How to improve MongoDB queries with multikey index in array?

To improve MongoDB queries with multikey index in array fields, use $elemMatch operator combined with proper compound indexes on array elements. This ensures efficient querying of nested objects within arrays.

Syntax

db.collection.find({
    "arrayField": {
        $elemMatch: {
            "field1": "value1",
            "field2": "value2"
        }
    }
});

Sample Data

db.demo444.insertMany([
    {
        "Information": [{
            id: 1,
            Name: "Chris"
        }]
    },
    {
        "Information": [{
            id: 2,
            Name: "David"
        }]
    },
    {
        "Information": [{
            id: 3,
            Name: "Bob"
        }]
    }
]);

{
    "acknowledged": true,
    "insertedIds": [
        ObjectId("5e78ea87bbc41e36cc3caebf"),
        ObjectId("5e78ea87bbc41e36cc3caec0"),
        ObjectId("5e78ea88bbc41e36cc3caec1")
    ]
}

Display All Documents

db.demo444.find();

{ "_id": ObjectId("5e78ea87bbc41e36cc3caebf"), "Information": [ { "id": 1, "Name": "Chris" } ] }
{ "_id": ObjectId("5e78ea87bbc41e36cc3caec0"), "Information": [ { "id": 2, "Name": "David" } ] }
{ "_id": ObjectId("5e78ea88bbc41e36cc3caec1"), "Information": [ { "id": 3, "Name": "Bob" } ] }

Query with $elemMatch for Multikey Index

db.demo444.find({
    "Information": {
        $elemMatch: {
            id: 2,
            Name: "David"
        }
    }
});

{ "_id": ObjectId("5e78ea87bbc41e36cc3caec0"), "Information": [ { "id": 2, "Name": "David" } ] }

Create Multikey Index for Optimization

db.demo444.createIndex({
    "Information.id": 1,
    "Information.Name": 1
});

Key Points

• $elemMatch ensures all conditions match within the same array element.
• Multikey indexes on array fields automatically index each array element separately.
• Compound indexes on nested fields improve query performance significantly.

Conclusion

Use $elemMatch with proper compound indexes on array fields to optimize MongoDB queries. This combination ensures efficient filtering and leverages multikey indexing for better performance on nested array elements.