ARROW-6738: [Java] Fix problems with current union comparison logic

There are some problems with the current union comparison logic. For example:
1. For type check, we should not require fields to be equal. It is possible that two vectors' value ranges are equal but their fields are different.
2. We should not compare the number of sub vectors, as it is possible that two union vectors have different numbers of sub vectors, but have equal values in the range.

Closes #5544 from liyafan82/fly_0930_share and squashes the following commits:

d6ef3d2 <liyafan82>  Refine test case
c008289 <liyafan82>  Resolve test failure after rebasing
c515393 <liyafan82>  Rule out the change for union type comparison
bab7402 <liyafan82>  Compare fields for all vectors except union vectors
5b2225e <liyafan82>  Fix the bug with decimal vector
4d8b570 <liyafan82>  Fix problems with current union comparison logic

Authored-by: liyafan82 <fan_li_ya@foxmail.com>
Signed-off-by: Micah Kornfield <emkornfield@gmail.com>

java/vector/src/main/codegen/templates/UnionVector.java

@@ -550,7 +550,7 @@ public Iterator<ValueVector> iterator() {
     return vectors.iterator();
   }
 
-    private ValueVector getVector(int index) {
+    public ValueVector getVector(int index) {
       int type = typeBuffer.getByte(index * TYPE_WIDTH);
       switch (MinorType.values()[type]) {
         case NULL:

java/vector/src/main/java/org/apache/arrow/vector/compare/ApproxEqualsVisitor.java

@@ -109,8 +109,10 @@ public Boolean visit(BaseFixedWidthVector left, Range range) {
   }
 
   @Override
-  protected ApproxEqualsVisitor createInnerVisitor(ValueVector left, ValueVector right) {
-    return new ApproxEqualsVisitor(left, right, floatDiffFunction.clone(), doubleDiffFunction.clone());
+  protected ApproxEqualsVisitor createInnerVisitor(
+      ValueVector left, ValueVector right,
+      BiFunction<ValueVector, ValueVector, Boolean> typeComparator) {
+    return new ApproxEqualsVisitor(left, right, floatDiffFunction.clone(), doubleDiffFunction.clone(), typeComparator);
   }
 
   private boolean float4ApproxEquals(Range range) {

java/vector/src/main/java/org/apache/arrow/vector/compare/RangeEqualsVisitor.java

@@ -24,7 +24,6 @@
 import org.apache.arrow.util.Preconditions;
 import org.apache.arrow.vector.BaseFixedWidthVector;
 import org.apache.arrow.vector.BaseVariableWidthVector;
-import org.apache.arrow.vector.FieldVector;
 import org.apache.arrow.vector.NullVector;
 import org.apache.arrow.vector.ValueVector;
 import org.apache.arrow.vector.complex.BaseRepeatedValueVector;
@@ -193,29 +192,33 @@ public Boolean visit(NullVector left, Range range) {
     return true;
   }
 
-  /**
-   * Creates a visitor to visit child vectors.
-   * It is used for complex vector types.
-   * @return the visitor for child vecors.
-   */
-  protected RangeEqualsVisitor createInnerVisitor(ValueVector leftInner, ValueVector rightInner) {
+  protected RangeEqualsVisitor createInnerVisitor(
+          ValueVector leftInner, ValueVector rightInner,
+          BiFunction<ValueVector, ValueVector, Boolean> typeComparator) {
     return new RangeEqualsVisitor(leftInner, rightInner, typeComparator);
   }
 
   protected boolean compareUnionVectors(Range range) {
     UnionVector leftVector = (UnionVector) left;
     UnionVector rightVector = (UnionVector) right;
 
-    List<FieldVector> leftChildren = leftVector.getChildrenFromFields();
-    List<FieldVector> rightChildren = rightVector.getChildrenFromFields();
-
-    if (leftChildren.size() != rightChildren.size()) {
-      return false;
-    }
-
-    for (int k = 0; k < leftChildren.size(); k++) {
-      RangeEqualsVisitor visitor = createInnerVisitor(leftChildren.get(k), rightChildren.get(k));
-      if (!visitor.rangeEquals(range)) {
+    Range subRange = new Range(0, 0, 1);
+    for (int i = 0; i < range.getLength(); i++) {
+      subRange.setLeftStart(range.getLeftStart() + i).setRightStart(range.getRightStart() + i);
+      ValueVector leftSubVector = leftVector.getVector(range.getLeftStart() + i);
+      ValueVector rightSubVector = rightVector.getVector(range.getRightStart() + i);
+
+      if (leftSubVector == null || rightSubVector == null) {
+        if (leftSubVector == rightSubVector) {
+          continue;
+        } else {
+          return false;
+        }
+      }
+      TypeEqualsVisitor typeVisitor = new TypeEqualsVisitor(rightSubVector);
+      RangeEqualsVisitor visitor =
+          createInnerVisitor(leftSubVector, rightSubVector, (left, right) -> typeVisitor.equals(left));
+      if (!visitor.rangeEquals(subRange)) {
         return false;
       }
     }
@@ -232,7 +235,8 @@ protected boolean compareStructVectors(Range range) {
     }
 
     for (String name : leftChildNames) {
-      RangeEqualsVisitor visitor = createInnerVisitor(leftVector.getChild(name), rightVector.getChild(name));
+      RangeEqualsVisitor visitor =
+          createInnerVisitor(leftVector.getChild(name), rightVector.getChild(name), /*type comparator*/ null);
       if (!visitor.rangeEquals(range)) {
         return false;
       }
@@ -311,7 +315,8 @@ protected boolean compareListVectors(Range range) {
     ListVector leftVector = (ListVector) left;
     ListVector rightVector = (ListVector) right;
 
-    RangeEqualsVisitor innerVisitor = createInnerVisitor(leftVector.getDataVector(), rightVector.getDataVector());
+    RangeEqualsVisitor innerVisitor =
+        createInnerVisitor(leftVector.getDataVector(), rightVector.getDataVector(), /*type comparator*/ null);
     Range innerRange = new Range();
 
     for (int i = 0; i < range.getLength(); i++) {
@@ -357,7 +362,8 @@ protected boolean compareFixedSizeListVectors(Range range) {
     }
 
     int listSize = leftVector.getListSize();
-    RangeEqualsVisitor innerVisitor = createInnerVisitor(leftVector.getDataVector(), rightVector.getDataVector());
+    RangeEqualsVisitor innerVisitor =
+        createInnerVisitor(leftVector.getDataVector(), rightVector.getDataVector(), /*type comparator*/ null);
     Range innerRange = new Range(0, 0, listSize);
 
     for (int i = 0; i < range.getLength(); i++) {

java/vector/src/test/java/org/apache/arrow/vector/compare/TestRangeEqualsVisitor.java

@@ -282,6 +282,57 @@ public void testUnionVectorRangeEquals() {
     }
   }
 
+  /**
+   * Test comparing two union vectors.
+   * The two vectors are different in total, but have a range with equal values.
+   */
+  @Test
+  public void testUnionVectorSubRangeEquals() {
+    try (final UnionVector vector1 = new UnionVector("union", allocator, null, null);
+         final UnionVector vector2 = new UnionVector("union", allocator, null, null);) {
+
+      final NullableUInt4Holder uInt4Holder = new NullableUInt4Holder();
+      uInt4Holder.value = 10;
+      uInt4Holder.isSet = 1;
+
+      final NullableIntHolder intHolder = new NullableIntHolder();
+      intHolder.value = 20;
+      intHolder.isSet = 1;
+
+      vector1.setType(0, Types.MinorType.UINT4);
+      vector1.setSafe(0, uInt4Holder);
+
+      vector1.setType(1, Types.MinorType.INT);
+      vector1.setSafe(1, intHolder);
+
+      vector1.setType(2, Types.MinorType.INT);
+      vector1.setSafe(2, intHolder);
+
+      vector1.setType(3, Types.MinorType.INT);
+      vector1.setSafe(3, intHolder);
+
+      vector1.setValueCount(4);
+
+      vector2.setType(0, Types.MinorType.UINT4);
+      vector2.setSafe(0, uInt4Holder);
+
+      vector2.setType(1, Types.MinorType.INT);
+      vector2.setSafe(1, intHolder);
+
+      vector2.setType(2, Types.MinorType.INT);
+      vector2.setSafe(2, intHolder);
+
+      vector2.setType(3, Types.MinorType.UINT4);
+      vector2.setSafe(3, uInt4Holder);
+
+      vector2.setValueCount(4);
+
+      RangeEqualsVisitor visitor = new RangeEqualsVisitor(vector1, vector2);
+      assertFalse(visitor.rangeEquals(new Range(0, 0, 4)));
+      assertTrue(visitor.rangeEquals(new Range(1, 1, 2)));
+    }
+  }
+
   @Ignore
   @Test
   public void testEqualsWithOutTypeCheck() {