Is this migration safe to run?

Before running a schema change against real data.

Review this database migration for safety against a production database with real data.

Check:
- Does it lock tables, and for how long at realistic row counts?
- Is it backwards compatible with the currently deployed code? Can old and new run simultaneously during deploy?
- Is it reversible? If not, say so loudly.
- What happens if it fails halfway through?
- Any data loss — dropped columns, narrowed types, truncation, constraint violations on existing rows?

Give a verdict: safe, safe with caveats, or do not run. If it needs splitting into multiple deploys, show the sequence.

Migration:

Reviewing a change. Passes you run over a diff before it merges: correctness, security, concurrency, migrations.