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skills/smart-debugging/examples/performance-bug-debug.md

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+# Performance Bug Debug Example
+
+Debugging slow database queries and N+1 problems.
+
+## Symptom
+
+API endpoint taking 4.5 seconds to respond (target: < 200ms).
+
+## Profiling
+
+```python
+import cProfile
+import pstats
+
+profiler = cProfile.Profile()
+profiler.enable()
+
+response = await get_users_with_posts()
+
+profiler.disable()
+stats = pstats.Stats(profiler)
+stats.sort_stats('cumulative')
+stats.print_stats(10)
+```
+
+### Profile Output
+
+```
+   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
+      100    4.321    0.043    4.321    0.043 database.py:42(execute_query)
+        1    0.089    0.089    4.410    4.410 users.py:15(get_users_with_posts)
+```
+
+**Issue**: Database query called 100 times! (N+1 problem)
+
+## Code Analysis
+
+```python
+# BAD: N+1 Query Problem
+async def get_users_with_posts():
+    users = await db.users.find_all()  # 1 query
+
+    result = []
+    for user in users:  # 100 iterations
+        posts = await db.posts.find({"user_id": user.id})  # N queries!
+        result.append({"user": user, "posts": posts})
+
+    return result  # Total: 101 queries (1 + 100)
+```
+
+## Fix: Use Join/Eager Loading
+
+```python
+# GOOD: Single Query with Join
+async def get_users_with_posts():
+    query = """
+        SELECT
+            users.*,
+            json_agg(posts.*) as posts
+        FROM users
+        LEFT JOIN posts ON posts.user_id = users.id
+        GROUP BY users.id
+    """
+    result = await db.execute(query)  # 1 query total!
+    return result
+```
+
+## Performance Comparison
+
+| Approach | Queries | Time |
+|----------|---------|------|
+| **Before (N+1)** | 101 | 4.5s ❌ |
+| **After (Join)** | 1 | 85ms ✅ |
+
+**Improvement**: 53x faster!
+
+## Prevention
+
+1. **Query logging**: Log all database queries in development
+2. **Performance tests**: Assert query count < threshold
+3. **APM monitoring**: Track query patterns in production (Datadog, New Relic)
+
+```python
+# Performance test
+def test_get_users_with_posts_query_count(query_counter):
+    get_users_with_posts()
+    assert query_counter.count <= 1, f"Expected 1 query, got {query_counter.count}"
+```
+
+---
+
+**Result**: N+1 detected and fixed. Performance SLA met (< 200ms).