If there are multiple $match stages in a subpipeline, this prevents join optimization from kicking in:

 db.many_rows.aggregate([{"$lookup":{"from":"many_rows","as":"right","pipeline":[{"$match":{foo: 1}}, {$match: {bar: 1}}]}},{"$unwind":"$right"}]).explain()

The following cases all work and allow join optimization:

• multiple $match stages at the top-level (they are fused)
• $and of two predicates inside a single $match
• two predicates inside a single $match with implicit $and

Could this be to a missing rewrite where $match stages in subpipelines are not fused?