SERVER-40805 Indicate reason for replanning

1 files changed, 13 insertions, 4 deletions

diff --git a/src/mongo/db/exec/cached_plan.cpp b/src/mongo/db/exec/cached_plan.cpp
index 9738936f4b7..7f35e28a63d 100644
--- a/src/mongo/db/exec/cached_plan.cpp
+++ b/src/mongo/db/exec/cached_plan.cpp
@@ -147,7 +147,10 @@ Status CachedPlanStage::pickBestPlan(PlanYieldPolicy* yieldPolicy) {
                    << " status: " << redact(statusObj);
 
             const bool shouldCache = false;
-            return replan(yieldPolicy, shouldCache);
+            return replan(yieldPolicy,
+                          shouldCache,
+                          str::stream() << "cached plan returned: "
+                                        << WorkingSetCommon::toStatusString(statusObj));
         } else {
             invariant(PlanStage::NEED_TIME == state);
         }
@@ -162,7 +165,13 @@ Status CachedPlanStage::pickBestPlan(PlanYieldPolicy* yieldPolicy) {
            << " plan summary before replan: " << Explain::getPlanSummary(child().get());
 
     const bool shouldCache = true;
-    return replan(yieldPolicy, shouldCache);
+    return replan(
+        yieldPolicy,
+        shouldCache,
+        str::stream()
+            << "cached plan was less efficient than expected: expected trial execution to take "
+            << _decisionWorks << " works but it took at least " << maxWorksBeforeReplan
+            << " works");
 }
 
 Status CachedPlanStage::tryYield(PlanYieldPolicy* yieldPolicy) {
@@ -179,7 +188,7 @@ Status CachedPlanStage::tryYield(PlanYieldPolicy* yieldPolicy) {
     return Status::OK();
 }
 
-Status CachedPlanStage::replan(PlanYieldPolicy* yieldPolicy, bool shouldCache) {
+Status CachedPlanStage::replan(PlanYieldPolicy* yieldPolicy, bool shouldCache, std::string reason) {
     // We're going to start over with a new plan. Clear out info from our old plan.
     {
         std::queue<WorkingSetID> emptyQueue;
@@ -188,7 +197,7 @@ Status CachedPlanStage::replan(PlanYieldPolicy* yieldPolicy, bool shouldCache) {
     _ws->clear();
     _children.clear();
 
-    _specificStats.replanned = true;
+    _specificStats.replanReason = std::move(reason);
 
     if (shouldCache) {
         // Deactivate the current cache entry.