NO AUTO UNDO

The NO AUTO UNDO option affects the handling of record versions (garbage) produced by the transaction in the event of rollback.With NO AUTO UNDO flagged, the ROLLBACK statement marks the transaction as rolled back without deleting the record versions created in the transaction.They are left to be mopped up later by garbage collection.

NO AUTO UNDO might be useful when a lot of separate statements are executed that change data in conditions where the transaction is likely to be committed successfully most of the time.

The NO AUTO UNDO option is ignored for transactions where no changes are made.

According to the Firebird sources, this will

The exact semantics and effects of this clause are not clear, and we recommend you do not use this clause.