Transactional DDL rollback behavior under concurrent load

Transactional DDL Rollback Behavior

Goal: Determine if rolling back a large schema change is truly safe when other sessions are active.

Constraints: The rollback must not hold Sch‑M locks for an unacceptable duration, and should not cause significant blocking of readers or writers on the affected object.

Open Issues

SQL Server supports transactional DDL, but undoing a large change can be as expensive as the original operation and keeps Sch‑M locks until completion. The impact on concurrent workloads is unclear.

• Will a rollback of a large ALTER TABLE executed inside a transaction hold Sch‑M locks long enough to block other sessions?
• How does the rollback duration scale with the size of the table or the complexity of the change?
• Are there version‑specific limits or recommended patterns to mitigate blocking during rollback?