From 9ae29cb8c40a9b0595a04461ca990f6d55f6ee69 Mon Sep 17 00:00:00 2001
From: Tobias Schwarzinger <tobias.schwarzinger@tuwien.ac.at>
Date: Mon, 24 Nov 2025 09:19:10 +0100
Subject: [PATCH 1/2] Allocate a buffer of the correct length for
 ScalarValue::FixedSizeBinary in ScalarValue::to_array_of_size

---
 datafusion/common/src/scalar/mod.rs | 28 +++++++++++++++++++++++++++-
 1 file changed, 27 insertions(+), 1 deletion(-)

diff --git a/datafusion/common/src/scalar/mod.rs b/datafusion/common/src/scalar/mod.rs
index 3fd5a3722455..354656b2672e 100644
--- a/datafusion/common/src/scalar/mod.rs
+++ b/datafusion/common/src/scalar/mod.rs
@@ -21,6 +21,8 @@ mod cache;
 mod consts;
 mod struct_builder;
 
+use arrow::buffer::MutableBuffer;
+use arrow::buffer::NullBuffer;
 use std::borrow::Borrow;
 use std::cmp::Ordering;
 use std::collections::{HashSet, VecDeque};
@@ -3055,7 +3057,19 @@ impl ScalarValue {
                     )
                     .unwrap(),
                 ),
-                None => Arc::new(FixedSizeBinaryArray::new_null(*s, size)),
+                None => {
+                    // TODO: Replace with FixedSizeBinaryArray::new_null once a fix for
+                    // https://github.com/apache/arrow-rs/issues/8900 is in the used arrow-rs
+                    // version.
+                    let capacity_in_bytes =
+                        s.to_usize().unwrap().checked_mul(size).unwrap();
+                    Arc::new(FixedSizeBinaryArray::try_new(
+                        *s,
+                        // MutableBuffer::new_null is in bits.
+                        MutableBuffer::new_null(capacity_in_bytes * 8).into(),
+                        Some(NullBuffer::new_null(size)),
+                    )?)
+                }
             },
             ScalarValue::LargeBinary(e) => match e {
                 Some(value) => Arc::new(
@@ -5314,6 +5328,18 @@ mod tests {
         assert_eq!(empty_array.len(), 0);
     }
 
+    /// See https://github.com/apache/datafusion/issues/18870
+    #[test]
+    fn test_to_array_of_size_for_none_fsb() {
+        let sv = ScalarValue::FixedSizeBinary(5, None);
+        let result = sv
+            .to_array_of_size(2)
+            .expect("Failed to convert to array of size");
+        assert_eq!(result.len(), 2);
+        assert_eq!(result.null_count(), 2);
+        assert_eq!(result.as_fixed_size_binary().values().len(), 10);
+    }
+
     #[test]
     fn test_list_to_array_string() {
         let scalars = vec![

From b79d12210d00c0ae21092dbcd2871f19edf45dc9 Mon Sep 17 00:00:00 2001
From: Tobias Schwarzinger <tobias.schwarzinger@tuwien.ac.at>
Date: Wed, 26 Nov 2025 10:18:34 +0100
Subject: [PATCH 2/2] Use FixedSizeBinaryBuilder instead of directly
 constructing the FixedSizeBinaryArray in `ScalarValue::to_array_of_size`

---
 datafusion/common/src/scalar/mod.rs | 31 +++++++++++------------------
 1 file changed, 12 insertions(+), 19 deletions(-)

diff --git a/datafusion/common/src/scalar/mod.rs b/datafusion/common/src/scalar/mod.rs
index 354656b2672e..ddb8b9a71fc9 100644
--- a/datafusion/common/src/scalar/mod.rs
+++ b/datafusion/common/src/scalar/mod.rs
@@ -21,8 +21,6 @@ mod cache;
 mod consts;
 mod struct_builder;
 
-use arrow::buffer::MutableBuffer;
-use arrow::buffer::NullBuffer;
 use std::borrow::Borrow;
 use std::cmp::Ordering;
 use std::collections::{HashSet, VecDeque};
@@ -63,15 +61,15 @@ use arrow::array::{
     Date64Array, Decimal128Array, Decimal256Array, Decimal32Array, Decimal64Array,
     DictionaryArray, DurationMicrosecondArray, DurationMillisecondArray,
     DurationNanosecondArray, DurationSecondArray, FixedSizeBinaryArray,
-    FixedSizeListArray, Float16Array, Float32Array, Float64Array, GenericListArray,
-    Int16Array, Int32Array, Int64Array, Int8Array, IntervalDayTimeArray,
-    IntervalMonthDayNanoArray, IntervalYearMonthArray, LargeBinaryArray, LargeListArray,
-    LargeStringArray, ListArray, MapArray, MutableArrayData, OffsetSizeTrait,
-    PrimitiveArray, Scalar, StringArray, StringViewArray, StructArray,
-    Time32MillisecondArray, Time32SecondArray, Time64MicrosecondArray,
-    Time64NanosecondArray, TimestampMicrosecondArray, TimestampMillisecondArray,
-    TimestampNanosecondArray, TimestampSecondArray, UInt16Array, UInt32Array,
-    UInt64Array, UInt8Array, UnionArray,
+    FixedSizeBinaryBuilder, FixedSizeListArray, Float16Array, Float32Array, Float64Array,
+    GenericListArray, Int16Array, Int32Array, Int64Array, Int8Array,
+    IntervalDayTimeArray, IntervalMonthDayNanoArray, IntervalYearMonthArray,
+    LargeBinaryArray, LargeListArray, LargeStringArray, ListArray, MapArray,
+    MutableArrayData, OffsetSizeTrait, PrimitiveArray, Scalar, StringArray,
+    StringViewArray, StructArray, Time32MillisecondArray, Time32SecondArray,
+    Time64MicrosecondArray, Time64NanosecondArray, TimestampMicrosecondArray,
+    TimestampMillisecondArray, TimestampNanosecondArray, TimestampSecondArray,
+    UInt16Array, UInt32Array, UInt64Array, UInt8Array, UnionArray,
 };
 use arrow::buffer::{BooleanBuffer, ScalarBuffer};
 use arrow::compute::kernels::cast::{cast_with_options, CastOptions};
@@ -3061,14 +3059,9 @@ impl ScalarValue {
                     // TODO: Replace with FixedSizeBinaryArray::new_null once a fix for
                     // https://github.com/apache/arrow-rs/issues/8900 is in the used arrow-rs
                     // version.
-                    let capacity_in_bytes =
-                        s.to_usize().unwrap().checked_mul(size).unwrap();
-                    Arc::new(FixedSizeBinaryArray::try_new(
-                        *s,
-                        // MutableBuffer::new_null is in bits.
-                        MutableBuffer::new_null(capacity_in_bytes * 8).into(),
-                        Some(NullBuffer::new_null(size)),
-                    )?)
+                    let mut builder = FixedSizeBinaryBuilder::new(*s);
+                    builder.append_nulls(size);
+                    Arc::new(builder.finish())
                 }
             },
             ScalarValue::LargeBinary(e) => match e {