N+1 Query Problem Causing Performance Degradation
Application performance degrades significantly after adding a feature that loads related data. Profiling shows massive number of database queries (1 main query + N queries for each result). Adding 50 users slows down response by seconds instead of milliseconds.
The feature works correctly but is fundamentally inefficient, querying the database hundreds or thousands of times for what should be a few queries.
Error Messages You Might See
Common Causes
- Lazy loading relationship in loop: for (user in users) { user.profile.name } triggers query per user
- Separate query for each item instead of batch loading
- Eager loading not configured, falling back to lazy loading
- No database indexes on foreign keys causing full table scans
- JOINs not used despite being available, falling back to multiple queries
How to Fix It
Enable SQL query logging to see all queries. Count them: if 51+ for 50 items, you have N+1. Use eager loading (JOIN FETCH) or explicit batch loading. Add database indexes on foreign keys. Use query analysis tools (EXPLAIN PLAN) to see join strategies. Consider denormalization or caching for expensive relationships.
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Get HelpFrequently Asked Questions
How to detect N+1 problems?
Enable SQL query logging. For N items, you should see ~O(1) or O(log N) queries, not O(N). If query count = 1 + N, that's N+1 problem.
How to fix with JOINs?
Use JOIN FETCH in query: SELECT user FROM User user JOIN FETCH user.profile WHERE ... (fetches user and related profile in single query).
Should all relationships be eager loaded?
No. Load eagerly only if always used. For conditional access, use explicit loading or batch loading when needed.