safe-migrationslisted

# Safe Migrations ## Purpose Provide a standardized, static methodology for turning a hazardous schema migration into a safe, reversible, zero-downtime change, without a database connection. The core idea: most outages come from a DDL statement that takes a long-held **ACCESS EXCLUSIVE** (Postgres) or table-copy (MySQL) lock and blocks all traffic while it rewrites the table. The fix is almost always the same shape, so this skill encodes it once. ## The Expand-Contract (Parallel-Change) Principle Never change a schema object in place while the app runs. Instead, evolve in additive steps so old and new code both work at every moment: 1. **Expand**: add the new structure (nullable column, new index built concurrently, `NOT VALID` constraint) without touching existing behavior. 2. **Migrate**: backfill data in bounded batches; dual-write from the app so old and new stay consistent. 3. **Contract**: once all deploys read the new structure, remove the old one in a *later* deploy. Each step is independently deployable and reversible. A rename or type change is never one migration, it is a sequence spanning multiple deploys. ## The Golden Rules - Add columns **nullable**; never `NOT NULL DEFAULT <volatile>` in one shot on a large table. - Build indexes **concurrently** (Postgres) or **in place, lock-free** (MySQL); otherwise use an online-DDL tool. - Add constraints as **`NOT VALID`** first, then **`VALIDATE`** in a separate statement (short lock, then a scan that does not bloc