Does MongoDB track how many times each index is used in a query?

Yes, MongoDB tracks index usage statistics through the $indexStats aggregation stage. This allows you to monitor how frequently each index is accessed, helping optimize database performance by identifying unused or heavily used indexes.

Syntax

db.collection.aggregate([
    { $indexStats: {} }
]);

Create Sample Data

First, let's create an index on the FirstName field ?

db.demo508.createIndex({"FirstName": 1});

{
    "createdCollectionAutomatically": true,
    "numIndexesBefore": 1,
    "numIndexesAfter": 2,
    "ok": 1
}

Now insert sample documents ?

db.demo508.insertMany([
    {"FirstName": "John"},
    {"FirstName": "Chris"},
    {"FirstName": "David"}
]);

{
    "acknowledged": true,
    "insertedIds": [
        ObjectId("5e883818987b6e0e9d18f578"),
        ObjectId("5e88381b987b6e0e9d18f579"),
        ObjectId("5e88381f987b6e0e9d18f57a")
    ]
}

View Collection Data

db.demo508.find();

{ "_id": ObjectId("5e883818987b6e0e9d18f578"), "FirstName": "John" }
{ "_id": ObjectId("5e88381b987b6e0e9d18f579"), "FirstName": "Chris" }
{ "_id": ObjectId("5e88381f987b6e0e9d18f57a"), "FirstName": "David" }

Track Index Usage Statistics

Use $indexStats to see how many times each index has been accessed ?

db.demo508.aggregate([
    { $indexStats: {} }
]);

{
    "name": "_id_",
    "key": {
        "_id": 1
    },
    "host": "DESKTOP-QN2RB3H:27017",
    "accesses": {
        "ops": NumberLong(0),
        "since": ISODate("2020-04-04T07:32:27.394Z")
    }
}
{
    "name": "FirstName_1",
    "key": {
        "FirstName": 1
    },
    "host": "DESKTOP-QN2RB3H:27017",
    "accesses": {
        "ops": NumberLong(0),
        "since": ISODate("2020-04-04T07:32:27.527Z")
    }
}

Key Points

• ops: Number of operations that used this index
• since: Timestamp when tracking started (server restart or index creation)
• name: Index name (auto-generated or custom)
• Statistics reset when the server restarts or index is rebuilt

Conclusion

The $indexStats aggregation stage provides valuable insights into index usage patterns. Monitor these statistics regularly to identify unused indexes for removal and heavily used indexes that may need optimization.