Stop using UUID v4 for database primary keys — UUIDv7 is the 2026 default

Your B-tree indexes hate UUID v4. They've hated it for years. You probably haven't noticed because the slowdown is gradual — until your table hits 10 million rows and inserts start taking 200ms instead of 5ms. UUIDv7 fixes this. RFC 9562 made it official in May 2024. PostgreSQL 18 ships native support. MySQL 8.4 has a helper. Most ORM and language libraries added v7 in 2024–2025. By 2026, v4 should be the exception, not the default. This post is the "why", the "when", and the "how to migrate w...