MongoDB index not used depending on query formulation

Question

We see in one of our services that certain queries always result in a COLLSCAN, instead of the appropriate index being used. The data has the form

{_id: DocumentID, "field": {"child": "Cross-App-ID"}}

with an Index {"field.child":1}.

Running the query for "field.child", i.e. db.collection.find({"field.child":"CA-ID"}) works as expected with the index being used. However, for some reason the app accessing the database creates the queries as db.collection.find({"field":{"child":"CA-ID"}}), which results in a COLLSCAN instead of an IXSCAN.

2 questions for me from this:

• Shouldn't these queries be equivalent and thus treated the same?
• How can I create an index for the second use-case? All examples I found create the index like we already did.

Answer 1

2 questions for me from this:

• Shouldn't these queries be equivalent and thus treated the same?
• How can I create an index for the second use-case?

The queries return same results, but from indexing point they are considered different.

Index { "field.child": 1 }, works with queries of the shape db.test.find({"field.child": "ABCD" }).

And, the index { field: 1 } works with the query shape db.test.find({field: { child: "ABCD" }}).

You can create both the indexes, and they will be used with their respective queries.