Can I query on a MongoDB index if my query contains the $or operator?

Yes, you can query on MongoDB indexes with the $or operator. MongoDB can effectively use separate indexes for each condition in the $or query, combining results through an OR stage in the execution plan.

Syntax

db.collection.find({ $or: [
    { field1: value1 },
    { field2: value2 }
]}).explain();

Create Sample Indexes

First, create indexes on the fields you'll query ?

db.indexOrQueryDemo.createIndex({"First": 1});

{
    "createdCollectionAutomatically": false,
    "numIndexesBefore": 2,
    "numIndexesAfter": 3,
    "ok": 1
}

db.indexOrQueryDemo.createIndex({"Second": 1});

{
    "createdCollectionAutomatically": false,
    "numIndexesBefore": 3,
    "numIndexesAfter": 4,
    "ok": 1
}

Query with $or and Explain Plan

Now query using $or operator and analyze the execution plan ?

db.indexOrQueryDemo.find({$or: [{First: 1}, {Second: 2}]}).explain();

{
    "queryPlanner": {
        "plannerVersion": 1,
        "namespace": "test.indexOrQueryDemo",
        "winningPlan": {
            "stage": "SUBPLAN",
            "inputStage": {
                "stage": "FETCH",
                "inputStage": {
                    "stage": "OR",
                    "inputStages": [
                        {
                            "stage": "IXSCAN",
                            "keyPattern": {"First": 1},
                            "indexName": "First_1",
                            "indexBounds": {
                                "First": ["[1.0, 1.0]"]
                            }
                        },
                        {
                            "stage": "IXSCAN",
                            "keyPattern": {"Second": 1},
                            "indexName": "Second_1",
                            "indexBounds": {
                                "Second": ["[2.0, 2.0]"]
                            }
                        }
                    ]
                }
            }
        }
    },
    "ok": 1
}

How It Works

The execution plan shows MongoDB uses both indexes effectively:

• OR stage: Combines results from multiple index scans
• IXSCAN: Each condition uses its respective index (First_1 and Second_1)
• FETCH: Retrieves full documents after index filtering

Conclusion

MongoDB efficiently supports $or queries with indexes by using separate index scans for each condition and combining results. The explain() method confirms index usage through the OR execution stage.