Display MongoDB explain query plan?

To analyze MongoDB query performance and execution strategy, use the explain() method. This provides detailed information about how MongoDB processes your queries, including execution stages, index usage, and performance metrics.

Syntax

db.collection.find(query).explain();
db.collection.find(query).explain("executionStats");
db.collection.find(query).explain("allPlansExecution");

Sample Data

db.demo202.insertMany([
    {"StudentFirstName": "Chris", "StudentAge": 21},
    {"StudentFirstName": "David", "StudentAge": 23},
    {"StudentFirstName": "Bob", "StudentAge": 22}
]);

{
    "acknowledged": true,
    "insertedIds": [
        ObjectId("5e3c3bd103d395bdc21346e8"),
        ObjectId("5e3c3bd803d395bdc21346e9"),
        ObjectId("5e3c3bde03d395bdc21346ea")
    ]
}

Example: Basic Query Plan

db.demo202.find({"StudentFirstName": "David"}).explain();

{
    "queryPlanner": {
        "plannerVersion": 1,
        "namespace": "test.demo202",
        "indexFilterSet": false,
        "parsedQuery": {
            "StudentFirstName": {
                "$eq": "David"
            }
        },
        "winningPlan": {
            "stage": "COLLSCAN",
            "filter": {
                "StudentFirstName": {
                    "$eq": "David"
                }
            },
            "direction": "forward"
        },
        "rejectedPlans": []
    },
    "ok": 1
}

Key Points

• COLLSCAN indicates a collection scan (no index used)
• IXSCAN would indicate index usage for better performance
• winningPlan shows the execution strategy MongoDB selected
• Use explain("executionStats") for detailed performance metrics

Conclusion

The explain() method is essential for query optimization in MongoDB. It reveals execution stages, helping identify when indexes are needed to improve query performance from collection scans to efficient index scans.