Cache ImmutableRoaringBitmap in NullValueVectorReaderImpl to avoid repeated deserialization (#18145)

Author: deeppatel710

pinot-segment-local/src/main/java/org/apache/pinot/segment/local/segment/index/readers/NullValueVectorReaderImpl.java

@@ -26,6 +26,9 @@
 public class NullValueVectorReaderImpl implements NullValueVectorReader {
 
   private final PinotDataBuffer _dataBuffer;
+  // Cached bitmap to avoid re-deserializing from the underlying buffer on every call.
+  // ImmutableRoaringBitmap is thread-safe once constructed, so a volatile field is sufficient.
+  private volatile ImmutableRoaringBitmap _nullBitmap;
 
   public NullValueVectorReaderImpl(PinotDataBuffer dataBuffer) {
     _dataBuffer = dataBuffer;
@@ -37,6 +40,11 @@ public boolean isNull(int docId) {
 
   @Override
   public ImmutableRoaringBitmap getNullBitmap() {
-    return new ImmutableRoaringBitmap(_dataBuffer.toDirectByteBuffer(0, (int) _dataBuffer.size()));
+    ImmutableRoaringBitmap nullBitmap = _nullBitmap;
+    if (nullBitmap == null) {
+      nullBitmap = new ImmutableRoaringBitmap(_dataBuffer.toDirectByteBuffer(0, (int) _dataBuffer.size()));
+      _nullBitmap = nullBitmap;
+    }
+    return nullBitmap;
   }
 }

pinot-segment-local/src/test/java/org/apache/pinot/segment/local/segment/index/readers/NullValueVectorReaderImplTest.java

@@ -62,6 +62,19 @@ public void testNullValueVectorReader()
     }
   }
 
+  @Test
+  public void testNullBitmapIsCached()
+      throws IOException {
+    File nullValueFile = new File(TEMP_DIR, TEMP_DIR.list()[0]);
+    try (PinotDataBuffer buffer = PinotDataBuffer.loadBigEndianFile(nullValueFile)) {
+      NullValueVectorReaderImpl reader = new NullValueVectorReaderImpl(buffer);
+      // Repeated calls to getNullBitmap() must return the same instance, not reconstruct it
+      org.roaringbitmap.buffer.ImmutableRoaringBitmap first = reader.getNullBitmap();
+      org.roaringbitmap.buffer.ImmutableRoaringBitmap second = reader.getNullBitmap();
+      Assert.assertSame(first, second, "getNullBitmap() should return the cached instance on repeated calls");
+    }
+  }
+
   @AfterClass
   public void tearDown()
       throws Exception {