Currently $group SBE stage builder (SlotBasedStageBuilder::buildGroup) does not support object form of the '_id' group-by expression like _id: {a: "$a", b: "$b"}. Two ideas were proposed so far.

1) One idea is to pass through ExpressionObject from DocumentSourceGroup to GroupNode QSN to SlotBasedStageBuilder::buildGroup and implement the expression walker for ExpressionObject.

• We would get $object implementation almost for free too.
• The group stage's group key becomes a full document with field names which would be less performing compared to multiple group-by keys just like the second idea.
• We would get the same behavior as the classic engine for generated _id documents.
• We need to modify DocumentSourceGroup and GroupNode. There would be chaining changes.

2) Another idea is to insert a mkbson stage manually to compose a _id document out of multiple group-by key expressions before returning a result document.

• We don't need to modify DocumentSourceGroup and GroupNode. We just need to add some logic to SlotBasedStageBuilder::buildGroup.
• The group stage's group key becomes multiple group-by key without field names which would be better performing compared to the first idea.
• We would not get the same behavior as the classic engine since passed-through group-by key expressions do not follow the original order in _id document specification.

Maybe we can follow a hybrid approach to get the best out of two approaches above though code changes would be bigger.

1. Pass through ExpressionObject from DocumentSourceGroup to GroupNode QSN to SlotBasedStageBuilder::buildGroup and implement the expression walker for ExpressionObject.
2. Extract slots and expressions for _id document fields from the generated SBE PlanStage tree from ExpressionObject to pass them to makeHashAgg inside SlotBasedStageBuilder::buildGroup.

Pros:

• We would get the same behavior as the classic engine for generated _id documents.
• We would get the better performance for group stage.
• We would get $object implementation almost for free too.

Cons:

• Code changes would be bigger like more changes to SlotBasedStageBuilder::buildGroup and DocumentSourceGroup and GroupNode and chaining changes.