1 files changed, 9 insertions, 0 deletions
diff --git a/src/mongo/db/query/plan_ranker.cpp b/src/mongo/db/query/plan_ranker.cpp
index e2ac018a67e..01dc6d3cd90 100644
--- a/src/mongo/db/query/plan_ranker.cpp
+++ b/src/mongo/db/query/plan_ranker.cpp
@@ -27,6 +27,7 @@
  */
 
 #include <algorithm>
+#include <math.h>
 #include <vector>
 #include <utility>
 
@@ -107,6 +108,14 @@ namespace mongo {
         std::stable_sort(scoresAndCandidateindices.begin(), scoresAndCandidateindices.end(),
                          scoreComparator);
 
+        // Determine whether plans tied for the win.
+        if (scoresAndCandidateindices.size() > 1) {
+            double bestScore = scoresAndCandidateindices[0].first;
+            double runnerUpScore = scoresAndCandidateindices[1].first;
+            static const double epsilon = 1e-10;
+            why->tieForBest = fabs(bestScore - runnerUpScore) < epsilon;
+        }
+
         // Update results in 'why'
         // Stats and scores in 'why' are sorted in descending order by score.
         why->stats.clear();