概述
undo保存修改數據的前鏡像，用來回滾或者一致性讀。
那麼undo裏存的具體內容是什麼？是block的全鏡像嗎？
解答這個問題，需要知道：
- update的內部過程
- oracle中事務的關鍵信息
- 如何定位事務對應的undo block
- 如何查看undo block的信息
1)update的過程
- 事務表登記1條事務記錄
- 向undo block中保存舊值
- 修改data block，保存新值
- 其他信息。
2)oracle中事務的關鍵信息
可通過v$transaction的字段瞭解。
關鍵信息包括：
ADDR事物狀態地址、 id、UBAFIL+UBABLK undo block地址、STATUS 狀態、IO統計信息等
Column Datatype Description
ADDR RAW(4 | 8) Address of the transaction state object
XIDUSN NUMBER Undo segment number
XIDSLOT NUMBER Slot number
XIDSQN NUMBER Sequence number
UBAFIL NUMBER Undo block address (UBA) filenum
UBABLK NUMBER UBA block number
UBASQN NUMBER UBA sequence number
UBAREC NUMBER UBA record number
STATUS VARCHAR2(16) Status
START_TIME VARCHAR2(20) Start time (wall clock)
START_SCNB NUMBER Start system change number (SCN) base
START_SCNW NUMBER Start SCN wrap
START_UEXT NUMBER Start extent number
START_UBAFIL NUMBER Start UBA file number
START_UBABLK NUMBER Start UBA block number
START_UBASQN NUMBER Start UBA sequence number
START_UBAREC NUMBER Start UBA record number
SES_ADDR RAW(4 | 8) User session object address
FLAG NUMBER Flag
SPACE VARCHAR2(3) YES if a space transaction
RECURSIVE VARCHAR2(3) YES if a recursive transaction
NOUNDO VARCHAR2(3) YES if a no undo transaction
PTX VARCHAR 2(3) YES if parallel transaction
NAME VARCHAR2(256) Name of a named transaction
PRV_XIDUSN NUMBER Previous transaction undo segment number
PRV_XIDSLT NUMBER Previous transaction slot number
PRV_XIDSQN NUMBER Previous transaction sequence number
PTX_XIDUSN NUMBER Rollback segment number of the parent XID
PTX_XIDSLT NUMBER Slot number of the parent XID
PTX_XIDSQN NUMBER Sequence number of the parent XID
DSCN-B NUMBER This column is obsolete and maintained for backward compatibility. The value of this column is always equal to the value in DSCN_BASE.
DSCN-W NUMBER This column is obsolete and maintained for backward compatibility. The value of this column is always equal to the value in DSCN_WRAP.
USED_UBLK NUMBER Number of undo blocks used
USED_UREC NUMBER Number of undo records used
LOG_IO NUMBER Logical I/O
PHY_IO NUMBER Physical I/O
CR_GET NUMBER Consistent gets
CR_CHANGE NUMBER Consistent changes
START_DATE DATE Start time (wall clock)
DSCN_BASE NUMBER Dependent SCN base
DSCN_WRAP NUMBER Dependent SCN wrap
START_SCN NUMBER Start SCN
DEPENDENT_SCN NUMBER Dependent SCN
XID RAW(8) Transaction XID
PRV_XID RAW(8) Previous transaction XID
PTX_XID RAW(8) Parent transaction XID
3) 如何定位事務對應的undo block
2個方法：
法1：
通過 v$transaction，可用v$session查找事物address。
--查詢本會話id，sid 132
select * from v$mystat;
--根據會話id，查看事物狀態地址：000007FF7ACE7A88
select * from v$session s where s.SID=132;
--查看事務信息
select * from v$transaction t where t.ADDR='000007FF7ACE7A88';
法2:
通過data block中的itl信息。
根據修改記錄的rowid，找到fileno，blockno；dump block內容。
4)如何查看undo block的信息
--dump數據塊信息
alter system dump datafile 13 block 4996;
查看dump文件路徑，注意路徑一致，但文件不一定完全一致。可根據文件的更新時間判斷。
select * from v$diag_info t where t.NAME = 'Default Trace File';
設計實驗
實驗步驟
1.更新多條記錄不提交
2.從mystat查詢當前sessionid
3.根據sessionid從transaction表查詢xid、uba
4.dump datablock，從itl中看uba與事務表是否一致
5.dump undo block，查看其中的內容
具體過程：
1.更新多條記錄不提交
當前數據
| DEPT_NO | SAL EMP_NO | PADDING |
|---|---|---|
| 2 | 20200513153556 | 5000 |
| 3 | 9999 | 5001 |
| 4 | 9999 | 5002 |
創建事務：更新但不提交。
update EMP t set t.sal = 9999 where t.emp_no in (5000, 5001, 5002);
2.從事物表查詢對應的undoBlock
select tr.*,s.SID,s.SERIAL#
from (select * from v$mystat where rownum = 1) stat,
v$session s,
v$transaction tr
where stat.sid = s.SID
and s.TADDR = tr.ADDR ;
| ADDR | XIDUSN | XIDSLOT | XIDSQN | UBAFIL | UBABLK | UBASQN | UBAREC |
|---|---|---|---|---|---|---|---|
| 000007FF7AD348B0 | 9 | 3 | 269771 | 3 | 934 | 27824 | 6 |
本事務的 undo block 在 undofile 3,block sn 934.
3.從datablock中dump出undo block
查詢修改記錄對應rowid，轉換成fileno+blockNo
| DEPT_NO | SAL | EMP_NO | PADDING | ROWID | FNO BLOCKNO | ROWNO |
|---|---|---|---|---|---|---|
| 2 | 20 | 5000 | x | AAASQbAANAAABOEAAO | 13 | 4996 |
| 3 | 20 | 5001 | x | AAASQbAANAAABOEAAP | 13 | 4996 |
| 4 | 20 | 5002 | x | AAASQbAANAAABOEAAQ | 13 | 4996 |
dump data block
alter system dump datafile 13 block 4996;
Itl Xid Uba Flag Lck Scn/Fsc
0x01 0x0002.004.0004303b 0x00c0854a.7119.12 ---- 3 fsc 0x0000.00000000
0x02 0x0008.021.00000cf6 0x00c0008f.0a3c.0b --U- 2 fsc 0x0060.ebd5624b
看Flag=’----'的，標示事務還沒提交，就這實驗的事務。
解析Uba的信息
select dbms_utility.data_block_address_file(to_number('c0854a',
'xxxxxxxxxxxx')) file#,
dbms_utility.data_block_address_block(to_number('c0854a',
'xxxxxxxxxxxx')) block#
from dual;
FILE# BLOCK#
3 34122
對應的undo一致。
ps:後來整理筆記的時候，原來的日誌覆蓋了，這裏實際不一致。見諒，意在說明意思。。
4.dump undo block信息，查看
--dump數據塊信息
alter system dump datafile 3 block 934;
查看dump出的trace文件內容：
*-----------------------------
* Rec #0x4 slt: 0x03 objn: 74779(0x0001241b) objd: 74779 tblspc: 9(0x00000009)
* Layer: 11 (Row) opc: 1 rci 0x00
Undo type: Regular undo Begin trans Last buffer split: No
Temp Object: No
Tablespace Undo: No
rdba: 0x00000000Ext idx: 0
flg2: 0
*-----------------------------
uba: 0x00c003a6.6cb0.02 ctl max scn: 0x0f00.94c4621b prv tx scn: 0x0f00.94c46237
txn start scn: scn: 0x0f00.94c464f1 logon user: 93
prev brb: 12583845 prev bcl: 0
KDO undo record:
KTB Redo
op: 0x04 ver: 0x01
compat bit: 4 (post-11) padding: 0
op: L itl: xid: 0x0008.00c.00043465 uba: 0x00c01e79.78af.0d
flg: C--- lkc: 0 scn: 0x0f00.94c3a499
KDO Op code: URP row dependencies Disabled
xtype: XA flags: 0x00000000 bdba: 0x03401384 hdba: 0x03401383
itli: 1 ispac: 0 maxfr: 4858
tabn: 0 slot: 14(0xe) flag: 0x2c lock: 0 ckix: 0
ncol: 4 nnew: 1 size: 6
col 1: [ 8] c7 15 15 06 0e 10 24 39
*-----------------------------
* Rec #0x5 slt: 0x03 objn: 74779(0x0001241b) objd: 74779 tblspc: 9(0x00000009)
* Layer: 11 (Row) opc: 1 rci 0x04
Undo type: Regular undo Last buffer split: No
Temp Object: No
Tablespace Undo: No
rdba: 0x00000000
*-----------------------------
KDO undo record:
KTB Redo
op: 0x02 ver: 0x01
compat bit: 4 (post-11) padding: 0
op: C uba: 0x00c003a6.6cb0.04
KDO Op code: URP row dependencies Disabled
xtype: XA flags: 0x00000000 bdba: 0x03401384 hdba: 0x03401383
itli: 1 ispac: 0 maxfr: 4858
tabn: 0 slot: 15(0xf) flag: 0x2c lock: 0 ckix: 0
ncol: 4 nnew: 1 size: 1
col 1: [ 3] c2 64 64
*-----------------------------
* Rec #0x6 slt: 0x03 objn: 74779(0x0001241b) objd: 74779 tblspc: 9(0x00000009)
* Layer: 11 (Row) opc: 1 rci 0x05
Undo type: Regular undo Last buffer split: No
Temp Object: No
Tablespace Undo: No
rdba: 0x00000000
*-----------------------------
KDO undo record:
KTB Redo
op: 0x02 ver: 0x01
compat bit: 4 (post-11) padding: 0
op: C uba: 0x00c003a6.6cb0.05
KDO Op code: URP row dependencies Disabled
xtype: XA flags: 0x00000000 bdba: 0x03401384 hdba: 0x03401383
itli: 1 ispac: 0 maxfr: 4858
tabn: 0 slot: 16(0x10) flag: 0x2c lock: 0 ckix: 0
ncol: 4 nnew: 1 size: 1
col 1: [ 3] c2 64 64
值轉換
select UTL_RAW.CAST_TO_NUMBER(replace('c7 15 15 06 0e 10 24 39', ' ')) SAL_old1,
UTL_RAW.CAST_TO_NUMBER(replace('c2 64 64 ', ' ')) SAL_old2,
UTL_RAW.CAST_TO_NUMBER(replace('c2 64 64 ', ' ')) SAL_old3
from dual;
解析：與字段舊值一致。
| SAL_OLD1 | SAL_OLD2 | SAL_OLD3 |
|---|---|---|
| 20200513153556 | 9999 | 9999 |
總結
綜上
1.undo block是保存的數據前鏡像，但不是整block的前鏡像。
2.前鏡像只保存所修改字段的舊值。
3.rowid是是非常重要的定位信息，file、block、rownum定義一條記錄。
4.事務是修改數據的，一個重要屬性就是關聯的undo block。