lidge-jun 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.