From 745b977ce784aa49e65735767aff73f5760365f2 Mon Sep 17 00:00:00 2001
From: Adrien Grand <jpountz@gmail.com>
Date: Thu, 3 Sep 2015 16:09:58 +0200
Subject: [PATCH] Optimize scrolls for constant-score queries.

We currently optimize scroll when sort=_doc because docs are returned in order.
But documents are also returned in order when sorting by score and the query
gives constant scores. This optimization has the nice side-effect of also
optimizing scrolls with the default `match_all` query.
---
 .../org/elasticsearch/search/query/QueryPhase.java | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/core/src/main/java/org/elasticsearch/search/query/QueryPhase.java b/core/src/main/java/org/elasticsearch/search/query/QueryPhase.java
index a7c022aba28..054d33866c7 100644
--- a/core/src/main/java/org/elasticsearch/search/query/QueryPhase.java
+++ b/core/src/main/java/org/elasticsearch/search/query/QueryPhase.java
@@ -112,6 +112,18 @@ public class QueryPhase implements SearchPhase {
         aggregationPhase.execute(searchContext);
     }
 
+    private static boolean returnsDocsInOrder(Query query, Sort sort) {
+        if (sort == null || Sort.RELEVANCE.equals(sort)) {
+            // sort by score
+            // queries that return constant scores will return docs in index
+            // order since Lucene tie-breaks on the doc id
+            return query.getClass() == ConstantScoreQuery.class
+                    || query.getClass() == MatchAllDocsQuery.class;
+        } else {
+            return Sort.INDEXORDER.equals(sort);
+        }
+    }
+
     /**
      * In a package-private method so that it can be tested without having to
      * wire everything (mapperService, etc.)
@@ -165,7 +177,7 @@ public class QueryPhase implements SearchPhase {
                     numDocs = Math.min(searchContext.size(), totalNumDocs);
                     lastEmittedDoc = scrollContext.lastEmittedDoc;
 
-                    if (Sort.INDEXORDER.equals(searchContext.sort())) {
+                    if (returnsDocsInOrder(query, searchContext.sort())) {
                         if (scrollContext.totalHits == -1) {
                             // first round
                             assert scrollContext.lastEmittedDoc == null;