apacheGH-692: Preserve nullability information while transfering DecimalVector and Decimal256Vector (apache#693) (#3)

This PR proposes to use the "from" ValueVector's field while transferring DecimalVector and Decimal256Vector to preserve nullability information. This is to have similar behavior with all the other primitive value vectors. Note that the FieldType of the value vector has nullability information.

Closes apache#692 .

(cherry picked from commit c29fcfc)

Author: bodduv

vector/src/main/java/org/apache/arrow/vector/Decimal256Vector.java

@@ -567,8 +567,11 @@ private class TransferImpl implements TransferPair {
 
     public TransferImpl(String ref, BufferAllocator allocator) {
       to =
-          new Decimal256Vector(
-              ref, allocator, Decimal256Vector.this.precision, Decimal256Vector.this.scale);
+          (Decimal256Vector.this.field != null
+                  && Decimal256Vector.this.field.getFieldType() != null)
+              ? new Decimal256Vector(ref, Decimal256Vector.this.field.getFieldType(), allocator)
+              : new Decimal256Vector(
+                  ref, allocator, Decimal256Vector.this.precision, Decimal256Vector.this.scale);
     }
 
     public TransferImpl(Field field, BufferAllocator allocator) {

vector/src/main/java/org/apache/arrow/vector/DecimalVector.java

@@ -565,7 +565,10 @@ private class TransferImpl implements TransferPair {
 
     public TransferImpl(String ref, BufferAllocator allocator) {
       to =
-          new DecimalVector(ref, allocator, DecimalVector.this.precision, DecimalVector.this.scale);
+          (DecimalVector.this.field != null && DecimalVector.this.field.getFieldType() != null)
+              ? new DecimalVector(ref, DecimalVector.this.field.getFieldType(), allocator)
+              : new DecimalVector(
+                  ref, allocator, DecimalVector.this.precision, DecimalVector.this.scale);
     }
 
     public TransferImpl(Field field, BufferAllocator allocator) {

vector/src/test/java/org/apache/arrow/vector/TestDecimal256Vector.java

@@ -26,6 +26,7 @@
 import org.apache.arrow.memory.ArrowBuf;
 import org.apache.arrow.memory.BufferAllocator;
 import org.apache.arrow.vector.types.pojo.ArrowType;
+import org.apache.arrow.vector.types.pojo.FieldType;
 import org.apache.arrow.vector.util.TransferPair;
 import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.BeforeEach;
@@ -375,6 +376,33 @@ public void testGetTransferPairWithField() {
     assertSame(fromVector.getField(), toVector.getField());
   }
 
+  @Test
+  public void testGetTransferPairWithoutField() {
+    final Decimal256Vector fromVector = new Decimal256Vector("decimal", allocator, 10, scale);
+    final TransferPair transferPair =
+        fromVector.getTransferPair(fromVector.getField().getName(), allocator);
+    final Decimal256Vector toVector = (Decimal256Vector) transferPair.getTo();
+    // A new Field created inside a new vector should reuse the field type (should be the same in
+    // memory as the original Field's field type).
+    assertSame(fromVector.getField().getFieldType(), toVector.getField().getFieldType());
+  }
+
+  @Test
+  public void testGetTransferPairWithoutFieldNonNullable() {
+    final FieldType decimal256NonNullableType =
+        new FieldType(
+            false, new ArrowType.Decimal(10, scale, Decimal256Vector.TYPE_WIDTH * 8), null);
+    final Decimal256Vector fromVector =
+        new Decimal256Vector("decimal", decimal256NonNullableType, allocator);
+    final TransferPair transferPair =
+        fromVector.getTransferPair(fromVector.getField().getName(), allocator);
+    final Decimal256Vector toVector = (Decimal256Vector) transferPair.getTo();
+    // A new Field created inside a new vector should reuse the field type (should be the same in
+    // memory as the original Field's field type).
+    assertSame(fromVector.getField().getFieldType(), toVector.getField().getFieldType());
+    assertSame(decimal256NonNullableType, toVector.getField().getFieldType());
+  }
+
   private void verifyWritingArrowBufWithBigEndianBytes(
       Decimal256Vector decimalVector, ArrowBuf buf, BigDecimal[] expectedValues, int length) {
     decimalVector.allocateNew();

vector/src/test/java/org/apache/arrow/vector/TestDecimalVector.java

@@ -26,6 +26,7 @@
 import org.apache.arrow.memory.ArrowBuf;
 import org.apache.arrow.memory.BufferAllocator;
 import org.apache.arrow.vector.types.pojo.ArrowType;
+import org.apache.arrow.vector.types.pojo.FieldType;
 import org.apache.arrow.vector.util.TransferPair;
 import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.BeforeEach;
@@ -371,6 +372,31 @@ public void testGetTransferPairWithField() {
     assertSame(fromVector.getField(), toVector.getField());
   }
 
+  @Test
+  public void testGetTransferPairWithoutField() {
+    final DecimalVector fromVector = new DecimalVector("decimal", allocator, 10, scale);
+    final TransferPair transferPair =
+        fromVector.getTransferPair(fromVector.getField().getName(), allocator);
+    final DecimalVector toVector = (DecimalVector) transferPair.getTo();
+    // A new Field created inside a new vector should reuse the field type (should be the same in
+    // memory as the original Field's field type).
+    assertSame(fromVector.getField().getFieldType(), toVector.getField().getFieldType());
+  }
+
+  @Test
+  public void testGetTransferPairWithoutFieldNonNullable() {
+    final FieldType decimalNonNullableType =
+        new FieldType(false, new ArrowType.Decimal(10, scale), null);
+    final DecimalVector fromVector =
+        new DecimalVector("decimal", decimalNonNullableType, allocator);
+    final TransferPair transferPair =
+        fromVector.getTransferPair(fromVector.getField().getName(), allocator);
+    final DecimalVector toVector = (DecimalVector) transferPair.getTo();
+    // A new Field created inside a new vector should reuse the field type (should be the same in
+    // memory as the original Field's field type).
+    assertSame(fromVector.getField().getFieldType(), toVector.getField().getFieldType());
+  }
+
   private void verifyWritingArrowBufWithBigEndianBytes(
       DecimalVector decimalVector, ArrowBuf buf, BigDecimal[] expectedValues, int length) {
     decimalVector.allocateNew();