SERVER-64315 Re-enable caching of SBE plans when there is a single query solution

This reverts commit f8589f840c8fee60abc482d2d2c41979e356922a.

(cherry picked from commit f462237ac17a9c8a3e4a5a3fb6bbe6a966d4be85)

3 files changed, 10 insertions, 12 deletions

diff --git a/src/mongo/db/query/classic_plan_cache.cpp b/src/mongo/db/query/classic_plan_cache.cpp
index 31ac6d13550..5f33481fef6 100644
--- a/src/mongo/db/query/classic_plan_cache.cpp
+++ b/src/mongo/db/query/classic_plan_cache.cpp
@@ -150,12 +150,12 @@ bool shouldCacheQuery(const CanonicalQuery& query) {
     // 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.
     //
-    // In order to be able to correctly measure 'executionTimeMillis' stats we should only skip
-    // caching top-level plans. Otherwise, if we were to skip caching inner pipelines for $lookup
-    // queries, we could run through the multi-planner for each document coming from the outer side,
-    // completely skewing the 'executionTimeMillis' stats.
+    // There is one exception: $lookup's implementation in the DocumentSource engine relies on
+    // caching the plan on the inner side in order to avoid repeating the planning process for every
+    // document on the outer side. To ensure that the 'executionTimeMillis' value is accurate for
+    // $lookup, we allow the inner side to use the cache even if the query is an explain.
     tassert(6497600, "expCtx is null", query.getExpCtxRaw());
-    if (query.getExplain() && query.getExpCtxRaw()->subPipelineDepth == 0) {
+    if (query.getExplain() && !query.getExpCtxRaw()->inLookup) {
         return false;
     }
 
diff --git a/src/mongo/db/query/get_executor.cpp b/src/mongo/db/query/get_executor.cpp
index 3ea135da726..07b2e440e64 100644
--- a/src/mongo/db/query/get_executor.cpp
+++ b/src/mongo/db/query/get_executor.cpp
@@ -1343,11 +1343,10 @@ StatusWith<std::unique_ptr<PlanExecutor, PlanExecutor::Deleter>> getSlotBasedExe
         roots[0] = helper.buildExecutableTree(*(solutions[0]));
     }
     auto&& [root, data] = roots[0];
-    // TODO SERVER-64315: re-enable caching of single solution plans
-    // if (!planningResult->recoveredPinnedCacheEntry()) {
-    //    plan_cache_util::updatePlanCache(
-    //        opCtx, collections.getMainCollection(), *cq, *solutions[0], *root, data);
-    // }
+    if (!planningResult->recoveredPinnedCacheEntry()) {
+        plan_cache_util::updatePlanCache(
+            opCtx, collections.getMainCollection(), *cq, *solutions[0], *root, data);
+    }
 
     // Prepare the SBE tree for execution.
     stage_builder::prepareSlotBasedExecutableTree(
diff --git a/src/mongo/db/query/sbe_sub_planner.cpp b/src/mongo/db/query/sbe_sub_planner.cpp
index 377c65d2d3e..e5e714ad3aa 100644
--- a/src/mongo/db/query/sbe_sub_planner.cpp
+++ b/src/mongo/db/query/sbe_sub_planner.cpp
@@ -117,8 +117,7 @@ CandidatePlans SubPlanner::plan(
     // TODO SERVER-61507: do it unconditionally when $group pushdown is integrated with the SBE plan
     // cache.
     if (_cq.pipeline().empty()) {
-        // TODO SERVER-64315: re-enable caching of single solution plans
-        // plan_cache_util::updatePlanCache(_opCtx, mainColl, _cq, *compositeSolution, *root, data);
+        plan_cache_util::updatePlanCache(_opCtx, mainColl, _cq, *compositeSolution, *root, data);
     }
 
     return {makeVector(plan_ranker::CandidatePlan{