Fixed search_after for hybrid query

CHANGELOG.md

@@ -10,6 +10,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 ### Enhancements
 
 ### Bug Fixes
+* [Hybrid Query] Fix NoSuchElementException in hybrid query with sort/search_after when a shard returns no results ([#1939](https://github.com/opensearch-project/neural-search/pull/1939))
 * [SemanticHighlighter] Fix SemanticHighlighterExtBuilder.toXContent ([#1906](https://github.com/opensearch-project/neural-search/issues/1906)) (query-insights [#651](https://github.com/opensearch-project/query-insights/issues/651))
 * [Sparse ANN] Fold sparse vector tokens into the signed-short range (modulus 32768) so folded tokens are never sign-extended to a negative value when stored in short[] ([#1926](https://github.com/opensearch-project/neural-search/pull/1926))
 

src/main/java/org/opensearch/neuralsearch/processor/combination/ScoreCombiner.java

@@ -234,6 +234,12 @@ private Map<Integer, Object[]> getDocIdSortFieldsMap(
         // we're merging docs with normalized and combined scores. we need to have only maxHits results
         Map<Integer, Object[]> docIdSortFieldMap = new HashMap<>();
         final List<TopDocs> topFieldDocs = compoundTopDocs.getTopDocs();
+        // A shard that returned no results (e.g. one exhausted past a search_after cursor while other shards
+        // still have hits) arrives here with an empty top docs list but a non-zero total hits count. There are
+        // no sort fields to collect, so return the empty map and avoid getFirst() on an empty list. See issue #1934.
+        if (topFieldDocs.isEmpty()) {
+            return docIdSortFieldMap;
+        }
         final boolean isSortByScore = isSortOrderByScore(sort);
         final boolean isCollapseEnabled = topFieldDocs.getFirst() instanceof CollapseTopFieldDocs;
         for (TopDocs topDocs : topFieldDocs) {

src/test/java/org/opensearch/neuralsearch/processor/ScoreCombinationTechniqueTests.java

@@ -209,6 +209,60 @@ public void testCombination_whenMultipleSubqueriesWithSortingEnabled_thenScoresN
         }
     }
 
+    public void testCombination_whenSortEnabledAndShardHasNoResults_thenNoException() {
+        // Reproduces https://github.com/opensearch-project/neural-search/issues/1934:
+        // with sort enabled (as search_after requires), a shard that contributed no results past the cursor
+        // arrives at the coordinator with an empty top docs list but a non-zero total hits count (see
+        // CompoundTopDocs constructor, which preserves totalHits while initializing an empty list when
+        // scoreDocs.length < 2). Before the fix, getDocIdSortFieldsMap probed topFieldDocs.getFirst()
+        // unconditionally and threw a bare NoSuchElementException on that empty list.
+        ScoreCombiner scoreCombiner = new ScoreCombiner();
+
+        Object[] sortFields1 = new Object[] { new BytesRef("value1") };
+        Object[] sortFields2 = new Object[] { new BytesRef("value2") };
+        SortField[] sortFields = new SortField[] { new SortField("_id", SortField.Type.STRING, true) };
+        Sort sort = new Sort(sortFields);
+
+        // First shard has results, second shard is exhausted: empty top docs list but total hits > 0.
+        CompoundTopDocs shardWithResults = new CompoundTopDocs(
+            new TotalHits(2, TotalHits.Relation.EQUAL_TO),
+            List.of(
+                new TopFieldDocs(
+                    new TotalHits(2, TotalHits.Relation.EQUAL_TO),
+                    new FieldDoc[] { new FieldDoc(1, 0.5f, sortFields1), new FieldDoc(2, 0.3f, sortFields2) },
+                    sortFields
+                )
+            ),
+            true,
+            SEARCH_SHARD
+        );
+        CompoundTopDocs exhaustedShard = new CompoundTopDocs(
+            new TotalHits(3, TotalHits.Relation.EQUAL_TO),
+            Collections.emptyList(),
+            true,
+            SEARCH_SHARD
+        );
+        final List<CompoundTopDocs> queryTopDocs = List.of(shardWithResults, exhaustedShard);
+
+        // Must not throw NoSuchElementException.
+        scoreCombiner.combineScores(
+            CombineScoresDto.builder()
+                .queryTopDocs(queryTopDocs)
+                .scoreCombinationTechnique(ScoreCombinationFactory.DEFAULT_METHOD)
+                .querySearchResults(Collections.emptyList())
+                .sort(sort)
+                .isSingleShard(false)
+                .build()
+        );
+
+        // The shard with results is combined as usual.
+        assertEquals(2, queryTopDocs.get(0).getScoreDocs().size());
+        // The exhausted shard yields no score docs but keeps its total hit count (search_after semantics:
+        // total_hits is pagination-independent, so a shard past its cursor still reports its full match count).
+        assertTrue(queryTopDocs.get(1).getScoreDocs().isEmpty());
+        assertEquals(3, queryTopDocs.get(1).getTotalHits().value());
+    }
+
     public void testCombination_whenMultipleSubqueriesResultsWithMinScore_thenScoresCombined() {
         ScoreCombiner scoreCombiner = new ScoreCombiner();