Transactions

Each migration runs in a separate transaction. Savepoints are used to isolate steps within each migration.

If an error occurs during a step and the step has ignore_errors set, then that individual step will be rolled back and execution will pick up from the next step. If ignore_errors is not set then the entire migration will be rolled back and execution stopped.

Note that some databases (eg MySQL) do not support rollback on DDL statements (eg CREATE ... and ALTER ... statements). For these databases you may need to manually intervene to reset the database state should errors occur in your migration.

Using group allows you to nest steps, giving you control of where rollbacks happen. For example:

group([
    step("ALTER TABLE employees ADD tax_code TEXT",
    step("CREATE INDEX tax_code_idx ON employees (tax_code)")
    ], ignore_errors='all'
)
step("UPDATE employees SET tax_code='C' WHERE pay_grade < 4")
step("UPDATE employees SET tax_code='B' WHERE pay_grade >= 6")
step("UPDATE employees SET tax_code='A' WHERE pay_grade >= 8")