LUCENE-8607: MatchAllDocsQuery can skip counting hits

lucene/CHANGES.txt

@@ -220,6 +220,9 @@ Optimizations
   to early terminate the iterator if the minimum score is greater than the constant
   score. (Christophe Bismuth via Jim Ferenczi)
 
+* LUCENE-8607: MatchAllDocsQuery can shortcut when total hit count is not
+  required (Alan Woodward, Adrien Grand)
+
 ======================= Lucene 7.7.0 =======================
 
 Build

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

@@ -24,7 +24,6 @@ import org.apache.lucene.util.Bits;
 
 /**
  * A query that matches all documents.
- *
  */
 public final class MatchAllDocsQuery extends Query {
 
@@ -47,6 +46,9 @@ public final class MatchAllDocsQuery extends Query {
 
       @Override
       public BulkScorer bulkScorer(LeafReaderContext context) throws IOException {
+        if (scoreMode == ScoreMode.TOP_SCORES) {
+          return super.bulkScorer(context);
+        }
         final float score = score();
         final int maxDoc = context.reader().maxDoc();
         return new BulkScorer() {

lucene/core/src/test/org/apache/lucene/search/TestMatchAllDocsQuery.java

@@ -99,4 +99,27 @@ public class TestMatchAllDocsQuery extends LuceneTestCase {
     iw.addDocument(doc);
   }
 
+  public void testEarlyTermination() throws IOException {
+
+    Directory dir = newDirectory();
+    IndexWriter iw = new IndexWriter(dir, newIndexWriterConfig(analyzer).setMaxBufferedDocs(2).setMergePolicy(newLogMergePolicy()));
+    for (int i = 0; i < 500; i++) {
+      addDoc("doc" + i, iw);
+    }
+    IndexReader ir = DirectoryReader.open(iw);
+
+    IndexSearcher is = newSearcher(ir);
+
+    final int totalHitsThreshold = 200;
+    TopScoreDocCollector c = TopScoreDocCollector.create(10, null, totalHitsThreshold);
+
+    is.search(new MatchAllDocsQuery(), c);
+    assertEquals(totalHitsThreshold, c.totalHits);
+
+    iw.close();
+    ir.close();
+    dir.close();
+
+  }
+
 }