From af2b903f65e4451838fb3e93511329acec30a2a1 Mon Sep 17 00:00:00 2001
From: David Smiley <dsmiley@apache.org>
Date: Wed, 15 Nov 2017 08:57:47 -0500
Subject: [PATCH] LUCENE-8040: optimize IndexSearcher.collectionStatistics

---
 lucene/CHANGES.txt                            |  6 +++++
 .../apache/lucene/search/IndexSearcher.java   | 26 +++++++++----------
 2 files changed, 19 insertions(+), 13 deletions(-)

diff --git a/lucene/CHANGES.txt b/lucene/CHANGES.txt
index 784677e01cd..ff838dd9769 100644
--- a/lucene/CHANGES.txt
+++ b/lucene/CHANGES.txt
@@ -41,6 +41,12 @@ Improvements
   Add missing range checks for similarity parameters.
   Improve BM25 and ClassicSimilarity's explanations. (Robert Muir)
 
+Optimizations
+
+* LUCENE-8040: Optimize IndexSearcher.collectionStatistics, avoiding MultiFields/MultiTerms
+  (David Smiley, Robert Muir)
+
+
 ======================= Lucene 7.2.0 =======================
 
 API Changes
diff --git a/lucene/core/src/java/org/apache/lucene/search/IndexSearcher.java b/lucene/core/src/java/org/apache/lucene/search/IndexSearcher.java
index e2283eadff4..e35e70bbd2d 100644
--- a/lucene/core/src/java/org/apache/lucene/search/IndexSearcher.java
+++ b/lucene/core/src/java/org/apache/lucene/search/IndexSearcher.java
@@ -37,7 +37,6 @@ import org.apache.lucene.index.IndexReader;
 import org.apache.lucene.index.IndexReaderContext;
 import org.apache.lucene.index.IndexWriter;
 import org.apache.lucene.index.LeafReaderContext;
-import org.apache.lucene.index.MultiFields;
 import org.apache.lucene.index.ReaderUtil;
 import org.apache.lucene.index.StoredFieldVisitor;
 import org.apache.lucene.index.Term;
@@ -778,21 +777,22 @@ public class IndexSearcher {
    * @lucene.experimental
    */
   public CollectionStatistics collectionStatistics(String field) throws IOException {
-    final int docCount;
-    final long sumTotalTermFreq;
-    final long sumDocFreq;
-
     assert field != null;
-    
-    Terms terms = MultiFields.getTerms(reader, field);
-    if (terms == null) {
+    long docCount = 0;
+    long sumTotalTermFreq = 0;
+    long sumDocFreq = 0;
+    for (LeafReaderContext leaf : reader.leaves()) {
+      final Terms terms = leaf.reader().terms(field);
+      if (terms == null) {
+        continue;
+      }
+      docCount += terms.getDocCount();
+      sumTotalTermFreq += terms.getSumTotalTermFreq();
+      sumDocFreq += terms.getSumDocFreq();
+    }
+    if (docCount == 0) {
       return null;
     }
-
-    docCount = terms.getDocCount();
-    sumTotalTermFreq = terms.getSumTotalTermFreq();
-    sumDocFreq = terms.getSumDocFreq();
-
     return new CollectionStatistics(field, reader.maxDoc(), docCount, sumTotalTermFreq, sumDocFreq);
   }
 }