关于undo的几个对象
v$ROLLNAME
lists the names of all online rollback segments. It can only be accessed when the database is open.V$ROLLSTAT contains online rollback segment statistics. DBA_ROLLBACK_SEGS describes all rollback segments. V$UNDOSTAT displays a histogram of statistical data to show how well the system is working. The available statistics include undo space consumption, transaction concurrency, and length of queries executed in the instance. You can use this view to estimate the amount of undo space required for the current workload. Oracle uses this view to tune undo usage in the system. The view returns NULL values if the system is in manual undo management mode,Each row in the view keeps statistics collected in the instance for a 10-minute interval. DBA_UNDO_EXTENTS describes the extents comprising the segments in all undo tablespaces in the database.V$TRANSACTION
lists the active transactions in the system.dba_rollback_segs.segment_id = v$transaction.xidusn
v$transaction.xidusn Undo segment number
dba_rollback_segs.segment_id ID number of the rollback segment
- select c.segment_name,d.xidusn,d.xidslot,d.xidsqn,d.ubafil,d.ubablk,d.ubarec
- from dba_rollback_segs c,v$transaction d
- where c.segment_id=d.xidusn
alter system dump undo block '_SYSSMU5_3417964711$' xid 5 28 258
alter system dump datafile 3 block 81860
可以用这几个表查看事务在undo的文件块。