From 3f33d3e7c42c886ab829cd78eda58fdd92196049 Mon Sep 17 00:00:00 2001
From: SeoYoung Lee <lsyhime@snu.ac.kr>
Date: Mon, 25 Aug 2025 12:58:43 +0000
Subject: [PATCH] fix incorrect memory accounting for StringViewArray

---
 .../physical-plan/src/spill/spill_manager.rs  | 79 +++++++++++++++++++
 1 file changed, 79 insertions(+)

diff --git a/datafusion/physical-plan/src/spill/spill_manager.rs b/datafusion/physical-plan/src/spill/spill_manager.rs
index 467a7aa72b9c..ad23bd66a021 100644
--- a/datafusion/physical-plan/src/spill/spill_manager.rs
+++ b/datafusion/physical-plan/src/spill/spill_manager.rs
@@ -17,6 +17,7 @@
 
 //! Define the `SpillManager` struct, which is responsible for reading and writing `RecordBatch`es to raw files based on the provided configurations.
 
+use arrow::array::StringViewArray;
 use arrow::datatypes::SchemaRef;
 use arrow::record_batch::RecordBatch;
 use datafusion_execution::runtime_env::RuntimeEnv;
@@ -185,6 +186,8 @@ impl SpillManager {
 
 pub(crate) trait GetSlicedSize {
     /// Returns the size of the `RecordBatch` when sliced.
+    /// Note: if multiple arrays or even a single array share the same data buffers, we may double count each buffer.
+    /// Therefore, make sure we call gc() or organize_stringview_arrays() before using this method.
     fn get_sliced_size(&self) -> Result<usize>;
 }
 
@@ -194,7 +197,83 @@ impl GetSlicedSize for RecordBatch {
         for array in self.columns() {
             let data = array.to_data();
             total += data.get_slice_memory_size()?;
+
+            // While StringViewArray holds large data buffer for non inlined string, the Arrow layout (BufferSpec)
+            // does not include any data buffers. Currently, ArrayData::get_slice_memory_size()
+            // under-counts memory size by accounting only views buffer although data buffer is cloned during slice()
+            //
+            // Therefore, we manually add the sum of the lengths used by all non inlined views
+            // on top of the sliced size for views buffer. This matches the intended semantics of
+            // "bytes needed if we materialized exactly this slice into fresh buffers".
+            // This is a workaround until https://github.com/apache/arrow-rs/issues/8230
+            if let Some(sv) = array.as_any().downcast_ref::<StringViewArray>() {
+                for buffer in sv.data_buffers() {
+                    total += buffer.capacity();
+                }
+            }
         }
         Ok(total)
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use crate::spill::{get_record_batch_memory_size, spill_manager::GetSlicedSize};
+    use arrow::datatypes::{DataType, Field, Schema};
+    use arrow::{
+        array::{ArrayRef, StringViewArray},
+        record_batch::RecordBatch,
+    };
+    use datafusion_common::Result;
+    use std::sync::Arc;
+
+    #[test]
+    fn check_sliced_size_for_string_view_array() -> Result<()> {
+        let array_length = 50;
+        let short_len = 8;
+        let long_len = 25;
+
+        // Build StringViewArray that includes both inline strings and non inlined strings
+        let strings: Vec<String> = (0..array_length)
+            .map(|i| {
+                if i % 2 == 0 {
+                    "a".repeat(short_len)
+                } else {
+                    "b".repeat(long_len)
+                }
+            })
+            .collect();
+
+        let string_array = StringViewArray::from(strings);
+        let array_ref: ArrayRef = Arc::new(string_array);
+        let batch = RecordBatch::try_new(
+            Arc::new(Schema::new(vec![Field::new(
+                "strings",
+                DataType::Utf8View,
+                false,
+            )])),
+            vec![array_ref],
+        )
+        .unwrap();
+
+        // We did not slice the batch, so these two memory size should be equal
+        assert_eq!(
+            batch.get_sliced_size().unwrap(),
+            get_record_batch_memory_size(&batch)
+        );
+
+        // Slice the batch into half
+        let half_batch = batch.slice(0, array_length / 2);
+        // Now sliced_size is smaller because the views buffer is sliced
+        assert!(
+            half_batch.get_sliced_size().unwrap()
+                < get_record_batch_memory_size(&half_batch)
+        );
+        let data = arrow::array::Array::to_data(&half_batch.column(0));
+        let views_sliced_size = data.get_slice_memory_size()?;
+        // The sliced size should be larger than sliced views buffer size
+        assert!(views_sliced_size < half_batch.get_sliced_size().unwrap());
+
+        Ok(())
+    }
+}