When you undo (roll back) a transaction, you reverse the transaction. The database is switched to the consistent state it had before the transaction was started.
To enable a transaction to be undone, the original values (those that existed before the transaction was started) are saved for all database objects that will be changed by the transaction. This information is known as an undo log entry (or “before-image” entry). Before an SQL statement that changes database objects is executed, the undo log entries are stored in an undo log file in the data area. Every transaction can access its undo log entries at any time.
liveCache database instances also use undo log entries to read data consistently and without locks.
See also:
Concepts of the Database System
· Using Log Entries for a Restart or Recovery
· Example: Reading Consistent Data Without Locking (liveCache)