Fixed merge logic for multiple shards case (#877) (#878)

* Fixed merge logic for multiple shards case

Signed-off-by: Martin Gaievski <gaievski@amazon.com>
(cherry picked from commit 3a6bdc7)

Co-authored-by: Martin Gaievski <gaievski@amazon.com>

CHANGELOG.md

@@ -16,6 +16,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 ### Features
 ### Enhancements
 ### Bug Fixes
+- Fixed merge logic in hybrid query for multiple shards case ([#877](/~https://github.com/opensearch-project/neural-search/pull/877))
 ### Infrastructure
 ### Documentation
 ### Maintenance

src/main/java/org/opensearch/neuralsearch/search/query/TopDocsMerger.java

@@ -55,9 +55,15 @@ class TopDocsMerger {
      * @return merged TopDocsAndMaxScore object
      */
     public TopDocsAndMaxScore merge(final TopDocsAndMaxScore source, final TopDocsAndMaxScore newTopDocs) {
-        if (Objects.isNull(newTopDocs) || Objects.isNull(newTopDocs.topDocs) || newTopDocs.topDocs.totalHits.value == 0) {
+        // we need to check if any of source and destination top docs are empty. This is needed for case when concurrent segment search
+        // is enabled. In such case search is done by multiple workers, and results are saved in multiple doc collectors. Any on those
+        // results can be empty, in such case we can skip actual merge logic and just return result object.
+        if (isEmpty(newTopDocs)) {
             return source;
         }
+        if (isEmpty(source)) {
+            return newTopDocs;
+        }
         TotalHits mergedTotalHits = getMergedTotalHits(source, newTopDocs);
         TopDocsAndMaxScore result = new TopDocsAndMaxScore(
             getTopDocs(getMergedScoreDocs(source.topDocs.scoreDocs, newTopDocs.topDocs.scoreDocs), mergedTotalHits),
@@ -66,6 +72,20 @@ public TopDocsAndMaxScore merge(final TopDocsAndMaxScore source, final TopDocsAn
         return result;
     }
 
+    /**
+     * Checks if TopDocsAndMaxScore is null, has no top docs or zero total hits
+     * @param topDocsAndMaxScore
+     * @return
+     */
+    private static boolean isEmpty(final TopDocsAndMaxScore topDocsAndMaxScore) {
+        if (Objects.isNull(topDocsAndMaxScore)
+            || Objects.isNull(topDocsAndMaxScore.topDocs)
+            || topDocsAndMaxScore.topDocs.totalHits.value == 0) {
+            return true;
+        }
+        return false;
+    }
+
     private TotalHits getMergedTotalHits(final TopDocsAndMaxScore source, final TopDocsAndMaxScore newTopDocs) {
         // merged value is a lower bound - if both are equal_to than merged will also be equal_to,
         // otherwise assign greater_than_or_equal

src/test/java/org/opensearch/neuralsearch/search/query/TopDocsMergerTests.java

@@ -176,6 +176,65 @@ public void testMergeScoreDocs_whenBothTopDocsHasNoHits_thenSuccessful() {
         assertEquals(MAGIC_NUMBER_START_STOP, scoreDocs[3].score, 0);
     }
 
+    @SneakyThrows
+    public void testMergeScoreDocs_whenSomeSegmentsHasNoHits_thenSuccessful() {
+        // Given
+        TopDocsMerger topDocsMerger = new TopDocsMerger(null);
+
+        // When
+        // first segment has no results, and we merge with non-empty segment
+        TopDocs topDocsOriginal = new TopDocs(new TotalHits(0, TotalHits.Relation.EQUAL_TO), new ScoreDoc[] {});
+        TopDocsAndMaxScore topDocsAndMaxScoreOriginal = new TopDocsAndMaxScore(topDocsOriginal, 0);
+        TopDocs topDocsNew = new TopDocs(
+            new TotalHits(2, TotalHits.Relation.EQUAL_TO),
+            new ScoreDoc[] {
+                createStartStopElementForHybridSearchResults(0),
+                createDelimiterElementForHybridSearchResults(0),
+                new ScoreDoc(0, 0.5f),
+                new ScoreDoc(2, 0.3f),
+                createStartStopElementForHybridSearchResults(0) }
+        );
+        TopDocsAndMaxScore topDocsAndMaxScoreNew = new TopDocsAndMaxScore(topDocsNew, 0.5f);
+        TopDocsAndMaxScore mergedTopDocsAndMaxScore = topDocsMerger.merge(topDocsAndMaxScoreOriginal, topDocsAndMaxScoreNew);
+
+        // Then
+        assertNotNull(mergedTopDocsAndMaxScore);
+
+        assertEquals(0.5f, mergedTopDocsAndMaxScore.maxScore, DELTA_FOR_ASSERTION);
+        assertEquals(2, mergedTopDocsAndMaxScore.topDocs.totalHits.value);
+        assertEquals(TotalHits.Relation.EQUAL_TO, mergedTopDocsAndMaxScore.topDocs.totalHits.relation);
+        assertEquals(5, mergedTopDocsAndMaxScore.topDocs.scoreDocs.length);
+        // check format, all elements one by one
+        ScoreDoc[] scoreDocs = mergedTopDocsAndMaxScore.topDocs.scoreDocs;
+        assertEquals(MAGIC_NUMBER_START_STOP, scoreDocs[0].score, 0);
+        assertEquals(MAGIC_NUMBER_DELIMITER, scoreDocs[1].score, 0);
+        assertScoreDoc(scoreDocs[2], 0, 0.5f);
+        assertScoreDoc(scoreDocs[3], 2, 0.3f);
+        assertEquals(MAGIC_NUMBER_START_STOP, scoreDocs[4].score, 0);
+
+        // When
+        // source object has results, and we merge with empty segment
+        TopDocs topDocsNewEmpty = new TopDocs(new TotalHits(0, TotalHits.Relation.EQUAL_TO), new ScoreDoc[] {});
+        TopDocsAndMaxScore topDocsAndMaxScoreNewEmpty = new TopDocsAndMaxScore(topDocsNewEmpty, 0);
+        TopDocsAndMaxScore finalMergedTopDocsAndMaxScore = topDocsMerger.merge(mergedTopDocsAndMaxScore, topDocsAndMaxScoreNewEmpty);
+
+        // Then
+        // merged object remains unchanged
+        assertNotNull(finalMergedTopDocsAndMaxScore);
+
+        assertEquals(0.5f, finalMergedTopDocsAndMaxScore.maxScore, DELTA_FOR_ASSERTION);
+        assertEquals(2, finalMergedTopDocsAndMaxScore.topDocs.totalHits.value);
+        assertEquals(TotalHits.Relation.EQUAL_TO, finalMergedTopDocsAndMaxScore.topDocs.totalHits.relation);
+        assertEquals(5, finalMergedTopDocsAndMaxScore.topDocs.scoreDocs.length);
+        // check format, all elements one by one
+        ScoreDoc[] finalScoreDocs = finalMergedTopDocsAndMaxScore.topDocs.scoreDocs;
+        assertEquals(MAGIC_NUMBER_START_STOP, finalScoreDocs[0].score, 0);
+        assertEquals(MAGIC_NUMBER_DELIMITER, finalScoreDocs[1].score, 0);
+        assertScoreDoc(finalScoreDocs[2], 0, 0.5f);
+        assertScoreDoc(finalScoreDocs[3], 2, 0.3f);
+        assertEquals(MAGIC_NUMBER_START_STOP, finalScoreDocs[4].score, 0);
+    }
+
     @SneakyThrows
     public void testThreeSequentialMerges_whenAllTopDocsHasHits_thenSuccessful() {
         TopDocsMerger topDocsMerger = new TopDocsMerger(null);