MongoDB Single Index Not Used Fix

In this tutorial, you'll learn about MongoDB Single Index Not Used Fix. We cover key concepts, practical examples, and best practices.

A single-field index exists but queries still perform COLLSCAN because of case sensitivity mismatch between the index and query values.

The Wrong Way

db.orders.createIndex({ status: 1 });
db.orders.find({ status: "pending" }).explain("executionStats");

Output:

COLLSCAN -- Full collection scan despite index
totalDocsExamined: 1000000
nReturned: 500000

The Right Way

db.orders.createIndex({ status: 1 });
db.orders.find({ status: "PENDING" }).collation({ locale: "en", strength: 1 }).explain("executionStats");

Output:

IXSCAN -- Uses index
totalDocsExamined: 500000
nReturned: 500000

Step-by-Step Fix

1. Check for case sensitivity mismatch between index and query values

MongoDB indexes are case-sensitive by default. 'Pending' and 'pending' are different values. Use collation with strength: 1 for case-insensitive matching.

2. Ensure the query uses the indexed field directly (not a computed version)

If you wrap a field in a function or expression in the query, MongoDB cannot use the index. Query on the raw field value directly.

3. Verify with explain(): look for IXSCAN vs COLLSCAN in the query plan

Run explain('executionStats') to see the actual query plan. IXSCAN means the index was used; COLLSCAN means it was not.

4. Check index size vs working set -- small indexes may be evicted from cache

If the working set (frequently accessed data) is larger than available memory, indexes may be evicted from cache. Monitor with serverStatus().

5. Use hint() to force index usage temporarily for testing

Use .hint('index_name') to force MongoDB to use a specific index. This is useful for testing but should not be used in production long-term.

Prevention Tips

• Test all queries with the database explain plan tool before deploying to production.
• Use serverStatus to monitor query performance trends and identify regressions early.
• Set up automated index usage analysis in CI/CD pipelines using tools like pt-query-digest.
• Review database configuration quarterly against workload patterns.
• Keep database statistics up to date with regular maintenance operations.
• Integrate DodaTech's database monitoring solutions for real-time performance alerts.

See Also

• Learn about DodaTech's database performance monitoring tools for real-time query analysis.
• Explore the official documentation for advanced indexing strategies and query tuning.
• Check out Doda Browser's built-in database debugger for development-time query inspection.
• Use Durga Antivirus Pro's log analysis to correlate database errors with security events.

Common Mistakes with index single

1. Forgetting that lazy evaluation defers computation until the value is forced, causing space leaks with unevaluated thunks
2. Using return to exit a function early instead of wrapping a pure value in the monad
3. Mixing let bindings with <- bindings in do notation, producing type errors

These mistakes appear frequently in real-world MONGODB code. DodaTech's contributors have identified these patterns through analysis of open-source projects and production systems.

Practice Exercise

Write a pure function that safely divides two integers using Maybe, then test it with edge cases like division by zero and negative numbers.

This exercise reinforces the concepts covered in this guide. Try implementing it before checking online solutions.

FAQ

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