Mongo $ in with order indexing

Question

I have a collection following this "schema":

{ _id: ObjectId, order: Number, fieldA: ObjectId, fieldB: Array[ObjectId] } 

And an index defined as follows:

 { fieldA: 1, fieldB: 1, order: 1 } 

When running a search query like this:

 { $and: [ {fieldA: {$in: [{"$oid":"592edae196232608d00f78f5"},{"$oid":"592edadc96232608d00f5614"}]}}, {fieldB: {$in:[{"$oid":"592edace96232608d00ef77f"},{"$oid":"592edacd96232608d00ef34b"}]}} ] } 

with sort defined as

 { order: 1 } 

The query runs fine, the index covers the query , and the explanation plan shows me:

• 4 IXSCANs
• 1 SORT_MERGE
• 1 FETCH

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Problem. If I ask fieldA and / or fieldB with a huge $in (I'm trying to do something like a join), then the indexing doesn't work the same, the query doesn't cover , the sorting is done in memory , and the explanation plan shows me:

• 1 IXSCAN
• 1 FETCH
• 1 SORT_KEY_GENERATOR
• 1 SORT

Even worse, if I run this query from Mongoose, the sort operation goes out of memory: /