Rollback of Transactions (144)

Rolling back means undoing any changes to data that have been performed by SQL
statements within an uncommitted transaction. Oracle uses undo tablespaces (or
rollback segments) to store old values. The redo log contains a record of changes.
Oracle lets you roll back an entire uncommitted transaction. Alternatively, you can roll
back the trailing portion of an uncommitted transaction to a marker called a savepoint.
All types of rollbacks use the same procedures:
■ Statement-level rollback (due to statement or deadlock execution error)
■ Rollback to a savepoint
■ Rollback of a transaction due to user request
■ Rollback of a transaction due to abnormal process termination
■ Rollback of all outstanding transactions when an instance terminates abnormally
■ Rollback of incomplete transactions during recovery
In rolling back an entire transaction, without referencing any savepoints, the
following occurs:
1. Oracle undoes all changes made by all the SQL statements in the transaction by
using the corresponding undo tablespace.
2. Oracle releases all the transaction’s locks of data.
3. The transaction ends.

回滚事务
1. Oracle使用回滚表空间来撤销所有改变
2. Oracle释放锁
3. 事务结束

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