fix: use spill writer's schema instead of the first batch schema for spill files

datafusion/core/tests/memory_limit/mod.rs

@@ -24,6 +24,7 @@ use std::sync::{Arc, LazyLock};
 #[cfg(feature = "extended_tests")]
 mod memory_limit_validation;
 mod repartition_mem_limit;
+mod union_nullable_spill;
 use arrow::array::{ArrayRef, DictionaryArray, Int32Array, RecordBatch, StringViewArray};
 use arrow::compute::SortOptions;
 use arrow::datatypes::{Int32Type, SchemaRef};

datafusion/core/tests/memory_limit/union_nullable_spill.rs

@@ -0,0 +1,162 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+use std::sync::Arc;
+
+use arrow::array::{Array, Int64Array, RecordBatch};
+use arrow::compute::SortOptions;
+use arrow::datatypes::{DataType, Field, Schema};
+use datafusion::datasource::memory::MemorySourceConfig;
+use datafusion_execution::config::SessionConfig;
+use datafusion_execution::memory_pool::FairSpillPool;
+use datafusion_execution::runtime_env::RuntimeEnvBuilder;
+use datafusion_physical_expr::expressions::col;
+use datafusion_physical_expr::{LexOrdering, PhysicalSortExpr};
+use datafusion_physical_plan::repartition::RepartitionExec;
+use datafusion_physical_plan::sorts::sort::sort_batch;
+use datafusion_physical_plan::union::UnionExec;
+use datafusion_physical_plan::{ExecutionPlan, Partitioning};
+use futures::StreamExt;
+
+const NUM_BATCHES: usize = 200;
+const ROWS_PER_BATCH: usize = 10;
+
+fn non_nullable_schema() -> Arc<Schema> {
+    Arc::new(Schema::new(vec![
+        Field::new("key", DataType::Int64, false),
+        Field::new("val", DataType::Int64, false),
+    ]))
+}
+
+fn nullable_schema() -> Arc<Schema> {
+    Arc::new(Schema::new(vec![
+        Field::new("key", DataType::Int64, false),
+        Field::new("val", DataType::Int64, true),
+    ]))
+}
+
+fn non_nullable_batches() -> Vec<RecordBatch> {
+    (0..NUM_BATCHES)
+        .map(|i| {
+            let start = (i * ROWS_PER_BATCH) as i64;
+            let keys: Vec<i64> = (start..start + ROWS_PER_BATCH as i64).collect();
+            RecordBatch::try_new(
+                non_nullable_schema(),
+                vec![
+                    Arc::new(Int64Array::from(keys)),
+                    Arc::new(Int64Array::from(vec![0i64; ROWS_PER_BATCH])),
+                ],
+            )
+            .unwrap()
+        })
+        .collect()
+}
+
+fn nullable_batches() -> Vec<RecordBatch> {
+    (0..NUM_BATCHES)
+        .map(|i| {
+            let start = (i * ROWS_PER_BATCH) as i64;
+            let keys: Vec<i64> = (start..start + ROWS_PER_BATCH as i64).collect();
+            let vals: Vec<Option<i64>> = (0..ROWS_PER_BATCH)
+                .map(|j| if j % 3 == 1 { None } else { Some(j as i64) })
+                .collect();
+            RecordBatch::try_new(
+                nullable_schema(),
+                vec![
+                    Arc::new(Int64Array::from(keys)),
+                    Arc::new(Int64Array::from(vals)),
+                ],
+            )
+            .unwrap()
+        })
+        .collect()
+}
+
+fn build_task_ctx(pool_size: usize) -> Arc<datafusion_execution::TaskContext> {
+    let session_config = SessionConfig::new().with_batch_size(2);
+    let runtime = RuntimeEnvBuilder::new()
+        .with_memory_pool(Arc::new(FairSpillPool::new(pool_size)))
+        .build_arc()
+        .unwrap();
+    Arc::new(
+        datafusion_execution::TaskContext::default()
+            .with_session_config(session_config)
+            .with_runtime(runtime),
+    )
+}
+
+/// Exercises spilling through UnionExec -> RepartitionExec where union children
+/// have mismatched nullability (one child's `val` is non-nullable, the other's
+/// is nullable with NULLs). A tiny FairSpillPool forces all batches to spill.
+///
+/// UnionExec returns child streams without schema coercion, so batches from
+/// different children carry different per-field nullability into the shared
+/// SpillPool. The IPC writer must use the SpillManager's canonical (nullable)
+/// schema — not the first batch's schema — so readback batches are valid.
+///
+/// Otherwise, sort_batch will panic with
+/// `Column 'val' is declared as non-nullable but contains null values`
+#[tokio::test]
+async fn test_sort_union_repartition_spill_mixed_nullability() {
+    let non_nullable_exec = MemorySourceConfig::try_new_exec(
+        &[non_nullable_batches()],
+        non_nullable_schema(),
+        None,
+    )
+    .unwrap();
+
+    let nullable_exec =
+        MemorySourceConfig::try_new_exec(&[nullable_batches()], nullable_schema(), None)
+            .unwrap();
+
+    let union_exec = UnionExec::try_new(vec![non_nullable_exec, nullable_exec]).unwrap();
+    assert!(union_exec.schema().field(1).is_nullable());
+
+    let repartition = Arc::new(
+        RepartitionExec::try_new(union_exec, Partitioning::RoundRobinBatch(1)).unwrap(),
+    );
+
+    let task_ctx = build_task_ctx(200);
+    let mut stream = repartition.execute(0, task_ctx).unwrap();
+
+    let sort_expr = LexOrdering::new(vec![PhysicalSortExpr {
+        expr: col("key", &nullable_schema()).unwrap(),
+        options: SortOptions::default(),
+    }])
+    .unwrap();
+
+    let mut total_rows = 0usize;
+    let mut total_nulls = 0usize;
+    while let Some(result) = stream.next().await {
+        let batch = result.unwrap();
+
+        let batch = sort_batch(&batch, &sort_expr, None).unwrap();
+
+        total_rows += batch.num_rows();
+        total_nulls += batch.column(1).null_count();
+    }
+
+    assert_eq!(
+        total_rows,
+        NUM_BATCHES * ROWS_PER_BATCH * 2,
+        "All rows from both UNION branches should be present"
+    );
+    assert!(
+        total_nulls > 0,
+        "Expected some null values in output (i.e. nullable batches were processed)"
+    );
+}

datafusion/physical-plan/src/spill/in_progress_spill_file.rs

@@ -62,7 +62,11 @@ impl InProgressSpillFile {
             ));
         }
         if self.writer.is_none() {
-            let schema = batch.schema();
+            // Use the SpillManager's declared schema rather than the batch's schema.
+            // Individual batches may have different schemas (e.g., different nullability)
+            // when they come from different branches of a UnionExec. The SpillManager's
+            // schema represents the canonical schema that all batches should conform to.
+            let schema = self.spill_writer.schema();
             if let Some(in_progress_file) = &mut self.in_progress_file {
                 self.writer = Some(IPCStreamWriter::new(
                     in_progress_file.path(),
@@ -138,3 +142,77 @@ impl InProgressSpillFile {
         Ok(self.in_progress_file.take())
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use arrow::array::Int64Array;
+    use arrow_schema::{DataType, Field, Schema};
+    use datafusion_execution::runtime_env::RuntimeEnvBuilder;
+    use datafusion_physical_expr_common::metrics::{
+        ExecutionPlanMetricsSet, SpillMetrics,
+    };
+    use futures::TryStreamExt;
+
+    #[tokio::test]
+    async fn test_spill_file_uses_spill_manager_schema() -> Result<()> {
+        let nullable_schema = Arc::new(Schema::new(vec![
+            Field::new("key", DataType::Int64, false),
+            Field::new("val", DataType::Int64, true),
+        ]));
+        let non_nullable_schema = Arc::new(Schema::new(vec![
+            Field::new("key", DataType::Int64, false),
+            Field::new("val", DataType::Int64, false),
+        ]));
+
+        let runtime = Arc::new(RuntimeEnvBuilder::new().build()?);
+        let metrics_set = ExecutionPlanMetricsSet::new();
+        let spill_metrics = SpillMetrics::new(&metrics_set, 0);
+        let spill_manager = Arc::new(SpillManager::new(
+            runtime,
+            spill_metrics,
+            Arc::clone(&nullable_schema),
+        ));
+
+        let mut in_progress = spill_manager.create_in_progress_file("test")?;
+
+        // First batch: non-nullable val (simulates literal-0 UNION branch)
+        let non_nullable_batch = RecordBatch::try_new(
+            Arc::clone(&non_nullable_schema),
+            vec![
+                Arc::new(Int64Array::from(vec![1, 2, 3])),
+                Arc::new(Int64Array::from(vec![0, 0, 0])),
+            ],
+        )?;
+        in_progress.append_batch(&non_nullable_batch)?;
+
+        // Second batch: nullable val with NULLs (simulates table UNION branch)
+        let nullable_batch = RecordBatch::try_new(
+            Arc::clone(&nullable_schema),
+            vec![
+                Arc::new(Int64Array::from(vec![4, 5, 6])),
+                Arc::new(Int64Array::from(vec![Some(10), None, Some(30)])),
+            ],
+        )?;
+        in_progress.append_batch(&nullable_batch)?;
+
+        let spill_file = in_progress.finish()?.unwrap();
+
+        let stream = spill_manager.read_spill_as_stream(spill_file, None)?;
+
+        // Stream schema should be nullable
+        assert_eq!(stream.schema(), nullable_schema);
+
+        let batches = stream.try_collect::<Vec<_>>().await?;
+        assert_eq!(batches.len(), 2);
+
+        // Both batches must have the SpillManager's nullable schema
+        assert_eq!(
+            batches[0],
+            non_nullable_batch.with_schema(Arc::clone(&nullable_schema))?
+        );
+        assert_eq!(batches[1], nullable_batch);
+
+        Ok(())
+    }
+}