SERVER-26113 explain() for aggregation request does not propogate to corresponding 'QueryRequest'

Small change to bug fix, and adding explain.js test.

Formatting changes

Missed formatting

Incorporating CR feedback

2 files changed, 5 insertions, 5 deletions

diff --git a/src/mongo/db/pipeline/pipeline_d.cpp b/src/mongo/db/pipeline/pipeline_d.cpp
index c37c1301198..57cfe0f53d7 100644
--- a/src/mongo/db/pipeline/pipeline_d.cpp
+++ b/src/mongo/db/pipeline/pipeline_d.cpp
@@ -304,6 +304,7 @@ StatusWith<std::unique_ptr<PlanExecutor>> attemptToGetExecutor(
     qr->setProj(projectionObj);
     qr->setSort(sortObj);
     if (aggRequest) {
+        qr->setExplain(static_cast<bool>(aggRequest->getExplain()));
         qr->setHint(aggRequest->getHint());
     }
 
diff --git a/src/mongo/db/query/plan_cache.cpp b/src/mongo/db/query/plan_cache.cpp
index ff8b32f6a6e..b0a80b537ee 100644
--- a/src/mongo/db/query/plan_cache.cpp
+++ b/src/mongo/db/query/plan_cache.cpp
@@ -283,11 +283,10 @@ bool PlanCache::shouldCacheQuery(const CanonicalQuery& query) {
         return false;
     }
 
-    // Explain queries are not-cacheable. This is primarily because of
-    // the need to generate current and accurate information in allPlans.
-    // If the explain report is generated by the cached plan runner using
-    // stale information from the cache for the losing plans, allPlans would
-    // simply be wrong.
+    // We don't read or write from the plan cache for explain. This ensures
+    // that explain queries don't affect cache state, and it also makes
+    // sure that we can always generate information regarding rejected plans
+    // and/or trial period execution of candidate plans.
     if (qr.isExplain()) {
         return false;
     }