[GitHub] [arrow] jacques-n commented on a change in pull request #6402: ARROW-7831: [Java] do not allocate a new offset buffer if the slice starts at 0 since the relative offset pointer would be unchanged

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jacques-n commented on a change in pull request #6402:
URL: https://github.com/apache/arrow/pull/6402#discussion_r444514257



##########
File path: 
java/vector/src/main/java/org/apache/arrow/vector/BaseVariableWidthVector.java
##########
@@ -751,55 +757,57 @@ private void splitAndTransferOffsetBuffer(int startIndex, 
int length, BaseVariab
    */
   private void splitAndTransferValidityBuffer(int startIndex, int length,
                                               BaseVariableWidthVector target) {
-    int firstByteSource = BitVectorHelper.byteIndex(startIndex);
-    int lastByteSource = BitVectorHelper.byteIndex(valueCount - 1);
-    int byteSizeTarget = getValidityBufferSizeFromCount(length);
-    int offset = startIndex % 8;
+    if (length <= 0) {

Review comment:
       I'm not understanding why this is okay. It seems like you're leaving 
this function with a different number of references than the other exit points.

##########
File path: 
java/vector/src/main/java/org/apache/arrow/vector/BaseVariableWidthVector.java
##########
@@ -751,55 +757,57 @@ private void splitAndTransferOffsetBuffer(int startIndex, 
int length, BaseVariab
    */
   private void splitAndTransferValidityBuffer(int startIndex, int length,
                                               BaseVariableWidthVector target) {
-    int firstByteSource = BitVectorHelper.byteIndex(startIndex);
-    int lastByteSource = BitVectorHelper.byteIndex(valueCount - 1);
-    int byteSizeTarget = getValidityBufferSizeFromCount(length);
-    int offset = startIndex % 8;
+    if (length <= 0) {
+      return;
+    }
 
-    if (length > 0) {
-      if (offset == 0) {
-        // slice
-        if (target.validityBuffer != null) {
-          target.validityBuffer.getReferenceManager().release();
-        }
-        target.validityBuffer = validityBuffer.slice(firstByteSource, 
byteSizeTarget);
-        target.validityBuffer.getReferenceManager().retain();
+    final int firstByteSource = BitVectorHelper.byteIndex(startIndex);
+    final int lastByteSource = BitVectorHelper.byteIndex(valueCount - 1);
+    final int byteSizeTarget = getValidityBufferSizeFromCount(length);
+    final int offset = startIndex % 8;
+
+    if (offset == 0) {
+      // slice
+      if (target.validityBuffer != null) {
+        target.validityBuffer.getReferenceManager().release();
+      }
+      final ArrowBuf slicedValidityBuffer = 
validityBuffer.slice(firstByteSource, byteSizeTarget);
+      target.validityBuffer = transferBuffer(slicedValidityBuffer, 
target.allocator);

Review comment:
       i think you can just return here (or make small modifications to do so) 
avoiding the large else block




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