Database Migration Rollback & Data Integrity Test Plan

You are a Principal Database Administrator and Migration Testing Specialist.

Review and test this database migration script for zero-downtime deployment safety:

Migration SQL (Up & Down):
```sql
-- V12__split_user_full_name.sql
ALTER TABLE users ADD COLUMN first_name VARCHAR(100);
ALTER TABLE users ADD COLUMN last_name VARCHAR(100);

UPDATE users SET 
  first_name = split_part(full_name, ' ', 1),
  last_name = split_part(full_name, ' ', 2);

ALTER TABLE users DROP COLUMN full_name;
```

Current Production Schema:
```sql
CREATE TABLE users (
  id BIGSERIAL PRIMARY KEY,
  full_name VARCHAR(255) NOT NULL,
  email VARCHAR(255) UNIQUE NOT NULL,
  created_at TIMESTAMPTZ DEFAULT NOW()
);
```

Table Volume & Traffic: 8 million rows, 200 writes/second during business hours

Analyze the migration against production safety standards:
1. **Locking & Table Rewrites**:
   - Will this migration acquire an `ACCESS EXCLUSIVE` lock that blocks reads/writes on high-traffic tables?
   - Does adding columns with default values trigger a full table rewrite?
   - Are indexes created using `CONCURRENTLY` (Postgres)?
2. **Backward Compatibility with Active App Version**:
   - Can the currently deployed version of the application run safely while this migration is applied (Expand/Contract Phase)?
3. **Data Integrity & Rollback Verification**:
   - If the rollback script executes, is there any permanent data loss for records created during the migration window?
4. **Automated Migration Test Script**:
   - Provide a test script (in SQL or Python/Testcontainers) that runs Up -> inserts dummy data -> runs Down -> asserts schema & data integrity.