Mongo $in with order indexing

问题

I have a collection following this "schema" :

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

And an index defined like this :

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

When running a find query like this one :

{
  $and: [
    {fieldA: {$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 explain plan shows me :

• 4 IXSCANs
• 1 SORT_MERGE
• 1 FETCH

---

Issue: If I query fieldA and/or fieldB with a huge $in (I'm trying to make something like a join), then the indexing does not act the same, the query is not covered, the sort is made in memory and the explain 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 :/

来源：https://stackoverflow.com/questions/46318304/mongo-in-with-order-indexing