Django Perf Review by @sentry · Skillet

Django Perf Review

Django performance code review. Use when asked to "review Django performance", "find N+1 queries", "optimize Django", "check queryset performance", "database performance", "Django ORM issues", or audit Django code for performance problems.

Permissions

Just instructions. No commands, network, or file access.

Files

SKILL.md

django-perf-review

allowed-tools: Read Grep Glob Bash Task
license: LICENSE

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

Research first - Trace data flow, check for existing optimizations, verify data volume
Validate before reporting - Pattern matching is not validation
Zero findings is acceptable - Don't manufacture issues to appear thorough
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	- Lock contention, slow requests

2 files · 11 KB11 KB

Version history

# 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})

# 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)

# 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

# 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

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

# SOLUTION: Stream with iterator()
for user in User.objects.iterator(chunk_size=1000):
    process(user)

# PROBLEM: Forces full evaluation into memory
all_users = list(User.objects.all())

# SOLUTION: Keep as queryset, slice if needed
users = User.objects.all()[:100]

# PROBLEM: Filtering on unindexed field
# User.objects.filter(email=email)  # full scan if no index

class User(models.Model):
    email = models.EmailField()  # ← no db_index

# SOLUTION: Add index
class User(models.Model):
    email = models.EmailField(db_index=True)

# PROBLEM: Sorting requires full scan without index
Order.objects.order_by('-created')

# SOLUTION: Index the sort field
class Order(models.Model):
    created = models.DateTimeField(db_index=True)

class Order(models.Model):
    user = models.ForeignKey(User)
    status = models.CharField(max_length=20)
    created = models.DateTimeField()

    class Meta:
        indexes = [
            models.Index(fields=['user', 'status']),  # for filter(user=x, status=y)
            models.Index(fields=['status', '-created']),  # for filter(status=x).order_by('-created')
        ]

# PROBLEM: N inserts, N round trips
for item in items:
    Model.objects.create(name=item['name'])

# SOLUTION: Single bulk insert
Model.objects.bulk_create([
    Model(name=item['name']) for item in items
])

# PROBLEM: N updates
for obj in queryset:
    obj.status = 'done'
    obj.save()

# SOLUTION A: Single UPDATE statement (same value for all)
queryset.update(status='done')

# SOLUTION B: bulk_update (different values)
for obj in objects:
    obj.status = compute_status(obj)
Model.objects.bulk_update(objects, ['status'], batch_size=500)

# PROBLEM: N deletes
for obj in queryset:
    obj.delete()

# SOLUTION: Single DELETE
queryset.delete()

# Slightly suboptimal
if queryset.count() > 0:
    do_thing()

# Marginally better
if queryset.exists():
    do_thing()

# Fetches all rows to count
if len(queryset) > 0:  # bad if queryset not yet evaluated

# Single COUNT query
if queryset.count() > 0:

# N queries, but if N is small (< 20), often fine
for id in ids:
    obj = Model.objects.get(id=id)

## Django Performance Review: [File/Component Name]

### Summary
Validated issues: X (Y Critical, Z High)

### Findings

#### [PERF-001] N+1 Query in UserListView (CRITICAL)
**Location:** `views.py:45`

**Issue:** Related field `profile` accessed in template loop without prefetch.

**Validation:**
- Traced: UserListView → users queryset → user_list.html → `{{ user.profile.bio }}` in loop
- Searched codebase: no select_related('profile') found
- User table: 50k+ rows (verified in admin)
- Hot path: linked from homepage navigation

**Evidence:**
```python
def get_queryset(self):
    return User.objects.filter(active=True)  # no select_related

def get_queryset(self):
    return User.objects.filter(active=True).select_related('profile')


If no issues found: "No performance issues identified after reviewing [files] and validating [what you checked]."

**Before submitting, sanity check each finding:**
- Does the severity match the actual impact? ("Minor inefficiency" ≠ CRITICAL)
- Is this a real performance issue or just a style preference?
- Would fixing this measurably improve performance?

If the answer to any is "no" - remove the finding.

---

## What NOT to Report

- Test files
- Admin-only views
- Management commands
- Migration files
- One-time scripts
- Code behind disabled feature flags
- Tables with <1000 rows that won't grow
- Patterns in cold paths (rarely executed code)
- Micro-optimizations (exists vs count, only/defer without evidence)

### False Positives to Avoid

**Queryset variable assignment is not an issue:**
```python
# This is FINE - no performance difference
projects_qs = Project.objects.filter(org=org)
projects = list(projects_qs)

# vs this - identical performance
projects = list(Project.objects.filter(org=org))

# This is ONE query, not N+1
projects = list(Project.objects.filter(org=org))

# This is 2 queries, not N+1 - report as LOW at most
state = AutofixState.objects.filter(pr_id=pr_id).first()
project_id = state.request.project_id  # second query

Django Perf Review

Django Performance Review

Review Approach

Impact Categories

Priority 1: N+1 Queries (CRITICAL)

Rule: Prefetch related data accessed in loops

Rule: Prefetch in serializers, not just views

Rule: Model properties that query are dangerous in loops

Validation Checklist for N+1

Priority 2: Unbounded Querysets (CRITICAL)

Rule: Always paginate list endpoints

Rule: Use iterator() for large batch processing

Rule: Never call list() on unbounded querysets

Validation Checklist for Unbounded Querysets

Priority 3: Missing Indexes (HIGH)

Rule: Index fields used in WHERE clauses on large tables

Rule: Index fields used in ORDER BY on large tables

Rule: Use composite indexes for common query patterns

Validation Checklist for Missing Indexes

Priority 4: Write Loops (HIGH)

Rule: Use bulk_create instead of create() in loops

Rule: Use update() or bulk_update instead of save() in loops

Rule: Use delete() on queryset, not in loops

Validation Checklist for Write Loops

Priority 5: Inefficient Patterns (LOW)

Pattern: count() vs exists()

Pattern: len(queryset) vs count()

Pattern: get() in small loops

Validation Requirements

Output Format