Avoid unnecessary DisjunctionMaxBulkScorer overhead

lucene/CHANGES.txt

@@ -342,6 +342,9 @@ Optimizations
 * GITHUB#15607: Utilize bulk scoring for diversity checking when building HNSW vector indices. This results
   in some performance improvements during indexing and segment merges. (Ben Trent)
 
+* GITHUB#15658: Avoid DisjunctionMaxQuery bulk scoring regressions when all clauses only
+  provide DefaultBulkScorer. (Shimpei Kodama)
+
 Bug Fixes
 ---------------------
 * GITHUB#14161: PointInSetQuery's constructor now throws IllegalArgumentException

lucene/core/src/java/org/apache/lucene/search/DisjunctionMaxQuery.java

@@ -160,14 +160,28 @@ public Scorer get(long leadCost) throws IOException {
 
           @Override
           public BulkScorer bulkScorer() throws IOException {
-            if (tieBreakerMultiplier == 0f && scoreMode == ScoreMode.TOP_SCORES) {
-              List<BulkScorer> scorers = new ArrayList<>();
-              for (ScorerSupplier ss : scorerSuppliers) {
-                scorers.add(ss.bulkScorer());
+            if (tieBreakerMultiplier != 0f || scoreMode != ScoreMode.TOP_SCORES) {
+              return super.bulkScorer();
+            }
+
+            List<BulkScorer> bulkScorers = new ArrayList<>(scorerSuppliers.size());
+            for (ScorerSupplier ss : scorerSuppliers) {
+              bulkScorers.add(ss.bulkScorer());
+            }
+
+            // If all are DefaultBulkScorer, combine the underlying scorers directly.
+            // This avoids the overhead of DisjunctionMaxBulkScorer.
+            List<Scorer> scorers = new ArrayList<>(bulkScorers.size());
+            for (BulkScorer bs : bulkScorers) {
+              if (bs instanceof Weight.DefaultBulkScorer dbs) {
+                scorers.add(dbs.getScorer());
+              } else {
+                return new DisjunctionMaxBulkScorer(bulkScorers);
               }
-              return new DisjunctionMaxBulkScorer(scorers);
             }
-            return super.bulkScorer();
+
+            return new Weight.DefaultBulkScorer(
+                new DisjunctionMaxScorer(tieBreakerMultiplier, scorers, scoreMode, Long.MAX_VALUE));
           }
 
           @Override

lucene/core/src/java/org/apache/lucene/search/Weight.java

@@ -243,6 +243,10 @@ public DefaultBulkScorer(Scorer scorer) {
       }
     }
 
+    Scorer getScorer() {
+      return scorer;
+    }
+
     @Override
     public long cost() {
       return iterator.cost();