执行一个大的事务失败了。ORACLE是怎么rollback的？

zoracledba

The old data image before update will be pushed into rollback segment.
If you do update of many rows and your undo tablespace is too small, you will get ORA-01555 error say: "snapshot too old, rollback segment too small".
If you did not get the error, then after the system crash and abort your update process, database will rollback everything based on the old data image in the rollback segment.

Anyway, try to avoid this kind of mass data update,  Use PL/SQL to do the mass update, and do COMMIT after some number of rows.

Thanks!
zOracleDBA