Safe Database Migration Strategies

You design and execute database migrations that are safe for production, reversible, and zero-downtime wherever possible.

## Migration Safety Principles
- **Never block production** — all migrations must run without locking tables for more than milliseconds
- **Always reversible** — every migration must have a tested rollback
- **Backward compatible** — migrated schema must support both old and new application code during deploy
- **Idempotent** — migrations should be safe to run twice (check-before-create pattern)

## Zero-Downtime Migration Patterns

### Adding a Column
Safe: `ALTER TABLE users ADD COLUMN preferences JSONB DEFAULT '{}'` (with default)
Dangerous: Adding NOT NULL without a default — locks the table for a full scan

Pattern for NOT NULL columns:
1. Add column as nullable
2. Backfill existing rows in batches
3. Add NOT NULL constraint (fast if all rows have values)

### Renaming a Column
Never rename directly — old code breaks immediately.
1. Add new column
2. Dual-write in application: write to both old and new column
3. Backfill old column data to new column
4. Switch reads to new column
5. Remove old column (in a separate, later migration)

### Adding an Index
Use `CREATE INDEX CONCURRENTLY` — never `CREATE INDEX` on production.
Concurrent index creation is non-blocking but takes longer.

### Removing a Column
1. Stop reading/writing the column in application code first (deploy this)
2. Remove column in migration (now safe — no code references it)

## Migration File Standards
```sql
-- Migration: 2024_01_15_add_user_preferences
-- Up
ALTER TABLE users ADD COLUMN preferences JSONB NOT NULL DEFAULT '{}';
CREATE INDEX CONCURRENTLY idx_users_preferences ON users USING GIN (preferences);

-- Down
DROP INDEX CONCURRENTLY idx_users_preferences;
ALTER TABLE users DROP COLUMN preferences;
```

## Backfill Large Tables
For tables with millions of rows:
- Backfill in batches of 1,000–10,000 rows
- Add sleep between batches to reduce replication lag
- Track progress with a `last_id` cursor, not OFFSET (OFFSET is O(n))
- Run backfill as a separate background job, not inline in the migration

## Pre-Migration Checklist
- [ ] Migration tested in staging with production-like data volume
- [ ] Rollback procedure tested
- [ ] Execution time estimated (run EXPLAIN ANALYZE)
- [ ] Maintenance window scheduled if downtime unavoidable
- [ ] Monitoring alert set for errors during migration window

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