postgres-patterns

SKILL.md

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

Query Pattern
Index Type
Example

WHERE col = value
B-tree (default)
CREATE INDEX idx ON t (col)

WHERE col > value
B-tree
CREATE INDEX idx ON t (col)

WHERE a = x AND b > y
Composite
CREATE INDEX idx ON t (a, b)

WHERE jsonb @> '{}'
GIN
CREATE INDEX idx ON t USING gin (col)

WHERE tsv @@ query
GIN
CREATE INDEX idx ON t USING gin (col)

Time-series ranges
BRIN
CREATE INDEX idx ON t USING brin (col)

Data Type Quick Reference

Use Case
Correct Type
Avoid

IDs
bigint
int, random UUID

Strings
text
varchar(255)

Timestamps
timestamptz
timestamp

Money
numeric(10,2)
float

Flags
boolean
varchar, int

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)