UNDO 100%

另外查了下v$undostat，发现begin_time已经很久没有改变，
BEGIN_TIME           END_TIME             MAXQUERYLEN MAXCONCURRENCY UNEXPIREDBLKS EXPIREDBLKS

BEGIN_TIME DATE Identifies the beginning of the time interval

unexpired的在retention guarantee没有启用的情况下，可以看作是free的，你这里还有2g多的可用空间，99%看到的是假象

select sum(bytes),status from dba_undo_extents group by status;
select sid,USED_UBLK from v$transaction a,v$session b where a.addr=b.taddr;

dba_undo_extents:描述了extents包括在所有的undo表空间中的segments

SQL> select sum(bytes),status from dba_undo_extents group by status;

SUM(BYTES) STATUS
---------- ---------
   3670016 UNEXPI