MongoDB Performance Issue with arrays

Question

I'm looking for some advice on how to improve the performance of my query.

I have this user model in mongoose and I'm indexing interested_cities .

 const firebaseToken = new Schema({ type: { type: String, default: "android", required: true, trim: true, }, device_id: { type: String, required: true, trim: true, }, fcm_token: { type: String, required: true, trim: true, }, }); const userSchema = new Schema({ name: { type: String, required: true, trim: true, }, interested_cities: { type: [{ type: String, trim: true, lowercase: true, unique: true }], required: false, default: [], }, push_notification_tokens_firebase: { type: [firebaseToken], required: false, default: [], }, }); userSchema.index({ interested_cities: 1 });

What I'm looking for is to query the users who have 'A' or 'B' in their array of interested_cities .

I'm querying something like this. I only need the firebase fcm_token from the query.

 const involvedUsers = await User.find( { $or: [ { interested_cities: { $in: ['A', 'B'] } }, { phone_number: { $in: adminPhoneNumbersList } }, ], }, { _id: 1, "push_notification_tokens_firebase.fcm_token": 1, } );

Currently, the query is taking 20 sec for 14k documents, which needs improvements. Any pointers would be appreciated.

Explain:

 { "explainVersion": "1", "queryPlanner": { "namespace": "production.users", "indexFilterSet": false, "parsedQuery": { "interested_cities": { "$in": [ "A", "B" ] } }, "maxIndexedOrSolutionsReached": false, "maxIndexedAndSolutionsReached": false, "maxScansToExplodeReached": false, "winningPlan": { "stage": "PROJECTION_DEFAULT", "transformBy": { "_id": 1, "push_notification_tokens_firebase.fcm_token": 1 }, "inputStage": { "stage": "FETCH", "inputStage": { "stage": "IXSCAN", "keyPattern": { "interested_cities": 1 }, "indexName": "interested_cities_1", "isMultiKey": true, "multiKeyPaths": { "interested_cities": [ "interested_cities" ] }, "isUnique": false, "isSparse": false, "isPartial": false, "indexVersion": 2, "direction": "forward", "indexBounds": { "interested_cities": [ "[\"A\", \"A\"]", "[\"B\", \"B\"]" ] } } } }, "rejectedPlans": [] }

 "executionStats": { "executionSuccess": true, "nReturned": 6497, "executionTimeMillis": 48, "totalKeysExamined": 0, "totalDocsExamined": 14827, "executionStages": { "stage": "SUBPLAN", "nReturned": 6497, "executionTimeMillisEstimate": 46, "works": 14829, "advanced": 6497, "needTime": 8331, "needYield": 0, "saveState": 14, "restoreState": 14, "isEOF": 1, "inputStage": { "stage": "PROJECTION_DEFAULT", "nReturned": 6497, "executionTimeMillisEstimate": 46, "works": 14829, "advanced": 6497, "needTime": 8331, "needYield": 0, "saveState": 14, "restoreState": 14, "isEOF": 1, "transformBy": { "_id": 1, "push_notification_tokens_firebase.fcm_token": 1 }, "inputStage": { "stage": "COLLSCAN", "filter": { "$or": [ { "interested_cities": { "$in": [ "A", "B" ] } }, { "phone_number": { "$in": [ "phone numbers", "phone number" ] } } ] }, "nReturned": 6497, "executionTimeMillisEstimate": 41, "works": 14829, "advanced": 6497, "needTime": 8331, "needYield": 0, "saveState": 14, "restoreState": 14, "isEOF": 1, "direction": "forward", "docsExamined": 14827 } } }, "allPlansExecution": [] }

Answer 1

Mongoose optimization:

By default, Mongoose queries return an instance of the Mongoose Document class. Documents are much heavier than vanilla JavaScript objects, because they have a lot of internal state for change tracking. Enabling the lean option tells Mongoose to skip instantiating a full Mongoose document and just give you the POJO.

https://mongoosejs.com/docs/tutorials/lean.html#using-lean

You can disable this behaviour on per-query basis by appending .lean() at the end. If your query is returning "a lot" of documents, this can really improve your speed. You should read more about lean() from the link above.

---

Query optimization:

When evaluating the clauses in the $or expression, MongoDB either performs a collection scan or, if all the clauses are supported by indexes, MongoDB performs index scans. That is, for MongoDB to use indexes to evaluate an $or expression, all the clauses in the $or expression must be supported by indexes. Otherwise, MongoDB will perform a collection scan.

https://www.mongodb.com/docs/manual/reference/operator/query/or/#-or-clauses-and-indexes

Query you shared looks like this:

const involvedUsers = await User.find({
  $or: [
    { interested_cities: { $in: citiesArr } },
    { phone_number: { $in: phonesArr } },
  ],
}, { _id: 1, "push_notification_tokens_firebase.fcm_token": 1 });

Based on the info above, you need to create following two indexes:

userSchema.index({ interested_cities: 1 });
userSchema.index({ phone_number: 1 });

This way, mongo will be able to "know" which documents are relevant, find them on disk, extract your projection ("_id" and "push_notification_tokens_firebase.fcm_token") and return it.

One step further in the optimization would be to create following indexes instead of the ones above:

userSchema.index({ interested_cities: 1, _id: 1, "push_notification_tokens_firebase.fcm_token": 1 });
userSchema.index({ phone_number: 1, _id: 1, "push_notification_tokens_firebase.fcm_token": 1 });

This way, mongo will have all the info it needs to fulfil your query from the indexes, meaning it will never even access the disk to fetch a document.

You can confirm this by running <your-query>.explain('executionStats') and confirming that totalDocsExamined is 0 .

Read more about executionStats here: https://www.mongodb.com/docs/manual/reference/explain-results/#mongodb-data-explain.executionStats

I hope this helps!