SERVER-38311 Change out merging strategy

Allows an $out stage to run in parallel on all shards if the target
collection is sharded and so is the input collection to the aggregate.

1 files changed, 3 insertions, 6 deletions

diff --git a/src/mongo/db/pipeline/document_source_group.cpp b/src/mongo/db/pipeline/document_source_group.cpp
index 318a67df1ff..dee7cd104d4 100644
--- a/src/mongo/db/pipeline/document_source_group.cpp
+++ b/src/mongo/db/pipeline/document_source_group.cpp
@@ -950,11 +950,7 @@ Document DocumentSourceGroup::makeDocument(const Value& id,
     return out.freeze();
 }
 
-intrusive_ptr<DocumentSource> DocumentSourceGroup::getShardSource() {
-    return this;  // No modifications necessary when on shard
-}
-
-NeedsMergerDocumentSource::MergingLogic DocumentSourceGroup::mergingLogic() {
+boost::optional<DocumentSource::MergingLogic> DocumentSourceGroup::mergingLogic() {
     intrusive_ptr<DocumentSourceGroup> mergingGroup(new DocumentSourceGroup(pExpCtx));
     mergingGroup->setDoingMerge(true);
 
@@ -973,7 +969,8 @@ NeedsMergerDocumentSource::MergingLogic DocumentSourceGroup::mergingLogic() {
         mergingGroup->addAccumulator(copiedAccumuledField);
     }
 
-    return {mergingGroup};
+    // {shardsStage, mergingStage, sortPattern}
+    return MergingLogic{this, mergingGroup, boost::none};
 }
 
 bool DocumentSourceGroup::pathIncludedInGroupKeys(const std::string& dottedPath) const {