Migration Skill

Purpose

Create safe, reversible database migrations.

Migration Template

Use: templates/migration-template.sql

-- Migration: [description]
-- Created: [date]
-- Author: [name]

-- ==================== UP ====================
BEGIN;

-- Your migration here
CREATE TABLE users (
  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
  email VARCHAR(255) NOT NULL UNIQUE,
  created_at TIMESTAMP DEFAULT NOW()
);

COMMIT;

-- ==================== DOWN ====================
BEGIN;

DROP TABLE IF EXISTS users;

COMMIT;

Pre-Migration Checklist

Use: checklists/pre-migration.md

• Down migration works
• Tested on production-like data
• Performance impact assessed
• Backup plan documented
• Deployment timing considered

Migration Types

Safe Migrations

• Add table
• Add nullable column
• Add index (CONCURRENTLY)
• Add foreign key (without validation)

Risky Migrations

• Drop table (verify no references)
• Drop column (verify no usage)
• Rename column (may break app)
• Change column type (may lose data)

Dangerous Migrations

• Truncate table
• Drop database
• Remove constraints

Large Table Migrations

For tables with >1M rows:

1. Create new structure
2. Backfill in batches
3. Add constraints
4. Switch over
5. Clean up old structure

Rollback Strategy

• Test down migration before running up
• Document manual rollback steps
• Have production backup
• Consider feature flags for code changes