Make the HitQueue size more appropriate for exact search

Signed-off-by: panguixin <[email protected]>

CHANGELOG.md

@@ -15,6 +15,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 ## [Unreleased 2.x](https://github.com/opensearch-project/k-NN/compare/2.13...2.x)
 ### Features
 ### Enhancements
+* Make the HitQueue size more appropriate for exact search [#1549](https://github.com/opensearch-project/k-NN/pull/1549)
 ### Bug Fixes
 ### Infrastructure
 ### Documentation

src/main/java/org/opensearch/knn/index/query/KNNWeight.java

@@ -117,7 +117,7 @@ public Scorer scorer(LeafReaderContext context) throws IOException {
          * This improves the recall.
          */
         if (filterWeight != null && canDoExactSearch(cardinality)) {
-            docIdsToScoreMap.putAll(doExactSearch(context, filterBitSet));
+            docIdsToScoreMap.putAll(doExactSearch(context, filterBitSet, cardinality));
         } else {
             Map<Integer, Float> annResults = doANNSearch(context, filterBitSet, cardinality);
             if (annResults == null) {
@@ -131,7 +131,7 @@ public Scorer scorer(LeafReaderContext context) throws IOException {
                     annResults.size(),
                     cardinality
                 );
-                annResults = doExactSearch(context, filterBitSet);
+                annResults = doExactSearch(context, filterBitSet, cardinality);
             }
             docIdsToScoreMap.putAll(annResults);
         }
@@ -309,10 +309,10 @@ private Map<Integer, Float> doANNSearch(final LeafReaderContext context, final B
             .collect(Collectors.toMap(KNNQueryResult::getId, result -> knnEngine.score(result.getScore(), spaceType)));
     }
 
-    private Map<Integer, Float> doExactSearch(final LeafReaderContext leafReaderContext, final BitSet filterIdsBitSet) {
+    private Map<Integer, Float> doExactSearch(final LeafReaderContext leafReaderContext, final BitSet filterIdsBitSet, int cardinality) {
         try {
             // Creating min heap and init with MAX DocID and Score as -INF.
-            final HitQueue queue = new HitQueue(this.knnQuery.getK(), true);
+            final HitQueue queue = new HitQueue(Math.min(this.knnQuery.getK(), cardinality), true);
             ScoreDoc topDoc = queue.top();
             final Map<Integer, Float> docToScore = new HashMap<>();
             FilteredIdsKNNIterator iterator = getFilteredKNNIterator(leafReaderContext, filterIdsBitSet);