django-perf-review

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• SKILL.md

SKILL.md (first lines)

# Django Performance Review

Review Django code for **validated** performance issues. Research the codebase to confirm issues before reporting. Report only what you can prove.

## Review Approach

1. **Research first** - Trace data flow, check for existing optimizations, verify data volume
2. **Validate before reporting** - Pattern matching is not validation
3. **Zero findings is acceptable** - Don't manufacture issues to appear thorough
4. **Severity must match impact** - If you catch yourself writing "minor" in a CRITICAL finding, it's not critical. Downgrade or skip it.

## Impact Categories

Issues are organized by impact. Focus on CRITICAL and HIGH - these cause real problems at scale.

| Priority | Category | Impact |
|----------|----------|--------|
| 1 | N+1 Queries | **CRITICAL** - Multiplies with data, causes timeouts |
| 2 | Unbounded Querysets | **CRITICAL** - Memory exhaustion, OOM kills |
| 3 | Missing Indexes | **HIGH** - Full table scans on large tables |
| 4 | Write Loops | **HIGH** - Lock contention, slow requests |
| 5 | Inefficient Patterns | **LOW** - Rarely worth reporting |

---

## Priority 1: N+1 Queries (CRITICAL)

**Impact:** Each N+1 adds `O(n)` database round trips. 100 rows = 100 extra queries. 10,000 rows = timeout.

### Rule: Prefetch related data accessed in loops

Validate by tracing: View → Queryset → Template/Serializer → Loop access

```python
# PROBLEM: N+1 - each iteration queries profile
def user_list(request):
    users = User.objects.all()
    return render(request, 'users.html', {'users': users})

# Template:
# {% for user in users %}
#     {{ user.profile.bio }}  ← triggers query per user
# {% endfor %}

# SOLUTION: Prefetch in view
def user_list(request):
    users = User.objects.select_related('profile')
    return render(request, 'users.html', {'users': users})
```

### Rule: Prefetch in serializers, not just views

DRF serializers accessing related fields cause N+1 if queryset isn't optimized.

```python
# PROBLEM: SerializerMethodField queries per object
class UserSerializer(serializers.ModelSerializer):
    order_count = serializers.SerializerMethodField()

    def get_order_count(self, obj):
        return obj.orders.count()  # ← query per user

# SOLUTION: Annotate in viewset, access in serializer
class UserViewSet(viewsets.ModelViewSet):
    def get_queryset(self):
        return User.objects.annotate(order_count=Count('orders'))

class UserSerializer(serializers.ModelSerializer):
    order_count = serializers.IntegerField(read_only=True)
```

### Rule: Model properties that query are dangerous in loops

```python
# PROBLEM: Property triggers query when accessed
class User(models.Model):
    @property
    def recent_orders(self):
        return self.orders.filter(created__gte=last_week)[:5]

# Used in template loop = N+1

# SOLUTION: Use Prefetch with custom queryset, or annotate
```

### Validation Checklist for N+1
- [ ] Traced data flow from view to template/serializer
- [ ] Confirmed related field is accessed inside a loop
- [ ] Searched codebase for existing select_related/prefetch_related
- [ ] Verified table has significant row count (1000+)
- [ ] Confirmed this is a hot path (not admin, not rare action)

---

## Priority 2: Unbounded Querysets (CRITICAL)

**Impact:** Loading entire tables exhausts memory. Large tables cause OOM kills and worker restarts.

### Rule: Always paginate list endpoints

```python
# PROBLEM: No pagination - loads all rows
class UserListView(ListView):
    model = User
    template_name = 'users.html'

# SOLUTION: Add pagination
class UserListView(ListView):
    model = User
    template_name = 'users.html'
    paginate_by = 25
```

### Rule: Use iterator() for large batch processing

```python
# PROBLEM: Loads all objects into memory at once
for user in User.objects.all():
    process(user)

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