lidge-jun/postgres-patterns

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.