From 3815f20da207c02852b443bccfad945305319a3b Mon Sep 17 00:00:00 2001
From: hutiefang76 <137664623+hutiefang76@users.noreply.github.com>
Date: Thu, 10 Sep 2026 16:08:11 +0800
Subject: [PATCH 1/2] GH-812: Fix appending to an empty UnionVector

---
 .../arrow/vector/util/VectorAppender.java     |  4 ++--
 .../arrow/vector/util/TestVectorAppender.java | 20 +++++++++++++++++++
 2 files changed, 22 insertions(+), 2 deletions(-)

diff --git a/vector/src/main/java/org/apache/arrow/vector/util/VectorAppender.java b/vector/src/main/java/org/apache/arrow/vector/util/VectorAppender.java
index 2cfeb0a04d..465d010bff 100644
--- a/vector/src/main/java/org/apache/arrow/vector/util/VectorAppender.java
+++ b/vector/src/main/java/org/apache/arrow/vector/util/VectorAppender.java
@@ -601,8 +601,8 @@ public ValueVector visit(UnionVector deltaVector, Void value) {
     UnionVector targetUnionVector = (UnionVector) targetVector;
     int newValueCount = targetVector.getValueCount() + deltaVector.getValueCount();
 
-    // make sure there is enough capacity
-    while (targetUnionVector.getValueCapacity() < newValueCount) {
+    // Child vectors are created and expanded below; an empty union has zero value capacity.
+    while (targetUnionVector.getTypeBuffer().capacity() / UnionVector.TYPE_WIDTH < newValueCount) {
       targetUnionVector.reAlloc();
     }
 
diff --git a/vector/src/test/java/org/apache/arrow/vector/util/TestVectorAppender.java b/vector/src/test/java/org/apache/arrow/vector/util/TestVectorAppender.java
index 9a8143f51b..57dd852e31 100644
--- a/vector/src/test/java/org/apache/arrow/vector/util/TestVectorAppender.java
+++ b/vector/src/test/java/org/apache/arrow/vector/util/TestVectorAppender.java
@@ -804,6 +804,26 @@ public void testAppendEmptyStructVector() {
     }
   }
 
+  @Test
+  public void testAppendToEmptyUnionVector() {
+    try (BufferAllocator limitedAllocator = allocator.newChildAllocator("empty union", 0, 1048576);
+        UnionVector target = UnionVector.empty("target", limitedAllocator);
+        UnionVector delta = UnionVector.empty("delta", limitedAllocator)) {
+      delta.setType(0, Types.MinorType.FLOAT4);
+      delta.setType(1, Types.MinorType.FLOAT4);
+      Float4Vector values = delta.getFloat4Vector();
+      values.allocateNew();
+      ValueVectorDataPopulator.setVector(values, 1f, 2f);
+      delta.setValueCount(2);
+
+      delta.accept(new VectorAppender(target), null);
+
+      assertEquals(2, target.getValueCount());
+      assertEquals(1f, target.getObject(0));
+      assertEquals(2f, target.getObject(1));
+    }
+  }
+
   @Test
   public void testAppendUnionVector() {
     final int length1 = 10;

From ea411ff7f7aa7cf1ddb8354d14633b38ebd5c6ea Mon Sep 17 00:00:00 2001
From: hutiefang76 <137664623+hutiefang76@users.noreply.github.com>
Date: Tue, 15 Sep 2026 00:47:19 +0800
Subject: [PATCH 2/2] GH-812: Remove redundant union capacity comment

---
 .../main/java/org/apache/arrow/vector/util/VectorAppender.java   | 1 -
 1 file changed, 1 deletion(-)

diff --git a/vector/src/main/java/org/apache/arrow/vector/util/VectorAppender.java b/vector/src/main/java/org/apache/arrow/vector/util/VectorAppender.java
index 465d010bff..7176e86c71 100644
--- a/vector/src/main/java/org/apache/arrow/vector/util/VectorAppender.java
+++ b/vector/src/main/java/org/apache/arrow/vector/util/VectorAppender.java
@@ -601,7 +601,6 @@ public ValueVector visit(UnionVector deltaVector, Void value) {
     UnionVector targetUnionVector = (UnionVector) targetVector;
     int newValueCount = targetVector.getValueCount() + deltaVector.getValueCount();
 
-    // Child vectors are created and expanded below; an empty union has zero value capacity.
     while (targetUnionVector.getTypeBuffer().capacity() / UnionVector.TYPE_WIDTH < newValueCount) {
       targetUnionVector.reAlloc();
     }