PostgreSQL Patterns

Quick reference for PostgreSQL best practices. For detailed guidance, use the database-reviewer agent.

When to Activate

• Writing SQL queries or migrations
• Designing database schemas
• Troubleshooting slow queries
• Implementing Row Level Security
• Setting up connection pooling

Quick Reference

#### Index Cheat Sheet

#### Data Type Quick Reference

#### Common Patterns

Composite Index Order:

-- Equality columns first, then range columns
CREATE INDEX idx ON orders (status, created_at);
-- Works for: WHERE status = 'pending' AND created_at > '2024-01-01'

Covering Index:

CREATE INDEX idx ON users (email) INCLUDE (name, created_at);
-- Avoids table lookup for SELECT email, name, created_at

Partial Index:

CREATE INDEX idx ON users (email) WHERE deleted_at IS NULL;
-- Smaller index, only includes active users

RLS Policy (Optimized):

CREATE POLICY policy ON orders
  USING ((SELECT auth.uid()) = user_id);  -- Wrap in SELECT!

UPSERT:

INSERT INTO settings (user_id, key, value)
VALUES (123, 'theme', 'dark')
ON CONFLICT (user_id, key)
DO UPDATE SET value = EXCLUDED.value;

Cursor Pagination:

SELECT * FROM products WHERE id > $last_id ORDER BY id LIMIT 20;
-- O(1) vs OFFSET which is O(n)

Queue Processing:

UPDATE jobs SET status = 'processing'
WHERE id = (
  SELECT id FROM jobs WHERE status = 'pending'
  ORDER BY created_at LIMIT 1
  FOR UPDATE SKIP LOCKED
) RETURNING *;

#### Anti-Pattern Detection

-- Find unindexed foreign keys
SELECT conrelid::regclass, a.attname
FROM pg_constraint c
JOIN pg_attribute a ON a.attrelid = c.conrelid AND a.attnum = ANY(c.conkey)
WHERE c.contype = 'f'
  AND NOT EXISTS (
    SELECT 1 FROM pg_index i
    WHERE i.indrelid = c.conrelid AND a.attnum = ANY(i.indkey)
  );

-- Find slow queries
SELECT query, mean_exec_time, calls
FROM pg_stat_statements
WHERE mean_exec_time > 100
ORDER BY mean_exec_time DESC;

-- Check table bloat
SELECT relname, n_dead_tup, last_vacuum
FROM pg_stat_user_tables
WHERE n_dead_tup > 1000
ORDER BY n_dead_tup DESC;

#### Configuration Template

-- Connection limits (adjust for RAM)
ALTER SYSTEM SET max_connections = 100;
ALTER SYSTEM SET work_mem = '8MB';

-- Timeouts
ALTER SYSTEM SET idle_in_transaction_session_timeout = '30s';
ALTER SYSTEM SET statement_timeout = '30s';

-- Monitoring
CREATE EXTENSION IF NOT EXISTS pg_stat_statements;

-- Security defaults
REVOKE ALL ON SCHEMA public FROM public;

SELECT pg_reload_conf();

Related

• Agent: database-reviewer - Full database review workflow
• Skill: clickhouse-io - ClickHouse analytics patterns
• Skill: backend-patterns - API and backend patterns

Based on Supabase Agent Skills (credit: Supabase team) (MIT License)

Read Replica Routing

Route queries based on consistency requirements:

• Writes → primary only
• Reads → replica (acceptable eventual consistency)
• Post-write reads within the same request → primary (to avoid read-your-own-writes issues)

Monitoring Alert Thresholds

Configure alerts for:

Use pg_stat_statements for slow query analysis.

EXPLAIN ANALYZE Rule

Before merging any query that touches more than 10,000 rows, run EXPLAIN ANALYZE and include the output in the PR description.

Backup Requirements

• Daily full backup + hourly WAL archiving
• Backups stored in a separate cloud account (not the application account)
• AES-256 encryption at rest
• Quarterly restore drills: verify RTO and RPO targets
• Document RTO (Recovery Time Objective) and RPO (Recovery Point Objective) in the runbook

GDPR, Right to Erasure

Implement within 30 days of erasure request. Options:

1. Soft-delete + background purge: set deleted_at, purge after 30-day window
2. Pseudonymization: replace PII with a non-reversible hash in-place

Never hard-delete without first checking cascade requirements and audit log retention.