How can I use the MongoDB explain plan to troubleshoot query performance

MongoDB's explain plan is a powerful tool for troubleshooting query performance. Here's how you can use it effectively:

Running an Explain Plan

To generate an explain plan, you can use the following methods:

1. For find queries:

   db.collection.find(<query>).explain()

1. For aggregation pipelines:

   db.collection.explain().aggregate([<pipeline>])

1. For other operations:

   db.collection.explain().<operation>

Understanding Explain Output

The explain plan provides detailed information about query execution in several key areas:

Query Planner

This section shows the plan selected by the query optimizer:

• winningPlan: Indicates the chosen execution plan
• rejectedPlans: Lists alternative plans that were considered but not used

Execution Stats

This part offers insights into the actual query execution:

• nReturned: Number of documents returned
• totalKeysExamined: Number of index keys scanned
• totalDocsExamined: Number of documents scanned
• executionTimeMillis: Total execution time

Index Usage

Look for the following indicators:

• IXSCAN: Indicates an index was used
• COLLSCAN: Suggests a full collection scan, which may be inefficient for large datasets

Troubleshooting Tips

1. Compare documents scanned vs. returned: A high ratio of examined to returned documents may indicate a need for better indexing[1].

2. Check for COLLSCAN: If you see this instead of IXSCAN, consider adding an appropriate index[1].

3. Analyze execution time: Look at the executionTimeMillis to identify slow queries[3].

4. Examine rejected plans: Understanding why certain plans were rejected can help in optimizing indexes or query structure[4].

5. Use different verbosity modes:

   • queryPlanner: Default mode, shows the winning plan
   • executionStats: Includes execution statistics
   • allPlansExecution: Provides data on all considered plans[4]

6. Iterate and refine: Use the explain plan results to make incremental improvements to your queries and indexes[7].

By leveraging the explain plan effectively, you can identify performance bottlenecks, optimize indexing strategies, and refine query structures to significantly improve MongoDB query performance.

Sources
[1] MongoDB: Investigate Queries with explain() and Index Usage (part 2) https://www.percona.com/blog/mongodb-investigate-queries-with-explain-index-usage-part-2/
[2] Index analysis using Explain command- Scaler Topics https://www.scaler.com/topics/mongodb/index-analysis-using-explain-command/
[3] Interpret Explain Plan Results - MongoDB Manual v8.0 https://www.mongodb.com/docs/manual/tutorial/analyze-query-plan/
[4] explain - MongoDB Manual v8.0 https://www.mongodb.com/docs/manual/reference/command/explain/
[5] Explain Results - MongoDB Manual v5.0 https://www.mongodb.com/docs/v5.0/reference/explain-results/
[6] Track and Troubleshoot MongoDB Performance With Datadog ... https://www.datadoghq.com/blog/mongodb-database-monitoring/
[7] How to Optimize MongoDB Queries for Peak Performance? https://embarkingonvoyage.com/blog/technologies/how-to-optimize-mongodb-queries-for-peak-performance/
[8] Using Explain Plans - Practical MongoDB Aggregations Book https://www.practical-mongodb-aggregations.com/guides/explain.html
[9] db.collection.explain() - MongoDB Manual v8.0 https://www.mongodb.com/docs/manual/reference/method/db.collection.explain/
[10] cursor.explain() - MongoDB Manual v8.0 https://www.mongodb.com/docs/manual/reference/method/cursor.explain/