ils15 Database Optimization Skill
When to Use
Use this skill when:
- Optimizing slow SQL queries
- Analyzing missing or redundant indexes
- Reviewing Alembic migrations for safety
- Identifying N+1 query problems
- Designing database schemas
- Planning data migrations
- Reviewing query execution plans
Optimization Checklist
1. Index Analysis
-- Check missing indexes
SELECT schemaname, tablename, indexname, indexdef
FROM pg_indexes
WHERE tablename = 'your_table';
-- Identify unused indexes
SELECT * FROM pg_stat_user_indexes
WHERE idx_scan = 0;
Index Recommendations:
✅ Always index:
- Primary keys (automatic)
- Foreign keys
- Columns in WHERE clauses
- Columns in ORDER BY
- Columns in JOIN conditions
❌ Avoid indexing:
- Low cardinality columns (boolean, status)
- Frequently updated columns
- Small tables (<1000 rows)
2. N+1 Query Detection
# ❌ N+1 Problem
users = await session.execute(select(User))
for user in users:
# This triggers N additional queries!
orders = await session.execute(
select(Order).where(Order.user_id == user.id)
)
# ✅ Solution: Eager loading
from sqlalchemy.orm import selectinload
users = await session.execute(
select(User).options(selectinload(User.orders))
)
3. Query Optimization Patterns
# ✅ Use pagination
from sqlalchemy import select
from app.models import Product
async def get_products(skip: int = 0, limit: int = 20):
query = select(Product).offset(skip).limit(limit)
result = await session.execute(query)
return result.scalars().all()
# ✅ Use specific columns (not SELECT *)
query = select(Product.id, Product.name, Product.price)
# ✅ Use exists() for existence checks
from sqlalchemy import exists
query = select(exists().where(User.email == email))
4. Migration Safety
# ✅ Safe migration patterns
def upgrade():
# Add column with default (no table lock)
op.add_column('users', sa.Column('status', sa.String(20),
server_default='active'))
# ❌ Dangerous patterns
def upgrade():
# Avoid: Rename column (breaks app)
op.alter_column('users', 'name', new_column_name='full_name')
# Avoid: Change column type (data loss risk)
op.alter_column('users', 'age', type_=sa.String())
5. Async Best Practices
# ✅ Proper async session handling
from sqlalchemy.ext.asyncio import AsyncSession
async def get_user(session: AsyncSession, user_id: int):
result = await session.execute(
select(User).where(User.id == user_id)
)
return result.scalar_one_or_none()
# ✅ Bulk operations
async def bulk_insert(session: AsyncSession, items: list):
session.add_all(items)
await session.commit()
Output Format
## Database Optimization Report
### Query Analysis
- Queries analyzed: X
- Slow queries (>100ms): X
- N+1 problems: X
### Index Recommendations
| Table | Column | Type | Reason |
|-------|--------|------|--------|
| users | email | UNIQUE | WHERE clause filter |
| orders | user_id | INDEX | Foreign key JOIN |
### Optimization Suggestions
1. [Query] - [Current time] - [Optimized time] - [How]
### Migration Review
- ✅ Safe to run
- ⚠️ Requires maintenance window
- ❌ Breaking change detected
Example Usage
@database Optimize the slow query in order_service.py:120
@database Review the new migration for safety issues
@database Find N+1 problems in the user module
@database Suggest indexes for the products table
LLM-Assisted Index Optimization
Automatic Index Discovery from EXPLAIN ANALYZE
import re
from dataclasses import dataclass, field
@dataclass
class IndexRecommendation:
table: str
columns: list[str]
reason: str
estimated_impact: str # HIGH / MEDIUM / LOW
query_pattern: str
class LLMIndexAdvisor:
"""Uses LLM to analyze EXPLAIN ANALYZE output and recommend indexes."""
INDEX_PROMPT = """Analyze this PostgreSQL EXPLAIN ANALYZE output and recommend indexes:
{explain_output}
The query was:
{query}
Rules:
1. Sequential scans on large tables -> recommend index
2. Missing composite indexes for multi-column WHERE clauses
3. Partial indexes for filtered queries (WHERE status = 'active')
4. Covering indexes for SELECT * queries with many columns
5. Consider existing indexes to avoid duplicates
Return recommendations as JSON array with: table, columns, reason, estimated_impact
"""
@classmethod
async def analyze(cls, explain_output: str, query: str) -> list[IndexRecommendation]:
"""Generate index recommendations from EXPLAIN ANALYZE output."""
# Uses LLM (via the configured model tier) to analyze the plan
# Returns structured recommendations
pass
@classmethod
async def validate_index_impact(cls, index_ddl: str) -> str:
"""Validate an index by running EXPLAIN ANALYZE before and after."""
# Run EXPLAIN ANALYZE with the query
# Create the index
# Run EXPLAIN ANALYZE again
# Compare costs and return diff
pass
Real-Time Query Pattern Detection
@dataclass
class QueryPattern:
fingerprint: str # Normalized query form
frequency: int # Times executed
avg_time_ms: float
table_scans: bool
missing_index_score: float # 0-1
class PatternAnalyzer:
"""Detect query patterns and suggest optimizations using LLM."""
@staticmethod
async def detect_patterns(pg_stat_statements: list[dict]) -> list[QueryPattern]:
"""Analyze pg_stat_statements output for optimization opportunities."""
# Group by normalized query
# Sort by total_time descending
# Flag sequential scans on large result sets
# Use LLM to suggest index strategies
pass
Migration Safety Review with LLM
class MigrationSafetyReviewer:
"""Review Alembic migrations for safety concerns using LLM."""
REVIEW_PROMPT = """Review this Alembic migration for safety:
{migration_code}
Check for:
1. Long-running locks (ADD COLUMN DEFAULT, ALTER TYPE)
2. Blocking operations (DROP COLUMN without expand-contract)
3. Data loss potential (DROP TABLE, TRUNCATE)
4. Missing indexes on new foreign key columns
5. Nullability changes that could break existing code
"""
@classmethod
async def review(cls, migration_sql: str) -> list[dict]:
"""Review migration for safety issues."""
# Pass migration to LLM for safety analysis
# Return structured findings with severity
pass
Appendix: General Performance Optimization
Additional performance optimization patterns beyond database-specific concerns.
Caching Strategy
class CachingService:
def __init__(self, redis_client):
self.redis = redis_client
async def get_user(self, user_id: int):
# Try cache first
cached = await self.redis.get(f"user:{user_id}")
if cached:
return json.loads(cached)
# Cache miss - fetch from DB
user = await db.query(User).filter(User.id == user_id).first()
# Cache for 1 hour
await self.redis.setex(
f"user:{user_id}",
3600,
json.dumps(user.dict())
)
return user
# Cache invalidation on update
async def update_user(user_id: int, data):
user = await db.query(User).filter(User.id == user_id).first()
user.update(data)
await db.commit()
# Invalidate cache
await self.redis.delete(f"user:{user_id}")
Monitoring Performance
# Add timing middleware
@app.middleware("http")
async def add_timing_header(request, call_next):
start = time.time()
response = await call_next(request)
duration = time.time() - start
response.headers["X-Process-Time"] = str(duration)
logger.info(f"{request.url.path} took {duration:.3f}s")
return response
Performance Checklist
- Identified slow queries (with EXPLAIN ANALYZE)
- Indexes added on WHERE/JOIN/ORDER BY columns
- N+1 queries fixed (eager loading)
- Caching implemented for hot data
- Pagination added to list endpoints
- Batch operations where possible
- Connection pooling configured
- Monitoring/alerting in place