SERVER-19835 change SubplanStage to skip creation of a plan cache entry in edge cases

Since the SubplanStage does not benefit from the CachedPlanStage's replanning, it should not create
a cache entry if there is a plan ranking tie or if zero results are produced during the plan ranking
trial period.

2 files changed, 15 insertions, 1 deletions

diff --git a/src/mongo/db/query/plan_ranker.cpp b/src/mongo/db/query/plan_ranker.cpp
index 57717d2427b..b89b9fe3f70 100644
--- a/src/mongo/db/query/plan_ranker.cpp
+++ b/src/mongo/db/query/plan_ranker.cpp
@@ -31,7 +31,7 @@
 #include "mongo/platform/basic.h"
 
 #include <algorithm>
-#include <math.h>
+#include <cmath>
 #include <vector>
 #include <utility>
 
@@ -111,6 +111,14 @@ size_t PlanRanker::pickBestPlan(const vector<CandidatePlan>& candidates, PlanRan
     std::stable_sort(
         scoresAndCandidateindices.begin(), scoresAndCandidateindices.end(), scoreComparator);
 
+    // Determine whether plans tied for the win.
+    if (scoresAndCandidateindices.size() > 1U) {
+        double bestScore = scoresAndCandidateindices[0].first;
+        double runnerUpScore = scoresAndCandidateindices[1].first;
+        const double epsilon = 1e-10;
+        why->tieForBest = std::abs(bestScore - runnerUpScore) < epsilon;
+    }
+
     // Update results in 'why'
     // Stats and scores in 'why' are sorted in descending order by score.
     why->stats.clear();
diff --git a/src/mongo/db/query/plan_ranker.h b/src/mongo/db/query/plan_ranker.h
index 1ae03b083ee..61db71caec2 100644
--- a/src/mongo/db/query/plan_ranker.h
+++ b/src/mongo/db/query/plan_ranker.h
@@ -137,6 +137,12 @@ struct PlanRankingDecision {
     // candidates[candidateOrder[1]] followed by
     // candidates[candidateOrder[2]], ...
     std::vector<size_t> candidateOrder;
+
+    // Whether two plans tied for the win.
+    //
+    // Reading this flag is the only reliable way for callers to determine if there was a tie,
+    // because the scores kept inside the PlanRankingDecision do not incorporate the EOF bonus.
+    bool tieForBest = false;
 };
 
 }  // namespace mongo