1 files changed, 17 insertions, 5 deletions
diff --git a/src/mongo/db/query/explain.h b/src/mongo/db/query/explain.h
index d41dd3a1725..1dcabdeb7e3 100644
--- a/src/mongo/db/query/explain.h
+++ b/src/mongo/db/query/explain.h
@@ -39,6 +39,7 @@ namespace mongo {
 
 class Collection;
 class CollectionPtr;
+class MultipleCollectionAccessor;
 class OperationContext;
 class PlanExecutorPipeline;
 struct PlanSummaryStats;
@@ -77,15 +78,26 @@ public:
                               BSONObj extraInfo,
                               const BSONObj& command,
                               BSONObjBuilder* out);
+
+    /**
+     * Similar to the above function, but takes in multiple collections instead to support
+     * aggregation that involves multiple collections (e.g. $lookup).
+     */
+    static void explainStages(PlanExecutor* exec,
+                              const MultipleCollectionAccessor& collections,
+                              ExplainOptions::Verbosity verbosity,
+                              BSONObj extraInfo,
+                              const BSONObj& command,
+                              BSONObjBuilder* out);
+
     /**
      * Adds "queryPlanner" and "executionStats" (if requested in verbosity) fields to 'out'. Unlike
      * the other overload of explainStages() above, this one does not add the "serverInfo" section.
      *
      * - 'exec' is the stage tree for the operation being explained.
-     * - 'collection' is the relevant collection. During this call it may be required to execute the
-     * plan to collect statistics. If the PlanExecutor uses 'kLockExternally' lock policy, the
-     * caller should hold at least an IS lock on the collection the that the query runs on, even if
-     * 'collection' parameter is nullptr.
+     * - 'collections' are the relevant main and secondary collections (e.g. for $lookup). If the
+     * PlanExecutor uses 'kLockExternally' lock policy, the caller should hold the necessary db_raii
+     * object on the involved collections.
      * - 'verbosity' is the verbosity level of the explain.
      * - 'extraInfo' specifies additional information to include into the output.
      * - 'executePlanStatus' is the status returned after executing the query (Status::OK if the
@@ -97,7 +109,7 @@ public:
      */
     static void explainStages(
         PlanExecutor* exec,
-        const CollectionPtr& collection,
+        const MultipleCollectionAccessor& collections,
         ExplainOptions::Verbosity verbosity,
         Status executePlanStatus,
         boost::optional<PlanExplainer::PlanStatsDetails> winningPlanTrialStats,