Use a migration runner with durable history for one-time, versioned changes, keep released migrations immutable, and design repeatable scripts to tolerate being applied again. These are two different kinds of “safe to rerun”: invoking the migration command again should skip completed work, while re-executing an individual script must be safe for the database’s current state.
What “safe to rerun” means
A migration command can be safe to invoke repeatedly without making every migration script repeatable. A runner checks its history, records completed migrations and applies only pending work. Separately, a script may need to run again after a partial failure or because it is configured as repeatable; in that case, the script’s operations must handle the existing schema and data correctly.
In Flyway, versioned migrations run in order and are applied once. Its schema history records applied migrations, checksums and success status, so the runner can distinguish completed work from pending changes. See Flyway’s migrations documentation for the distinction between versioned and repeatable migrations and how history is used.
Choose between a one-time and repeatable migration
Use a versioned migration for one-time work
Put a schema change or one-off data correction in a uniquely versioned migration. To answer the common question, “How do you add a migration script that should only run once?”: give it a new version and let the migration runner record its successful application. Calling the runner again should not execute that completed versioned migration again.
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Once a versioned migration has been used in an environment, treat it as immutable. Its checksum helps detect edits, but a checksum is a warning mechanism, not permission to rewrite deployed history. If a released migration needs correction, add a new versioned migration that brings the database from its actual state to the intended state. Do not manually mark work complete unless you have verified the database and understand the effect on future deployments.
Use a repeatable migration for definitions meant to be refreshed
Repeatable migrations suit definitions such as views and procedures that should be recreated when their contents change. The runner uses a checksum to determine whether a repeatable migration needs to be applied again. Flyway’s documentation states: “It is your responsibility to ensure the same repeatable migration can be applied multiple times.” A common approach for database definitions is supported CREATE OR REPLACE syntax.
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For data changes that may be repeated, choose behavior for the target engine and the intended outcome: for example, a condition, a uniqueness constraint, or an upsert. A guard such as IF NOT EXISTS can avoid a duplicate-object error but still leave an object with the wrong definition. Verify the resulting schema and data instead of treating a suppressed error as proof of correctness.
Make retries safe without assuming rollback
Transactions can make a migration more atomic, but only if the database supports transactional execution for the statements involved. Flyway ordinarily wraps a migration in a transaction where supported. Some statements cannot run transactionally, and some databases implicitly commit around DDL; in those cases, a failed migration can leave changes behind. Flyway describes the possibility of manual cleanup and history repair after a failed non-transactional migration in its migration documentation.
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Liquibase also warns that a multi-statement changeset configured not to run in a transaction can leave changelog state invalid if it fails partway through. Its runInTransaction reference, last updated January 21, 2026, explains the default transactional behavior and this failure risk. Neither tool’s transaction setting overrides the database’s statement-level capabilities.
Prepare a recovery path for non-transactional work
Before deploying a migration that cannot run transactionally, isolate the non-transactional step when the tool and database permit it. Document how to inspect whether it completed, what partial effects may remain, and how to clean up or finish the operation. Do this before production deployment, not while guessing after an error.
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Use special care with PostgreSQL concurrent indexes
PostgreSQL’s CREATE INDEX CONCURRENTLY has special transaction and locking considerations. Flyway’s PostgreSQL database reference notes that Flyway’s default transactional lock can cause issues for this statement and documents an alternative session-level lock setting. Confirm the installed Flyway version and deployment configuration before using that setting.
What to do after a migration fails
A failed migration does not prove that nothing changed. If the statements ran transactionally and the database rolled them back, a retry may be appropriate after confirming the runner’s history. If the migration was non-transactional or rollback is uncertain, inspect both the actual schema and data state and the migration ledger before retrying.
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- Inspect the database. Determine which statements took effect and whether they left partial schema or data changes.
- Inspect migration history. Check whether the runner recorded the migration as failed, successful or otherwise inconsistent with the database.
- Reconcile the state. Clean up or complete partial effects using a procedure appropriate to the actual database state.
- Repair bookkeeping only when it is accurate. Use the migration tool’s repair mechanism only after the database and history have been reconciled; then apply the intended migration or a new correction as appropriate.
An undo script is not a general substitute for inspection. A multi-statement migration may fail after some statements have succeeded, leaving a state that an undo for the full migration does not address. Prefer backward-compatible schema changes during rollout and a tested backup-and-restore process over assuming an undo can reverse every partial execution. Flyway covers rollout compatibility and recovery practices in its rolling out updates guidance.
Prevent concurrent migration runs
Serialize migration execution: use one migration runner for a database change window or rely on the migration tool’s supported locking. Flyway describes a database-level lock on its schema history table so that only one concurrent migrations-based deployment proceeds at a time; see its deployment guidance.
Locking details still depend on the database and the code acquiring locks. PostgreSQL documents that under repeatable read, a transaction’s snapshot may predate a lock acquired after an earlier query. If application code uses explicit locks to coordinate concurrent changes, lock acquisition order matters; see PostgreSQL’s application-level consistency checks documentation.
Test the clean path, retry path and rollout
Exercise both command-level reruns and script-level retries. A useful test matrix includes:
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- A database already at the target version, where the runner should have no completed versioned work to repeat.
- A deliberate failure after an early statement, followed by inspection and a retry that reflects the database’s actual state.
- Two deployment processes attempting to migrate concurrently, to confirm that the supported lock or deployment procedure serializes them.
- A staged application rollout in which old and new application versions remain compatible with the intermediate database schema.
- A restore from backup, so recovery is based on a tested procedure rather than an assumed ability to undo partial DDL.
Before relying on a migration’s rollback or retry behavior, confirm the database version, migration-tool version and transactional support of the specific statements it contains. DDL behavior is not uniform across engines.
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