LUCENE-6754: Optimized IndexSearcher.count for simple queries.

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1700791 13f79535-47bb-0310-9956-ffa450edef68

lucene/CHANGES.txt

@@ -83,6 +83,9 @@ Optimizations
 * LUCENE-6746: DisjunctionMaxQuery, BoostingQuery and BoostedQuery now create
   sub weights through IndexSearcher so that they can be cached. (Adrien Grand)
 
+* LUCENE-6754: Optimized IndexSearcher.count for the cases when it can use
+  index statistics instead of collecting all matches. (Adrien Grand)
+
 Bug Fixes
 
 * LUCENE-6730: Hyper-parameter c is ignored in term frequency NormalizationH1.

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

@@ -339,6 +339,29 @@ public class IndexSearcher {
    * Count how many documents match the given query.
    */
   public int count(Query query) throws IOException {
+    query = rewrite(query);
+    while (true) {
+      // remove wrappers that don't matter for counts
+      if (query instanceof ConstantScoreQuery) {
+        query = ((ConstantScoreQuery) query).getQuery();
+      } else {
+        break;
+      }
+    }
+
+    // some counts can be computed in constant time
+    if (query instanceof MatchAllDocsQuery) {
+      return reader.numDocs();
+    } else if (query instanceof TermQuery && reader.hasDeletions() == false) {
+      Term term = ((TermQuery) query).getTerm();
+      int count = 0;
+      for (LeafReaderContext leaf : reader.leaves()) {
+        count += leaf.reader().docFreq(term);
+      }
+      return count;
+    }
+
+    // general case: create a collecor and count matches
     final CollectorManager<TotalHitCountCollector, Integer> collectorManager = new CollectorManager<TotalHitCountCollector, Integer>() {
 
       @Override

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

@@ -18,6 +18,7 @@ package org.apache.lucene.search;
  */
 
 import java.io.IOException;
+import java.util.Arrays;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.LinkedBlockingQueue;
 import java.util.concurrent.ThreadPoolExecutor;
@@ -31,6 +32,7 @@ import org.apache.lucene.document.StringField;
 import org.apache.lucene.index.IndexReader;
 import org.apache.lucene.index.RandomIndexWriter;
 import org.apache.lucene.index.Term;
+import org.apache.lucene.search.BooleanClause.Occur;
 import org.apache.lucene.store.Directory;
 import org.apache.lucene.util.BytesRef;
 import org.apache.lucene.util.IOUtils;
@@ -144,15 +146,36 @@ public class TestIndexSearcher extends LuceneTestCase {
       if (random().nextBoolean()) {
         doc.add(new StringField("foo", "bar", Store.NO));
       }
+      if (random().nextBoolean()) {
+        doc.add(new StringField("foo", "baz", Store.NO));
+      }
+      if (rarely()) {
+        doc.add(new StringField("delete", "yes", Store.NO));
+      }
       w.addDocument(doc);
     }
-    w.commit();
+    for (boolean delete : new boolean[] {false, true}) {
+      if (delete) {
+        w.deleteDocuments(new Term("delete", "yes"));
+      }
       final IndexReader reader = w.getReader();
-    w.close();
       final IndexSearcher searcher = newSearcher(reader);
-    final Query query = new TermQuery(new Term("foo", "bar"));
+      // Test multiple queries, some of them are optimized by IndexSearcher.count()
+      for (Query query : Arrays.asList(
+          new MatchAllDocsQuery(),
+          new MatchNoDocsQuery(),
+          new TermQuery(new Term("foo", "bar")),
+          new ConstantScoreQuery(new TermQuery(new Term("foo", "baz"))),
+          new BooleanQuery.Builder()
+            .add(new TermQuery(new Term("foo", "bar")), Occur.SHOULD)
+            .add(new TermQuery(new Term("foo", "baz")), Occur.SHOULD)
+            .build()
+          )) {
         assertEquals(searcher.count(query), searcher.search(query, 1).totalHits);
+      }
       reader.close();
+    }
+    w.close();
     dir.close();
   }
 

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

@@ -971,7 +971,7 @@ public class TestLRUQueryCache extends LuceneTestCase {
     
     try {
       // trigger an eviction
-      searcher.count(new MatchAllDocsQuery());
+      searcher.search(new MatchAllDocsQuery(), new TotalHitCountCollector());
       fail();
     } catch (ConcurrentModificationException e) {
       // expected