Utilize exact kNN search when gathering k > numVectors in a segment (#12806)

When requesting for k >= numVectors, it doesn't make sense to go through the HNSW graph. Even without a user supplied filter, we should not explore the HNSW graph if it contains fewer than k vectors.

One scenario where we may still explore the graph if k >= numVectors is when not every document has a vector and there are deleted docs. But, this commit significantly improves things regardless.

lucene/CHANGES.txt

@@ -285,6 +285,8 @@ Optimizations
 
 * GITHUB#12784: Cache buckets to speed up BytesRefHash#sort. (Guo Feng)
 
+* GITHUB#12806: Utilize exact kNN search when gathering k >= numVectors in a segment (Ben Trent)
+
 Changes in runtime behavior
 ---------------------
 

lucene/core/src/java/org/apache/lucene/codecs/lucene99/Lucene99HnswVectorsReader.java

@@ -227,12 +227,22 @@ public final class Lucene99HnswVectorsReader extends KnnVectorsReader
         || fieldEntry.vectorEncoding != VectorEncoding.FLOAT32) {
       return;
     }
-    RandomVectorScorer scorer = flatVectorsReader.getRandomVectorScorer(field, target);
-    HnswGraphSearcher.search(
-        scorer,
-        new OrdinalTranslatedKnnCollector(knnCollector, scorer::ordToDoc),
-        getGraph(fieldEntry),
-        scorer.getAcceptOrds(acceptDocs));
+    final RandomVectorScorer scorer = flatVectorsReader.getRandomVectorScorer(field, target);
+    final KnnCollector collector =
+        new OrdinalTranslatedKnnCollector(knnCollector, scorer::ordToDoc);
+    final Bits acceptedOrds = scorer.getAcceptOrds(acceptDocs);
+    if (knnCollector.k() < scorer.maxOrd()) {
+      HnswGraphSearcher.search(scorer, collector, getGraph(fieldEntry), acceptedOrds);
+    } else {
+      // if k is larger than the number of vectors, we can just iterate over all vectors
+      // and collect them
+      for (int i = 0; i < scorer.maxOrd(); i++) {
+        if (acceptedOrds == null || acceptedOrds.get(i)) {
+          knnCollector.incVisitedCount(1);
+          knnCollector.collect(scorer.ordToDoc(i), scorer.score(i));
+        }
+      }
+    }
   }
 
   @Override
@@ -245,12 +255,22 @@ public final class Lucene99HnswVectorsReader extends KnnVectorsReader
         || fieldEntry.vectorEncoding != VectorEncoding.BYTE) {
       return;
     }
-    RandomVectorScorer scorer = flatVectorsReader.getRandomVectorScorer(field, target);
-    HnswGraphSearcher.search(
-        scorer,
-        new OrdinalTranslatedKnnCollector(knnCollector, scorer::ordToDoc),
-        getGraph(fieldEntry),
-        scorer.getAcceptOrds(acceptDocs));
+    final RandomVectorScorer scorer = flatVectorsReader.getRandomVectorScorer(field, target);
+    final KnnCollector collector =
+        new OrdinalTranslatedKnnCollector(knnCollector, scorer::ordToDoc);
+    final Bits acceptedOrds = scorer.getAcceptOrds(acceptDocs);
+    if (knnCollector.k() < scorer.maxOrd()) {
+      HnswGraphSearcher.search(scorer, collector, getGraph(fieldEntry), acceptedOrds);
+    } else {
+      // if k is larger than the number of vectors, we can just iterate over all vectors
+      // and collect them
+      for (int i = 0; i < scorer.maxOrd(); i++) {
+        if (acceptedOrds == null || acceptedOrds.get(i)) {
+          knnCollector.incVisitedCount(1);
+          knnCollector.collect(scorer.ordToDoc(i), scorer.score(i));
+        }
+      }
+    }
   }
 
   @Override

lucene/core/src/test/org/apache/lucene/search/BaseKnnVectorQueryTestCase.java

@@ -17,6 +17,8 @@
 package org.apache.lucene.search;
 
 import static com.carrotsearch.randomizedtesting.RandomizedTest.frequently;
+import static com.carrotsearch.randomizedtesting.RandomizedTest.randomBoolean;
+import static com.carrotsearch.randomizedtesting.RandomizedTest.randomIntBetween;
 import static org.apache.lucene.index.VectorSimilarityFunction.COSINE;
 import static org.apache.lucene.search.DocIdSetIterator.NO_MORE_DOCS;
 
@@ -222,7 +224,7 @@ abstract class BaseKnnVectorQueryTestCase extends LuceneTestCase {
             getIndexStore("field", new float[] {0, 1}, new float[] {1, 2}, new float[] {0, 0});
         IndexReader reader = DirectoryReader.open(indexStore)) {
       IndexSearcher searcher = newSearcher(reader);
-      AbstractKnnVectorQuery kvq = getKnnVectorQuery("field", new float[] {0}, 10);
+      AbstractKnnVectorQuery kvq = getKnnVectorQuery("field", new float[] {0}, 1);
       IllegalArgumentException e =
           expectThrows(IllegalArgumentException.class, () -> searcher.search(kvq, 10));
       assertEquals("vector query dimension: 1 differs from field dimension: 2", e.getMessage());
@@ -779,6 +781,16 @@ abstract class BaseKnnVectorQueryTestCase extends LuceneTestCase {
       doc.add(getKnnVectorField(field, contents[i], vectorSimilarityFunction));
       doc.add(new StringField("id", "id" + i, Field.Store.YES));
       writer.addDocument(doc);
+      if (randomBoolean()) {
+        // Add some documents without a vector
+        for (int j = 0; j < randomIntBetween(1, 5); j++) {
+          doc = new Document();
+          doc.add(new StringField("other", "value", Field.Store.NO));
+          // Add fields that will be matched by our test filters but won't have vectors
+          doc.add(new StringField("id", "id" + j, Field.Store.YES));
+          writer.addDocument(doc);
+        }
+      }
     }
     // Add some documents without a vector
     for (int i = 0; i < 5; i++) {