What is "save undo"?

上次查询seg$表，发现了有一种段类型为save undo,顺便上网查了下，记录分享一下
看tom 的回答
save undo is undo saved for offlined tablespaces. It is also known as deferred rollback
A "Deferred Rollback" segment is created for a tablespace
when a tablespace is taken offline. Undo records are written
to the deferred rollback segment of a tablespace when undo is being applied to a
tablespace which is offline, hence, the undo
cannot be applied.
The undo stored in the deferred rollback segment is applied when
the tablespace comes back online. The deferred rollback segment
is automatically removed after the undo has been applied.
Deferred rollback segments always reside in the system tablespace.
试验一下

sys@ORCL> select * from seg$ where type#=2;
no rows selected
sys@ORCL> select * from v$tablespace;
       TS# NAME                           INC BIG FLA ENC
---------- ------------------------------ --- --- --- ---
         0 SYSTEM                         YES NO  YES
         7 TEST                           YES NO  YES
         2 SYSAUX                         YES NO  YES
         4 USERS                          YES NO  YES
         6 EXAMPLE                        YES NO  YES
         9 TT                             YES NO  YES
        19 TBSLOGMNR                      YES NO  YES
         5 UNDOTBS2                       YES NO  YES
         3 TEMP                           NO  NO  YES
        20 SMAILTBS                       YES NO  YES
        24 INDEXTBS                       YES NO  YES
11 rows selected.
sys@ORCL> alter tablespace tt offline;
Tablespace altered.
sys@ORCL> select * from seg$ where type#=2;
     FILE#     BLOCK#      TYPE#        TS#     BLOCKS    EXTENTS    INIEXTS    MINEXTS    MAXEXTS    EXTSIZE     EXTPCT      USER#      LISTS
---------- ---------- ---------- ---------- ---------- ---------- ---------- ---------- ---------- ---------- ---------- ---------- ----------
    GROUPS BITMAPRANGES  CACHEHINT   SCANHINT    HWMINCR     SPARE1     SPARE2
---------- ------------ ---------- ---------- ---------- ---------- ----------
         1      68209          2          0          8          1          5          1 2147483645        128          0          0          0
         0            0          0          0          0          5
sys@ORCL> alter tablespace tt online;
Tablespace altered.
sys@ORCL> select * from seg$ where type#=2;
no rows selected