SERVER-22407 make COUNT_SCAN return RID_AND_OBJ instead of OWNED_OBJ

Fixes most of a 7% perf regression for COUNT_SCAN plans.

2 files changed, 4 insertions, 5 deletions

diff --git a/src/mongo/db/exec/count_scan.cpp b/src/mongo/db/exec/count_scan.cpp
index e65e7977c55..e460854542a 100644
--- a/src/mongo/db/exec/count_scan.cpp
+++ b/src/mongo/db/exec/count_scan.cpp
@@ -106,9 +106,7 @@ PlanStage::StageState CountScan::doWork(WorkingSetID* out) {
     }
 
     WorkingSetID id = _workingSet->allocate();
-    WorkingSetMember* member = _workingSet->get(id);
-    member->obj = Snapshotted<BSONObj>(SnapshotId(), BSONObj().getOwned());
-    member->transitionToOwnedObj();
+    _workingSet->transitionToRecordIdAndObj(id);
     *out = id;
     return PlanStage::ADVANCED;
 }
diff --git a/src/mongo/db/exec/count_scan.h b/src/mongo/db/exec/count_scan.h
index 7d40003fc4d..f53c5e3ab0f 100644
--- a/src/mongo/db/exec/count_scan.h
+++ b/src/mongo/db/exec/count_scan.h
@@ -58,8 +58,9 @@ struct CountScanParams {
 
 /**
  * Used by the count command. Scans an index from a start key to an end key. Creates a
- * WorkingSetMember for each matching index key in OWNED_OBJ state. It has an owned empty object
- * with a null snapshot id rather than real data, since all we need is the count.
+ * WorkingSetMember for each matching index key in RID_AND_OBJ state. It has a null record id and an
+ * empty object with a null snapshot id rather than real data. Returning real data is unnecessary
+ * since all we need is the count.
  *
  * Only created through the getExecutorCount() path, as count is the only operation that doesn't
  * care about its data.