MongoDB query response is really slow, when searching for a phone number through 2 documents with each having 10,000 phone numbers

Question

Current MongoDB query, takes upto 5 mins to search through 2 documents, when each document has 10,000 contacts, Please suggest ways to improve this significantly.

I am trying to search for a phone number in hundreds of documents. Each document belongs to a user and each user has a contacts array (as you can see in the below code) with 10,000 objects and each object can have 2 to 3 phone numbers. (See below document structure). If a phone number is found in multiple documents, I need the MongoDB query to return an array with userNumber's found in those documents.

Below is the structure of the document I have in MongoDB collection. For simplicity, I showed only one object in contacts array, infact there are thousands of objects

{
 "_id": { "$oid": "61d1f04266289f003452d705" },
 "userID": { "$oid": "61d1efea2c0fab00340f47c8" },
 "contacts": [
   {
     "emailAddresses": [
       { "id": "6884", "label": "email1", "email": "addedemail@gmail.com" }
     ],
     "phoneNumbers": [
       {
         "label": "other",
         "id": "4594",
         "number": "+918984292930"
       },
       {
         "label": "other",
         "id": "4595",
         "number": "+911234567890"
       }
     ],
     "_id": { "$oid": "61d1f04266289f003452d744" },
     "ContactName": "Sample User 1 Name Changed",
     "ContactNumber": "+918984292930",
     "recordID": "833"
   }
 ],
 "userNumber": "+911234567890",
 "__v": 7
}

Current MongoDB Query:

await ContactModel.aggregate([
       {
         $match: {
           userNumber: userNumber,
         },
       },
       {
         $unwind: "$contacts",
       },
       {
         $lookup: {
           from: "phonenumbers",
           let: {
             contactNumberVar: "$contacts.ContactNumber",
           },
           pipeline: [
             { $unwind: "$contacts" },
             {
               $project: {
                 userNumber: 1,
                 "contacts.ContactNumber": 1,
               },
             },
             {
               $match: {
                 $and: [
                   { $expr: { $eq: ["$$contactNumberVar", "$userNumber"] } },
                   {
                     $expr: {
                       $eq: [contactNumber, "$contacts.ContactNumber"],
                     },
                   },
                 ],
               },
             },
           ],
           as: "mutualContacts",
         },
       },
       {
         $project: {
           userID: 1,
           "mutualContacts.userNumber": 1,
         },
       },
       {
         $group: {
           _id: "$userID",
           mutualContacts: {
             $push: {
               $cond: [
                 { $gt: [{ $size: "$mutualContacts" }, 0] },
                 { $arrayElemAt: ["$mutualContacts.userNumber", 0] },
                 "$$REMOVE",
               ],
             },
           },
         },
       },
     ]).exec()

Answer 1

First of all ensure you have indexes that support the query on both collections.

{userNumber:1}

Should be a good candidate, but please test other options.

Next - query optimisation. In the lookup pipeline:

       pipeline: [
         { $unwind: "$contacts" },
         {
           $project: {
             userNumber: 1,
             "contacts.ContactNumber": 1,
           },
         },
         {
           $match: {
             $and: [
               { $expr: { $eq: ["$$contactNumberVar", "$userNumber"] } },
               {
                 $expr: {
                   $eq: [contactNumber, "$contacts.ContactNumber"],
                 },
               },
             ],
           },
         },
       ],

You unwind whole phonenumbers collection. Match it first and unwind/project only matching documents instead:

       pipeline: [
         {
           $match: {
             $and: [
               { $expr: { $eq: ["$$contactNumberVar", "$userNumber"] } },
               {
                 $expr: {
                   $eq: [contactNumber, "$contacts.ContactNumber"],
                 },
               },
             ],
           },
         },
         { $unwind: "$contacts" },
         {
           $project: {
             userNumber: 1,
             "contacts.ContactNumber": 1,
           },
         },
         {
           $match: {
             $and: [
               { $expr: { $eq: ["$$contactNumberVar", "$userNumber"] } },
               {
                 $expr: {
                   $eq: [contactNumber, "$contacts.ContactNumber"],
                 },
               },
             ],
           },
         },
       ],