SQL入門之12 Read Consistency

總結 回滾段用來儲存DML時的前映像，給查詢使用者提供一致讀，回退發生時，前映像用來寫回原資料。[@more@]

When an insert, update, or delete operation is made to the database, the Oracle Server takes a copy of the data before it is changed and writes it to a undo segment.
All readers, except the one who issued the change, still see the database as it existed before the changes started; they view the undo segment’s snapshot of the data.
When a DML statement is committed, the change made to the database becomes visible to anyone executing a SELECT statement. The space occupied by the "old" data in the undo segment file is freed for reuse.
If the transaction is rolled back, the changes are undone:
The original, older version, of the data in the undo segment is written back to the table.
All users see the database as it existed before the transaction began.