Hello team,

I was using $lookup in an aggregation query, but using

{ explain: true }

at the end of the aggregate() method, I was having trouble getting index stats for the foreign collection i.e. the collection in the "from" part of the $lookup. The index stats for the local collection were still available using

{ explain: true }

.

However, when I ran it like explain("allPlansExecution").aggregate(...), the explain() output had index usage stats for the foreign collection as well.

For reference, you can't run a query like .aggregate(...).explain() in the Mongo shell, so you need to put explain() before aggregate() like explain(...).aggregate(...).

I had some trouble figuring out how to find the index stats for $lookup because of the difference in results between

{ explain: true }

and .explain(), so I would really appreciate if this was documented somewhere. I think the customers would appreciate it too.

Thanks,
Matt