SimpleText[Float|Byte]VectorValues::scorer should return null when the vector values is empty

lucene/backward-codecs/src/java/org/apache/lucene/backward_codecs/lucene90/Lucene90HnswVectorsReader.java

@@ -445,6 +445,9 @@ public float[] vectorValue(int targetOrd) throws IOException {
 
     @Override
     public VectorScorer scorer(float[] target) {
+      if (size() == 0) {
+        return null;
+      }
       OffHeapFloatVectorValues values = this.copy();
       return new VectorScorer() {
         @Override

lucene/backward-codecs/src/java/org/apache/lucene/backward_codecs/lucene91/Lucene91HnswVectorsReader.java

@@ -494,6 +494,9 @@ public float[] vectorValue(int targetOrd) throws IOException {
 
     @Override
     public VectorScorer scorer(float[] target) {
+      if (size == 0) {
+        return null;
+      }
       OffHeapFloatVectorValues values = this.copy();
       return new VectorScorer() {
         @Override

lucene/backward-codecs/src/test/org/apache/lucene/backward_codecs/lucene90/TestLucene90HnswVectorsFormat.java

@@ -73,4 +73,9 @@ public void testSortedIndexBytes() throws Exception {
   public void testByteVectorScorerIteration() {
     // unimplemented
   }
+
+  @Override
+  public void testEmptyByteVectorData() {
+    // unimplemented
+  }
 }

lucene/backward-codecs/src/test/org/apache/lucene/backward_codecs/lucene91/TestLucene91HnswVectorsFormat.java

@@ -72,4 +72,9 @@ public void testSortedIndexBytes() throws Exception {
   public void testByteVectorScorerIteration() {
     // unimplemented
   }
+
+  @Override
+  public void testEmptyByteVectorData() {
+    // unimplemented
+  }
 }

lucene/backward-codecs/src/test/org/apache/lucene/backward_codecs/lucene92/TestLucene92HnswVectorsFormat.java

@@ -62,4 +62,9 @@ public void testSortedIndexBytes() throws Exception {
   public void testByteVectorScorerIteration() {
     // unimplemented
   }
+
+  @Override
+  public void testEmptyByteVectorData() {
+    // unimplemented
+  }
 }

lucene/codecs/src/java/org/apache/lucene/codecs/simpletext/SimpleTextKnnVectorsReader.java

@@ -360,6 +360,9 @@ public int advance(int target) throws IOException {
 
     @Override
     public VectorScorer scorer(float[] target) {
+      if (size() == 0) {
+        return null;
+      }
       SimpleTextFloatVectorValues simpleTextFloatVectorValues =
           new SimpleTextFloatVectorValues(this);
       return new VectorScorer() {
@@ -470,6 +473,9 @@ public int advance(int target) throws IOException {
 
     @Override
     public VectorScorer scorer(byte[] target) {
+      if (size() == 0) {
+        return null;
+      }
       SimpleTextByteVectorValues simpleTextByteVectorValues = new SimpleTextByteVectorValues(this);
       return new VectorScorer() {
         @Override

lucene/test-framework/src/java/org/apache/lucene/tests/index/BaseKnnVectorsFormatTestCase.java

@@ -17,6 +17,7 @@
 package org.apache.lucene.tests.index;
 
 import static java.nio.charset.StandardCharsets.UTF_8;
+import static org.apache.lucene.index.VectorSimilarityFunction.DOT_PRODUCT;
 import static org.apache.lucene.search.DocIdSetIterator.NO_MORE_DOCS;
 
 import java.io.ByteArrayOutputStream;
@@ -838,6 +839,58 @@ public void testByteVectorScorerIteration() throws Exception {
     }
   }
 
+  public void testEmptyFloatVectorData() throws Exception {
+    try (Directory dir = newDirectory();
+        IndexWriter w = new IndexWriter(dir, newIndexWriterConfig())) {
+      var doc1 = new Document();
+      doc1.add(new StringField("id", "0", Field.Store.NO));
+      doc1.add(new KnnFloatVectorField("v", new float[] {2, 3, 5, 6}, DOT_PRODUCT));
+      w.addDocument(doc1);
+
+      var doc2 = new Document();
+      doc2.add(new StringField("id", "1", Field.Store.NO));
+      w.addDocument(doc2);
+
+      w.deleteDocuments(new Term("id", Integer.toString(0)));
+      w.commit();
+      w.forceMerge(1);
+
+      try (DirectoryReader reader = DirectoryReader.open(w)) {
+        LeafReader r = getOnlyLeafReader(reader);
+        FloatVectorValues values = r.getFloatVectorValues("v");
+        assertNotNull(values);
+        assertEquals(0, values.size());
+        assertNull(values.scorer(new float[] {2, 3, 5, 6}));
+      }
+    }
+  }
+
+  public void testEmptyByteVectorData() throws Exception {
+    try (Directory dir = newDirectory();
+        IndexWriter w = new IndexWriter(dir, newIndexWriterConfig())) {
+      var doc1 = new Document();
+      doc1.add(new StringField("id", "0", Field.Store.NO));
+      doc1.add(new KnnByteVectorField("v", new byte[] {2, 3, 5, 6}, DOT_PRODUCT));
+      w.addDocument(doc1);
+
+      var doc2 = new Document();
+      doc2.add(new StringField("id", "1", Field.Store.NO));
+      w.addDocument(doc2);
+
+      w.deleteDocuments(new Term("id", Integer.toString(0)));
+      w.commit();
+      w.forceMerge(1);
+
+      try (DirectoryReader reader = DirectoryReader.open(w)) {
+        LeafReader r = getOnlyLeafReader(reader);
+        ByteVectorValues values = r.getByteVectorValues("v");
+        assertNotNull(values);
+        assertEquals(0, values.size());
+        assertNull(values.scorer(new byte[] {2, 3, 5, 6}));
+      }
+    }
+  }
+
   protected VectorSimilarityFunction randomSimilarity() {
     return VectorSimilarityFunction.values()[
         random().nextInt(VectorSimilarityFunction.values().length)];