
-            }
+            PrimitiveScalarType::Null => Self::Null,
             PrimitiveScalarType::Bool => Self::Boolean(None),
             PrimitiveScalarType::Uint8 => Self::UInt8(None),
             PrimitiveScalarType::Int8 => Self::Int8(None),

datafusion/proto/src/lib.rs

@@ -851,6 +851,26 @@ mod roundtrip_tests {
         roundtrip_expr_test(test_expr, ctx);
     }
 
+    #[test]
+    fn roundtrip_case_with_null() {
+        let test_expr = Expr::Case {
+            expr: Some(Box::new(lit(1.0_f32))),
+            when_then_expr: vec![(Box::new(lit(2.0_f32)), Box::new(lit(3.0_f32)))],
+            else_expr: Some(Box::new(Expr::Literal(ScalarValue::Null))),
+        };
+
+        let ctx = SessionContext::new();
+        roundtrip_expr_test(test_expr, ctx);
+    }
+
+    #[test]
+    fn roundtrip_null_literal() {
+        let test_expr = Expr::Literal(ScalarValue::Null);
+
+        let ctx = SessionContext::new();
+        roundtrip_expr_test(test_expr, ctx);
+    }
+
     #[test]
     fn roundtrip_cast() {
         let test_expr = Expr::Cast {

datafusion/proto/src/to_proto.rs

@@ -1096,6 +1096,9 @@ impl TryFrom<&ScalarValue> for protobuf::ScalarValue {
                     Value::IntervalDaytimeValue(*s)
                 })
             }
+            ScalarValue::Null => protobuf::ScalarValue {
+                value: Some(Value::NullValue(PrimitiveScalarType::Null as i32)),
+            },
             _ => {
                 return Err(Error::invalid_scalar_value(val));
             }