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

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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

# 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.