data-migrationlisted

# Data Migration ## The Law ``` EVERY MIGRATION NEEDS A ROLLBACK. A migration without a rollback plan is a one-way door. Before you open it, know how to close it. ``` ## When to Use Use this skill before: - Any `ALTER TABLE` in production - Any column rename, type change, or removal - Any large `UPDATE` or `DELETE` (more than 1000 rows) - Any index creation on a large table - Any data backfill or transformation script ## The Migration Process ### Step 1: Classify the Migration | Type | Risk | Strategy | |------|------|---------| | Additive (new table, new nullable column) | Low | Single deploy | | Non-breaking change (add index, add constraint) | Low–Medium | Verify impact first | | Column rename or type change | High | Multi-phase deploy | | Column or table removal | High | Multi-phase deploy | | Large data backfill (>100k rows) | Medium | Batched, off-peak | | Destructive (DROP TABLE, DELETE) | Critical | Backup + explicit approval | ### Step 2: Write the Migration Plan Save to `docs/migrations/YYYY-MM-DD-description.md`: ```markdown # Migration: [Title] ## Summary [What changes and why] ## Risk Level [Low / Medium / High / Critical] ## Migration Steps ### UP (apply) \```sql -- Step 1: Add new column (nullable — safe, no lock) ALTER TABLE orders ADD COLUMN discount_amount NUMERIC(10,2); -- Step 2: Backfill (batched — see script) -- Run: scripts/migrations/backfill-discount.sql -- Step 3: Add NOT NULL constraint after backfill ALTER TABLE orders ALTER COLUMN