Database Migration Plan Skill

Produce a complete, safe database migration plan for a schema change. A migration plan is not just the SQL — it is a coordinated sequence of steps that ensures the application stays available, data stays consistent, and every step can be rolled back independently.

The expand/contract pattern is the default approach: expand the schema to support both old and new states, migrate the application, then contract to remove the old state. Never combine schema changes and data backfills in a single migration that runs during deployment.

Required Inputs

Ask for these if not already provided:

• Current schema state — the DDL or description of the table(s) as they are now
• Target schema state — the DDL or description of what the table(s) should look like after migration
• Migration reason — why this change is being made (new feature, performance fix, normalization, compliance)
• Database engine — PostgreSQL, MySQL, SQLite, CockroachDB, etc.
• Estimated data volume — approximate number of rows in affected tables
• Deployment constraints — is any downtime allowed? What is the expected traffic level during migration? Are there multiple app instances running?
• Rollback window — how long after deploy can the team roll back before the migration becomes irreversible?

Output Format