Don't compute numDocs when size is set to 0 while creating query phase collector manager (#112488)

A simple call to `IndexReader#numDocs` can make us go through all docs and execute scripts in case
there are runtime fields involved in the search. This is necessary only when size is greater than 0,
otherwise it can be avoided. This commit moves the call to the two specific if branches in place
of computing numDocs before the conditional

Author: javanna

server/src/main/java/org/elasticsearch/search/query/QueryPhaseCollectorManager.java

@@ -220,8 +220,6 @@ static CollectorManager<Collector, QueryPhaseResult> createQueryPhaseCollectorMa
         );
         final IndexReader reader = searchContext.searcher().getIndexReader();
         final Query query = searchContext.rewrittenQuery();
-        // top collectors don't like a size of 0
-        final int totalNumDocs = Math.max(1, reader.numDocs());
         if (searchContext.size() == 0) {
             return new EmptyHits(
                 postFilterWeight,
@@ -232,31 +230,10 @@ static CollectorManager<Collector, QueryPhaseResult> createQueryPhaseCollectorMa
                 searchContext.sort(),
                 searchContext.trackTotalHitsUpTo()
             );
-        } else if (searchContext.scrollContext() != null) {
-            // we can disable the tracking of total hits after the initial scroll query
-            // since the total hits is preserved in the scroll context.
-            int trackTotalHitsUpTo = searchContext.scrollContext().totalHits != null
-                ? SearchContext.TRACK_TOTAL_HITS_DISABLED
-                : SearchContext.TRACK_TOTAL_HITS_ACCURATE;
-            // no matter what the value of from is
-            int numDocs = Math.min(searchContext.size(), totalNumDocs);
-            return forScroll(
-                postFilterWeight,
-                terminateAfterChecker,
-                aggsCollectorManager,
-                searchContext.minimumScore(),
-                searchContext.getProfilers() != null,
-                reader,
-                query,
-                searchContext.sort(),
-                numDocs,
-                searchContext.trackScores(),
-                trackTotalHitsUpTo,
-                hasFilterCollector,
-                searchContext.scrollContext(),
-                searchContext.numberOfShards()
-            );
-        } else {
+        }
+        // top collectors don't like a size of 0
+        final int totalNumDocs = Math.max(1, reader.numDocs());
+        if (searchContext.scrollContext() == null) {
             int numDocs = Math.min(searchContext.from() + searchContext.size(), totalNumDocs);
             final boolean rescore = searchContext.rescore().isEmpty() == false;
             if (rescore) {
@@ -265,38 +242,61 @@ static CollectorManager<Collector, QueryPhaseResult> createQueryPhaseCollectorMa
                     numDocs = Math.max(numDocs, rescoreContext.getWindowSize());
                 }
             }
-            if (searchContext.collapse() != null) {
-                boolean trackScores = searchContext.sort() == null || searchContext.trackScores();
-                return forCollapsing(
+            if (searchContext.collapse() == null) {
+                return new WithHits(
                     postFilterWeight,
                     terminateAfterChecker,
                     aggsCollectorManager,
                     searchContext.minimumScore(),
                     searchContext.getProfilers() != null,
-                    searchContext.collapse(),
+                    reader,
+                    query,
                     searchContext.sort(),
+                    searchContext.searchAfter(),
                     numDocs,
-                    trackScores,
-                    searchContext.searchAfter()
+                    searchContext.trackScores(),
+                    searchContext.trackTotalHitsUpTo(),
+                    hasFilterCollector
                 );
             } else {
-                return new WithHits(
+                boolean trackScores = searchContext.sort() == null || searchContext.trackScores();
+                return forCollapsing(
                     postFilterWeight,
                     terminateAfterChecker,
                     aggsCollectorManager,
                     searchContext.minimumScore(),
                     searchContext.getProfilers() != null,
-                    reader,
-                    query,
+                    searchContext.collapse(),
                     searchContext.sort(),
-                    searchContext.searchAfter(),
                     numDocs,
-                    searchContext.trackScores(),
-                    searchContext.trackTotalHitsUpTo(),
-                    hasFilterCollector
+                    trackScores,
+                    searchContext.searchAfter()
                 );
             }
         }
+        // we can disable the tracking of total hits after the initial scroll query
+        // since the total hits is preserved in the scroll context.
+        int trackTotalHitsUpTo = searchContext.scrollContext().totalHits != null
+            ? SearchContext.TRACK_TOTAL_HITS_DISABLED
+            : SearchContext.TRACK_TOTAL_HITS_ACCURATE;
+        // no matter what the value of from is
+        int numDocs = Math.min(searchContext.size(), totalNumDocs);
+        return forScroll(
+            postFilterWeight,
+            terminateAfterChecker,
+            aggsCollectorManager,
+            searchContext.minimumScore(),
+            searchContext.getProfilers() != null,
+            reader,
+            query,
+            searchContext.sort(),
+            numDocs,
+            searchContext.trackScores(),
+            trackTotalHitsUpTo,
+            hasFilterCollector,
+            searchContext.scrollContext(),
+            searchContext.numberOfShards()
+        );
     }
 
     /**