Optimize reduceToTopK in ResultUtil by removing pre-filling and reduc…

…ing peek calls (#2146)

Signed-off-by: Junqiu Lei <junqiu@amazon.com>

CHANGELOG.md

@@ -21,6 +21,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 * Add AVX512 support to k-NN for FAISS library [#2069](https://github.com/opensearch-project/k-NN/pull/2069)
 ### Enhancements
 * Add short circuit if no live docs are in segments [#2059](https://github.com/opensearch-project/k-NN/pull/2059)
+* Optimize reduceToTopK in ResultUtil by removing pre-filling and reducing peek calls [#2146](https://github.com/opensearch-project/k-NN/pull/2146)
 * Update Default Rescore Context based on Dimension [#2149](https://github.com/opensearch-project/k-NN/pull/2149)
 ### Bug Fixes
 * KNN80DocValues should only be considered for BinaryDocValues fields [#2147](https://github.com/opensearch-project/k-NN/pull/2147)

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

@@ -33,15 +33,14 @@ public final class ResultUtil {
     public static void reduceToTopK(List<Map<Integer, Float>> perLeafResults, int k) {
         // Iterate over all scores to get min competitive score
         PriorityQueue<Float> topKMinQueue = new PriorityQueue<>(k);
-        for (int i = 0; i < k; i++) {
-            topKMinQueue.add(-Float.MAX_VALUE);
-        }
 
         int count = 0;
         for (Map<Integer, Float> perLeafResult : perLeafResults) {
             count += perLeafResult.size();
             for (Float score : perLeafResult.values()) {
-                if (topKMinQueue.peek() != null && score > topKMinQueue.peek()) {
+                if (topKMinQueue.size() < k) {
+                    topKMinQueue.add(score);
+                } else if (topKMinQueue.peek() != null && score > topKMinQueue.peek()) {
                     topKMinQueue.poll();
                     topKMinQueue.add(score);
                 }