Django performance review

---
name: django-perf-review
description: 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.
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

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)

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

### Rule: Never call list() on unbounded querysets

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

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

### Validation Checklist for Unbounded Querysets
- [ ] Table is large (10k+ rows) or will grow unbounded
- [ ] No pagination class, paginate_by, or slicing
- [ ] This runs on user-facing request (not background job with chunking)

---

## Priority 3: Missing Indexes (HIGH)

**Impact:** Full table scans. Negligible on small tables, catastrophic on large ones.

### Rule: Index fields used in WHERE clauses on large tables

```python
# 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)
```

### Rule: Index fields used in ORDER BY on large tables

```python
# 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)
```

### Rule: Use composite indexes for common query patterns

```python
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')
        ]
```

### Validation Checklist for Missing Indexes
- [ ] Table has 10k+ rows
- [ ] Field is used in filter() or order_by() on hot path
- [ ] Checked model - no db_index=True or Meta.indexes entry
- [ ] Not a foreign key (already indexed automatically)

---

## Priority 4: Write Loops (HIGH)

**Impact:** N database writes instead of 1. Lock contention. Slow requests.

### Rule: Use bulk_create instead of create() in loops

```python
# 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
])
```

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

```python
# 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)
```

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

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

# SOLUTION: Single DELETE
queryset.delete()
```

### Validation Checklist for Write Loops
- [ ] Loop iterates over 100+ items (or unbounded)
- [ ] Each iteration calls create(), save(), or delete()
- [ ] This runs on user-facing request (not one-time migration script)

---

## Priority 5: Inefficient Patterns (LOW)

**Rarely worth reporting.** Include only as minor notes if you're already reporting real issues.

### Pattern: count() vs exists()

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

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

**Usually skip** - difference is <1ms in most cases.

### Pattern: len(queryset) vs count()

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

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

**Only flag** if queryset is large and not already evaluated.

### Pattern: get() in small loops

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

**Only flag** if loop is large or this is in a very hot path.

---

## Validation Requirements

Before reporting ANY issue:

1. **Trace the data flow** - Follow queryset from creation to consumption
2. **Search for existing optimizations** - Grep for select_related, prefetch_related, pagination
3. **Verify data volume** - Check if table is actually large
4. **Confirm hot path** - Trace call sites, verify this runs frequently
5. **Rule out mitigations** - Check for caching, rate limiting

**If you cannot validate all steps, do not report.**

---

## Output Format

```markdown
## 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
```

**Fix:**
```python
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))
```
Querysets are lazy. Assigning to a variable doesn't execute anything.

**Single query patterns are not N+1:**
```python
# This is ONE query, not N+1
projects = list(Project.objects.filter(org=org))
```
N+1 requires a loop that triggers additional queries. A single `list()` call is fine.

**Missing select_related on single object fetch is not N+1:**
```python
# 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
```
N+1 requires a loop. A single object doing 2 queries instead of 1 can be reported as LOW if relevant, but never as CRITICAL/HIGH.

**Style preferences are not performance issues:**
If your only suggestion is "combine these two lines" or "rename this variable" - that's style, not performance. Don't report it.

The download contains SKILL.md. Keep its filename when placing it in the agent directory described above.

By Functional Software, Inc. dba Sentry. Exact upstream source ↗ · Licence · Attribution

SHA-256 52283bff7b23f06f94bbb413d174fd1cbcf497f8e8a63d4f0d569ba6bfd949d4

                                 Apache License
                           Version 2.0, January 2004
                        http://www.apache.org/licenses/

   TERMS AND CONDITIONS FOR USE, REPRODUCTION, AND DISTRIBUTION

   1. Definitions.

      "License" shall mean the terms and conditions for use, reproduction,
      and distribution as defined by Sections 1 through 9 of this document.

      "Licensor" shall mean the copyright owner or entity authorized by
      the copyright owner that is granting the License.

      "Legal Entity" shall mean the union of the acting entity and all
      other entities that control, are controlled by, or are under common
      control with that entity. For the purposes of this definition,
      "control" means (i) the power, direct or indirect, to cause the
      direction or management of such entity, whether by contract or
      otherwise, or (ii) ownership of fifty percent (50%) or more of the
      outstanding shares, or (iii) beneficial ownership of such entity.

      "You" (or "Your") shall mean an individual or Legal Entity
      exercising permissions granted by this License.

      "Source" form shall mean the preferred form for making modifications,
      including but not limited to software source code, documentation
      source, and configuration files.

      "Object" form shall mean any form resulting from mechanical
      transformation or translation of a Source form, including but
      not limited to compiled object code, generated documentation,
      and conversions to other media types.

      "Work" shall mean the work of authorship, whether in Source or
      Object form, made available under the License, as indicated by a
      copyright notice that is included in or attached to the work
      (an example is provided in the Appendix below).

      "Derivative Works" shall mean any work, whether in Source or Object
      form, that is based on (or derived from) the Work and for which the
      editorial revisions, annotations, elaborations, or other modifications
      represent, as a whole, an original work of authorship. For the purposes
      of this License, Derivative Works shall not include works that remain
      separable from, or merely link (or bind by name) to the interfaces of,
      the Work and Derivative Works thereof.

      "Contribution" shall mean any work of authorship, including
      the original version of the Work and any modifications or additions
      to that Work or Derivative Works thereof, that is intentionally
      submitted to the Licensor for inclusion in the Work by the copyright owner
      or by an individual or Legal Entity authorized to submit on behalf of
      the copyright owner. For the purposes of this definition, "submitted"
      means any form of electronic, verbal, or written communication sent
      to the Licensor or its representatives, including but not limited to
      communication on electronic mailing lists, source code control systems,
      and issue tracking systems that are managed by, or on behalf of, the
      Licensor for the purpose of discussing and improving the Work, but
      excluding communication that is conspicuously marked or otherwise
      designated in writing by the copyright owner as "Not a Contribution."

      "Contributor" shall mean Licensor and any individual or Legal Entity
      on behalf of whom a Contribution has been received by Licensor and
      subsequently incorporated within the Work.

   2. Grant of Copyright License. Subject to the terms and conditions of
      this License, each Contributor hereby grants to You a perpetual,
      worldwide, non-exclusive, no-charge, royalty-free, irrevocable
      copyright license to reproduce, prepare Derivative Works of,
      publicly display, publicly perform, sublicense, and distribute the
      Work and such Derivative Works in Source or Object form.

   3. Grant of Patent License. Subject to the terms and conditions of
      this License, each Contributor hereby grants to You a perpetual,
      worldwide, non-exclusive, no-charge, royalty-free, irrevocable
      (except as stated in this section) patent license to make, have made,
      use, offer to sell, sell, import, and otherwise transfer the Work,
      where such license applies only to those patent claims licensable
      by such Contributor that are necessarily infringed by their
      Contribution(s) alone or by combination of their Contribution(s)
      with the Work to which such Contribution(s) was submitted. If You
      institute patent litigation against any entity (including a
      cross-claim or counterclaim in a lawsuit) alleging that the Work
      or a Contribution incorporated within the Work constitutes direct
      or contributory patent infringement, then any patent licenses
      granted to You under this License for that Work shall terminate
      as of the date such litigation is filed.

   4. Redistribution. You may reproduce and distribute copies of the
      Work or Derivative Works thereof in any medium, with or without
      modifications, and in Source or Object form, provided that You
      meet the following conditions:

      (a) You must give any other recipients of the Work or
          Derivative Works a copy of this License; and

      (b) You must cause any modified files to carry prominent notices
          stating that You changed the files; and

      (c) You must retain, in the Source form of any Derivative Works
          that You distribute, all copyright, patent, trademark, and
          attribution notices from the Source form of the Work,
          excluding those notices that do not pertain to any part of
          the Derivative Works; and

      (d) If the Work includes a "NOTICE" text file as part of its
          distribution, then any Derivative Works that You distribute must
          include a readable copy of the attribution notices contained
          within such NOTICE file, excluding those notices that do not
          pertain to any part of the Derivative Works, in at least one
          of the following places: within a NOTICE text file distributed
          as part of the Derivative Works; within the Source form or
          documentation, if provided along with the Derivative Works; or,
          within a display generated by the Derivative Works, if and
          wherever such third-party notices normally appear. The contents
          of the NOTICE file are for informational purposes only and
          do not modify the License. You may add Your own attribution
          notices within Derivative Works that You distribute, alongside
          or as an addendum to the NOTICE text from the Work, provided
          that such additional attribution notices cannot be construed
          as modifying the License.

      You may add Your own copyright statement to Your modifications and
      may provide additional or different license terms and conditions
      for use, reproduction, or distribution of Your modifications, or
      for any such Derivative Works as a whole, provided Your use,
      reproduction, and distribution of the Work otherwise complies with
      the conditions stated in this License.

   5. Submission of Contributions. Unless You explicitly state otherwise,
      any Contribution intentionally submitted for inclusion in the Work
      by You to the Licensor shall be under the terms and conditions of
      this License, without any additional terms or conditions.
      Notwithstanding the above, nothing herein shall supersede or modify
      the terms of any separate license agreement you may have executed
      with Licensor regarding such Contributions.

   6. Trademarks. This License does not grant permission to use the trade
      names, trademarks, service marks, or product names of the Licensor,
      except as required for reasonable and customary use in describing the
      origin of the Work and reproducing the content of the NOTICE file.

   7. Disclaimer of Warranty. Unless required by applicable law or
      agreed to in writing, Licensor provides the Work (and each
      Contributor provides its Contributions) on an "AS IS" BASIS,
      WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
      implied, including, without limitation, any warranties or conditions
      of TITLE, NON-INFRINGEMENT, MERCHANTABILITY, or FITNESS FOR A
      PARTICULAR PURPOSE. You are solely responsible for determining the
      appropriateness of using or redistributing the Work and assume any
      risks associated with Your exercise of permissions under this License.

   8. Limitation of Liability. In no event and under no legal theory,
      whether in tort (including negligence), contract, or otherwise,
      unless required by applicable law (such as deliberate and grossly
      negligent acts) or agreed to in writing, shall any Contributor be
      liable to You for damages, including any direct, indirect, special,
      incidental, or consequential damages of any character arising as a
      result of this License or out of the use or inability to use the
      Work (including but not limited to damages for loss of goodwill,
      work stoppage, computer failure or malfunction, or any and all
      other commercial damages or losses), even if such Contributor
      has been advised of the possibility of such damages.

   9. Accepting Warranty or Additional Liability. While redistributing
      the Work or Derivative Works thereof, You may choose to offer,
      and charge a fee for, acceptance of support, warranty, indemnity,
      or other liability obligations and/or rights consistent with this
      License. However, in accepting such obligations, You may act only
      on Your own behalf and on Your sole responsibility, not on behalf
      of any other Contributor, and only if You agree to indemnify,
      defend, and hold each Contributor harmless for any liability
      incurred by, or claims asserted against, such Contributor by reason
      of your accepting any such warranty or additional liability.

   END OF TERMS AND CONDITIONS

   Copyright 2025 Functional Software, Inc. dba Sentry

   Licensed under the Apache License, Version 2.0 (the "License");
   you may not use this file except in compliance with the License.
   You may obtain a copy of the License at

       http://www.apache.org/licenses/LICENSE-2.0

   Unless required by applicable law or agreed to in writing, software
   distributed under the License is distributed on an "AS IS" BASIS,
   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   See the License for the specific language governing permissions and
   limitations under the License.