A large production table needs a foreign key. The plain add takes an exclusive lock — reads and writes are blocked while it scans. The trick is two steps: first hang the key with «don't validate yet» (instant, doesn't block writers), then run validation against the old rows in a separate command (no write locks). Build parents and children, insert a few rows. Add a foreign key children_parent_id_fkey to children from column parent_id to parents(id) without immediate validation. Then run validation against this same key — it should end up marked as validated.