SERVER-36723 Push $limit beneath DocumentSourceCursor into the PlanStage layer.

In addition towards working towards the general goal of
doing as much query execution as possible with a PlanStage
tree, this should have a positive performance impact for
certain agg pipelines. Previously, a pipeline with a
$project (or a $project-like stage such as $addFields)
followed by a $limit might have applied this limit only
after a full batch of data was loaded by
DocumentSourceCursor. After this change, the limit will take
effect prior to DocumentSourceCursor batching, and thus may
reduce the amount of data processed by the query.

1 files changed, 2 insertions, 5 deletions

diff --git a/src/mongo/s/query/cluster_aggregation_planner.cpp b/src/mongo/s/query/cluster_aggregation_planner.cpp
index 1c03e375e00..19e24235923 100644
--- a/src/mongo/s/query/cluster_aggregation_planner.cpp
+++ b/src/mongo/s/query/cluster_aggregation_planner.cpp
@@ -128,15 +128,12 @@ boost::optional<long long> getPipelineLimit(Pipeline* pipeline) {
 
         auto sortStage = dynamic_cast<DocumentSourceSort*>(source);
         if (sortStage) {
-            return (sortStage->getLimit() >= 0) ? boost::optional<long long>(sortStage->getLimit())
-                                                : boost::none;
+            return sortStage->getLimit();
         }
 
         auto cursorStage = dynamic_cast<DocumentSourceSort*>(source);
         if (cursorStage) {
-            return (cursorStage->getLimit() >= 0)
-                ? boost::optional<long long>(cursorStage->getLimit())
-                : boost::none;
+            return cursorStage->getLimit();
         }
 
         // If this stage is one that can swap with a $limit stage, then we can look at the previous