postgres-patterns - SKILL.md Agent Skill

name: postgres-patterns description: PostgreSQL database patterns for query optimization, schema design, indexing, and security.

PostgreSQL Patterns

Quick reference for PostgreSQL best practices.

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)`
Multi-col leading skip	B-tree (PG 18 skip scan)	Automatic for `WHERE b = value` on index `(a, b)`

Data Type Quick Reference

Use Case	Correct Type	Avoid
IDs	`UUIDv7` (time-sortable, B-tree friendly)	UUIDv4 (random, fragments indexes)
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)

Virtual Generated Column (PG 18+):

-- Computed on read, no storage cost
ALTER TABLE users ADD COLUMN display_name TEXT
    GENERATED ALWAYS AS (first_name || ' ' || last_name) VIRTUAL;
-- On PG 12–17, use STORED (persisted to disk)

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();

Authentication

-- MD5 password auth is deprecated. Use SCRAM-SHA-256 (default since PG 14).
ALTER SYSTEM SET password_encryption = 'scram-sha-256';
-- Update pg_hba.conf: replace 'md5' with 'scram-sha-256'
-- Existing users must reset passwords after switching.