fix(spark-expr): preserve scalar tag in WideDecimalBinaryExpr when both inputs are scalars

native/spark-expr/src/math_funcs/wide_decimal_binary_expr.rs

@@ -184,6 +184,16 @@ impl PhysicalExpr for WideDecimalBinaryExpr {
         let left_val = self.left.evaluate(batch)?;
         let right_val = self.right.evaluate(batch)?;
 
+        // Track scalar-ness so we can return a Scalar when both inputs are scalars.
+        // Without this, a (Scalar op Scalar) result would be returned as a length-1
+        // Array, and downstream comparisons against full batches would incorrectly
+        // see two Array operands with mismatched lengths instead of (Array, Scalar).
+        // See https://github.com/apache/datafusion-comet/issues/1615 (the q23 BHJ
+        // join-filter scalar-subquery crash).
+        let both_scalar = matches!(
+            (&left_val, &right_val),
+            (ColumnarValue::Scalar(_), ColumnarValue::Scalar(_))
+        );
         let (left_arr, right_arr): (ArrayRef, ArrayRef) = match (&left_val, &right_val) {
             (ColumnarValue::Array(l), ColumnarValue::Array(r)) => (Arc::clone(l), Arc::clone(r)),
             (ColumnarValue::Scalar(l), ColumnarValue::Array(r)) => {
@@ -280,7 +290,16 @@ impl PhysicalExpr for WideDecimalBinaryExpr {
             result
         };
         let result = result.with_data_type(DataType::Decimal128(p_out, s_out));
-        Ok(ColumnarValue::Array(Arc::new(result)))
+        if both_scalar {
+            // Convert the length-1 result back into a Scalar so downstream
+            // expressions (binary ops, comparisons) can take the scalar fast-path
+            // and propagate the scalar tag (Datum::is_scalar) through arrow-rs
+            // kernels.
+            let scalar = datafusion::common::ScalarValue::try_from_array(&result, 0)?;
+            Ok(ColumnarValue::Scalar(scalar))
+        } else {
+            Ok(ColumnarValue::Array(Arc::new(result)))
+        }
     }
 
     fn children(&self) -> Vec<&Arc<dyn PhysicalExpr>> {
@@ -557,4 +576,66 @@ mod tests {
         let arr = result.as_primitive::<Decimal128Type>();
         assert_eq!(arr.value(0), 20000); // 2.0000
     }
+
+    /// Regression test for the TPC-DS q23 BroadcastHashJoin crash (issue #1615).
+    ///
+    /// When both inputs are `ColumnarValue::Scalar`, `evaluate` must return a
+    /// `ColumnarValue::Scalar` -- not a length-1 `ColumnarValue::Array`. Otherwise
+    /// downstream comparisons against full batches see two `Array` operands with
+    /// mismatched lengths and arrow-ord's `compare_op` rejects them with
+    /// "Cannot compare arrays of different lengths, got N vs 1".
+    #[test]
+    fn test_scalar_scalar_returns_scalar() {
+        use datafusion::common::ScalarValue;
+        use datafusion::physical_expr::expressions::Literal;
+
+        // 0.95 * 100.00 -- the same Scalar x Scalar decimal multiply pattern that
+        // appears in the q23 filter `0.95 * scalar_subquery > ssales`.
+        let left: Arc<dyn PhysicalExpr> =
+            Arc::new(Literal::new(ScalarValue::Decimal128(Some(95), 38, 2)));
+        let right: Arc<dyn PhysicalExpr> =
+            Arc::new(Literal::new(ScalarValue::Decimal128(Some(10000), 38, 2)));
+
+        let expr = WideDecimalBinaryExpr::new(
+            left,
+            right,
+            WideDecimalOp::Multiply,
+            38,
+            2,
+            EvalMode::Legacy,
+        );
+
+        // Empty schema -- both inputs are Literals so no columns are needed.
+        let batch = RecordBatch::new_empty(Arc::new(Schema::empty()));
+        match expr.evaluate(&batch).unwrap() {
+            ColumnarValue::Scalar(ScalarValue::Decimal128(Some(v), 38, 2)) => {
+                // 0.95 * 100.00 = 95.00 -> at scale 2, integer 9500
+                assert_eq!(v, 9500);
+            }
+            ColumnarValue::Scalar(other) => {
+                panic!("expected Decimal128(Some(_), 38, 2), got {other:?}");
+            }
+            ColumnarValue::Array(_) => {
+                panic!(
+                    "Scalar x Scalar must return ColumnarValue::Scalar, not Array.                      This is the q23 BHJ crash regression (issue #1615)."
+                );
+            }
+        }
+    }
+
+    /// Companion test: when at least one input is an Array, the result must remain an Array.
+    /// Guards against over-eager scalar-unwrapping in the fix.
+    #[test]
+    fn test_array_input_returns_array() {
+        let batch = make_batch(
+            vec![Some(150), Some(250)],
+            38,
+            2,
+            vec![Some(100), Some(200)],
+            38,
+            2,
+        );
+        let result = eval_expr(&batch, WideDecimalOp::Add, 38, 2, EvalMode::Legacy).unwrap();
+        assert_eq!(result.len(), 2);
+    }
 }