為了保證事務的回退和滿足多使用者的CR， oracle引入了undo 機制，由於undo是迴圈使用的，在一個事務完成過程中，它與redo相互配合，其中undo在一次事務中需要完成以下工作：

(1) Transaction 開始前回滾段獲取一個ITL(事務槽)，分配空間，記錄事務資訊

(2) Transaction 提交後，redo完成記錄，同時還清除回滾段的事務資訊包括行級鎖，ITL資訊(commit 標誌，SCN等)

清除這些事務段的資訊的過程就叫做塊清除，在完成塊清除時, 我們本事務修改的資料塊就會存在兩種可能(1) 所有的資料塊還儲存在 buffer cache 中， (2)部分資料塊或者是全部資料塊由於LRU管理已經被刷出了buffer cache。oracle為了考慮到塊清除的成本，以及效能，會作以下兩種方式的塊清除處理：

(1) 快速塊清除(fast block cleanout), 當事務修改的資料庫全部儲存在buffer cache 並且修改資料塊的資料量沒有超過 cache buffer 的 10%，快速清除事務資訊。

(2) 延遲塊清除(delayed block cleanout) 當修改的資料塊的閥值超過10% 或者本次事務相關的資料塊已經被刷出了 buffer cache， oracle 會下次訪問此block 時再清除事務資訊。

下面透過一個實驗測試，來熟悉一下delayed block cleanout的處理

SQL> select * from v$version ;

BANNER

----------------------------------------------------------------

Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - Prod

PL/SQL Release 10.2.0.1.0 - Production

CORE 10.2.0.1.0 Production

TNS for Linux: Version 10.2.0.1.0 - Production

NLSRTL Version 10.2.0.1.0 – Production

SQL> show parameter undo;

NAME TYPE VALUE

------------------------------------ ----------- ------------------------------

undo_management string AUTO

undo_retention integer 900

undo_tablespace string UNDOTBS1

SQL> conn gabriel/gabriel

Connected.

SQL> create table test_delayed as select * from user_objects;

Table created.

SQL> select count(1) from test_delayed;

COUNT(1)

----------

3

SQL> update test_delayed set object_id=1 where object_name='TEST_REDO';

1 row updated.

SQL> update test_delayed set object_id=2 where object_name='TEST_DELAYED';

1 row updated.

SQL> -----不提交

查詢回滾段資訊:

SQL> col segment_name for a20;

select owner,segment_name,SEGMENT_ID,FILE_ID,BLOCK_ID,STATUS

SQL> tablespace_name from dba_rollback_segs;

2

OWNER SEGMENT_NAME SEGMENT_ID FILE_ID BLOCK_ID TABLESPACE_NAME

------ -------------------- ---------- ---------- ---------- ----------------

SYS SYSTEM 0 1 9 ONLINE

PUBLIC _SYSSMU1$ 1 2 9 ONLINE

PUBLIC _SYSSMU2$ 2 2 25 ONLINE

PUBLIC _SYSSMU3$ 3 2 41 ONLINE

PUBLIC _SYSSMU4$ 4 2 57 ONLINE

PUBLIC _SYSSMU5$ 5 2 73 ONLINE

PUBLIC _SYSSMU6$ 6 2 89 ONLINE

PUBLIC _SYSSMU7$ 7 2 105 ONLINE

PUBLIC _SYSSMU8$ 8 2 121 ONLINE

PUBLIC _SYSSMU9$ 9 2 137 ONLINE

PUBLIC _SYSSMU10$ 10 2 153 ONLINE

11 rows selected.

查詢事務資訊

SQL> select xidusn,xidslot,xidsqn,ubablk,ubafil,ubarec from v$transaction;

XIDUSN XIDSLOT XIDSQN UBABLK UBAFIL UBAREC

---------- ---------- ---------- ---------- - --------- ----------

6 40 427 99 2 7

查詢該活動事務所在的回滾段

SQL> select * from v$rollname where usn = &usn;

Enter value for usn: 6

old 1: select * from v$rollname where usn = &usn

new 1: select * from v$rollname where usn = 6

USN NAME

---------- ------------------------------

6 _SYSSMU6$

查詢 test_delayed 物件所在的 fileid 和 blockid 由於資料物件還存在buffer

SQL> select b.segment_name,a.file#,a.dbarfil,a.dbablk,a.class,

2 a.state,decode(bitand(flag,1), 0, 'N', 'Y') DIRTY

3 from x$bh a,dba_extents b

4 where b.RELATIVE_FNO = a.dbarfil

5 and b.BLOCK_ID <= a.dbablk and b.block_id + b.blocks > a.dbablk

6 and b.owner='GABRIEL' and b.segment_name='TEST_DELAYED';

SEGMENT_NAME FILE# DBARFIL DBABLK CLASS STATE D

-------------------- ---------- ---------- ---------- ---------- ---------- -

TEST_DELAYED 8 8 28 1 1 N

TEST_DELAYED 8 8 28 1 3 N

TEST_DELAYED 8 8 27 4 1 N

TEST_DELAYED 8 8 27 4 3 N

由上可知：x$bh.class= 4 表示為 segment header x$bh.state =3 為前映象塊，因此file#=8

Dbablk=28 為資料塊

SQL> alter system dump datafile 8 block 28;

System altered.

SQL> alter system dump undo header '_SYSSMU6$';

System altered.

SQL> alter system dump datafile 2 block 99;

System altered.

SQL> @gettrname.sql

TRACE_FILE_NAME

--------------------------------------------------------------------------------

/u01/app/oracle/admin/gabriel/udump/gabriel_ora_4756.trc

以下為 trace 檔案中的截圖部分

Block header dump: 0x0200001c

Object id on Block? Y

seg/obj: 0xcf1a csc: 0x00.f4707 itc: 3 flg: E typ: 1 - DATA

brn: 0 bdba: 0x2000019 ver: 0x01 opc: 0

inc: 0 exflg: 0

Itl Xid Uba Flag Lck Scn/Fsc

0x01 0xffff.000.00000000 0x00000000.0000.00 C--- 0 scn 0x0000.000f4707

0x02 0x0006.028.000001ab 0x00800063.0156.07 ---- 2 fsc 0x0004.00000000

0x03 0x0000.000.00000000 0x00000000.0000.00 ---- 0 fsc 0x0000.00000000

---事務資訊存在

tab 0, row 1, @0x1cf8

tl: 76 fb: --H-FL-- lb: 0x2 cc: 12

col 0: [ 9] 54 45 53 54 5f 52 45 44 4f

col 1: *NULL*

col 2: [ 2] c1 02

col 3: [ 4] c3 06 1d 2d

col 4: [ 5] 54 41 42 4c 45

col 5: [ 7] 78 6f 06 17 0d 0c 23

col 6: [ 7] 78 6f 06 17 0d 0c 23

col 7: [19] 32 30 31 31 2d 30 36 2d 32 33 3a 31 32 3a 31 31 3a 33 34

col 8: [ 5] 56 41 4c 49 44

col 9: [ 1] 4e

col 10: [ 1] 4e

col 11: [ 1] 4e

tab 0, row 2, @0x1ca9

tl: 79 fb: --H-FL-- lb: 0x2 cc: 12

col 0: [12] 54 45 53 54 5f 44 45 4c 41 59 45 44

col 1: *NULL*

col 2: [ 2] c1 03

col 3: [ 4] c3 06 1f 13

col 4: [ 5] 54 41 42 4c 45

col 5: [ 7] 78 6f 08 14 16 02 35

col 6: [ 7] 78 6f 08 14 16 02 35

col 7: [19] 32 30 31 31 2d 30 38 2d 32 30 3a 32 31 3a 30 31 3a 35 32

col 8: [ 5] 56 41 4c 49 44

col 9: [ 1] 4e

col 10: [ 1] 4e

col 11: [ 1] 4e

end_of_block_dump

End dump data blocks tsn: 9 file#: 8 minblk 28 maxblk 28

*** 2011-08-20 23:25:38.403

---鎖資訊也存在

再來仔細看看undo segment header 資訊：

TRN TBL::

index state cflags wrap# uel scn dba parent-xid nub stmt_num cmt

------------------------------------------------------------------------------------------------

0x28 10 0x80 0x01ab 0x0002 0x0000.000f59ac 0x00800063 0x0000.000.00000000 0x00000001 0x00000000 0

0x28 轉化為十進位制為 40 剛好為前面查詢的事務槽， state 狀態為10 表示活動事務，dba 0x00800063 轉化為2進位制 0000 0000 1000 0000 0000 0000 0110 0011 根據dba的轉換 2號檔案的 64+32+2+1 =99

接下來我們看看 undo 段的轉存資訊(限於篇幅，只擷取了部分關鍵資訊)

Start dump data blocks tsn: 1 file#: 2 minblk 99 maxblk 99

buffer tsn: 1 rdba: 0x00800063 (2/99)

scn: 0x0000.000f59c1 seq: 0x01 flg: 0x04 tail: 0x59c10201

frmt: 0x02 chkval: 0xa7ae type: 0x02=KTU UNDO BLOCK

Hex dump of block: st=0, typ_found=1

…….

UNDO BLK:

xid: 0x0006.028.000001ab seq: 0x156 cnt: 0x7 irb: 0x7 icl: 0x0 flg: 0x0000

Rec Offset Rec Offset Rec Offset Rec Offset Rec Offset

---------------------------------------------------------------------------

0x01 0x1ecc 0x02 0x1e00 0x03 0x1d1c 0x04 0x1c80 0x05 0x1c04

0x06 0x1ad0 0x07 0x19c4

*-----------------------------

* Rec #0x1 slt: 0x06 objn: 49948(0x0000c31c) objd: 49948 tblspc: 2(0x00000002)

* Layer: 11 (Row) opc: 1 rci 0x00

Undo type: Regular undo Last buffer split: No

Temp Object: No

Tablespace Undo: No

rdba: 0x00800062

*-----------------------------

KDO undo record:

KTB Redo

op: 0x02 ver: 0x01

op: C uba: 0x00800062.0156.1e

KDO Op code: URP row dependencies Disabled

xtype: XA flags: 0x00000000 bdba: 0x00c05be8 hdba: 0x00c05be3

itli: 2 ispac: 0 maxfr: 4858

tabn: 0 slot: 7(0x7) flag: 0x2c lock: 0 ckix: 0

ncol: 12 nnew: 2 size: 0

col 9: [ 1] 80

col 10: [ 1] 80

*-----------------------------

* Rec #0x2 slt: 0x06 objn: 49871(0x0000c2cf) objd: 49871 tblspc: 2(0x00000002)

* Layer: 10 (Index) opc: 22 rci 0x01

Undo type: Regular undo Last buffer split: No

Temp Object: No

Tablespace Undo: No

rdba: 0x00000000

*-----------------------------

index undo for leaf key operations

KTB Redo

op: 0x04 ver: 0x01

op: L itl: xid: 0x0003.02a.000001b9 uba: 0x008000d7.0174.23

………..

*-----------------------------

* Rec #0x6 slt: 0x28 objn: 53018(0x0000cf1a) objd: 53018 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: 0x00000000

*-----------------------------

uba: 0x00800063.0156.04 ctl max scn: 0x0000.000f5354 prv tx scn: 0x0000.000f535c

txn start scn: scn: 0x0000.000f59ac logon user: 64

prev brb: 8388871 prev bcl: 0

KDO undo record:

KTB Redo

op: 0x03 ver: 0x01

op: Z

KDO Op code: URP row dependencies Disabled

xtype: XA flags: 0x00000000 bdba: 0x0200001c hdba: 0x0200001b

itli: 2 ispac: 0 maxfr: 4858

tabn: 0 slot: 1(0x1) flag: 0x2c lock: 0 ckix: 183

ncol: 12 nnew: 1 size: 2

col 2: [ 4] c3 06 1d 2d

*-----------------------------

* Rec #0x7 slt: 0x28 objn: 53018(0x0000cf1a) objd: 53018 tblspc: 9(0x00000009)

* Layer: 11 (Row) opc: 1 rci 0x06

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

op: C uba: 0x00800063.0156.06

KDO Op code: URP row dependencies Disabled

xtype: XA flags: 0x00000000 bdba: 0x0200001c hdba: 0x0200001b

itli: 2 ispac: 0 maxfr: 4858

tabn: 0 slot: 2(0x2) flag: 0x2c lock: 0 ckix: 0

ncol: 12 nnew: 1 size: 2

col 2: [ 4] c3 06 1f 13

End dump data blocks tsn: 1 file#: 2 minblk 99 maxblk 99

--- irb: 0x7 最近未提交事務的起始回滾點，回滾段資訊偏移量的最後偏移地址剛好相等

-- rci 0x06 代表undo chain下一偏移量地址

上面dump資訊是整個事務在沒有commit的情況下產生，下面我們flush 一下 buffer_cache 將 buffer cache 中的前資料塊寫入dbfile，然後將事務commit，我們再認真比對dump資訊，

SQL> alter system flush buffer_cache;

System altered.

------在事務窗體進行以下操作：

SQL> show user

USER is "GABRIEL"

SQL> commit;

Commit complete.

重複以上dump操作

…….

SQL> alter system dump datafile 2 block 99;

System altered.

SQL> @gettrname.sql

TRACE_FILE_NAME

--------------------------------------------------------------------------------

/u01/app/oracle/admin/gabriel/udump/gabriel_ora_4963.trc

先看看資料塊的dump資訊

Block header dump: 0x0200001c

Object id on Block? Y

seg/obj: 0xcf1a csc: 0x00.f4707 itc: 3 flg: E typ: 1 - DATA

brn: 0 bdba: 0x2000019 ver: 0x01 opc: 0

inc: 0 exflg: 0

Itl Xid Uba Flag Lck Scn/Fsc

0x01 0xffff.000.00000000 0x00000000.0000.00 C--- 0 scn 0x0000.000f4707

0x02 0x0006.028.000001ab 0x00800063.0156.07 ---- 2 fsc 0x0004.00000000

0x03 0x0000.000.00000000 0x00000000.0000.00 ---- 0 fsc 0x0000.00000000

tab 0, row 1, @0x1cf8

tl: 76 fb: --H-FL-- lb: 0x2 cc: 12

col 0: [ 9] 54 45 53 54 5f 52 45 44 4f

col 1: *NULL*

col 2: [ 2] c1 02

col 3: [ 4] c3 06 1d 2d

col 4: [ 5] 54 41 42 4c 45

col 5: [ 7] 78 6f 06 17 0d 0c 23

col 6: [ 7] 78 6f 06 17 0d 0c 23

col 7: [19] 32 30 31 31 2d 30 36 2d 32 33 3a 31 32 3a 31 31 3a 33 34

col 8: [ 5] 56 41 4c 49 44

col 9: [ 1] 4e

col 10: [ 1] 4e

col 11: [ 1] 4e

tab 0, row 2, @0x1ca9

tl: 79 fb: --H-FL-- lb: 0x2 cc: 12

col 0: [12] 54 45 53 54 5f 44 45 4c 41 59 45 44

col 1: *NULL*

col 2: [ 2] c1 03

col 3: [ 4] c3 06 1f 13

col 4: [ 5] 54 41 42 4c 45

col 5: [ 7] 78 6f 08 14 16 02 35

col 6: [ 7] 78 6f 08 14 16 02 35

col 7: [19] 32 30 31 31 2d 30 38 2d 32 30 3a 32 31 3a 30 31 3a 35 32

col 8: [ 5] 56 41 4c 49 44

col 9: [ 1] 4e

col 10: [ 1] 4e

col 11: [ 1] 4e

end_of_block_dump

End dump data blocks tsn: 9 file#: 8 minblk 28 maxblk 28

*** 2011-08-21 00:18:07.820

可以看出資料塊的資訊與 commit 之前的內容基本吻合

再來仔細看看undo segment header 資訊：

0x27 9 0x00 0x01ab 0x0015 0x0000.000f5655 0x00800062 0x0000.000.00000000 0x00000002 0x00000000 1313850640

0x28 9 0x00 0x01ab 0xffff 0x0000.000f6a64 0x00800063 0x0000.000.00000000 0x00000001 0x00000000 1313857062

0x29 9 0x00 0x01aa 0x000a 0x0000.000f54e4 0x00800108 0x0000.000.00000000 0x00000001 0x00000000 1313849990

-----事務已經提交

接下來我們看看 undo 段的轉存資訊(限於篇幅，只擷取了部分關鍵資訊)

*** 2011-08-21 00:18:20.252

Start dump data blocks tsn: 1 file#: 2 minblk 99 maxblk 99

buffer tsn: 1 rdba: 0x00800063 (2/99)

scn: 0x0000.000f59c1 seq: 0x01 flg: 0x04 tail: 0x59c10201

frmt: 0x02 chkval: 0xa7ae type: 0x02=KTU UNDO BLOCK

Hex dump of block: st=0, typ_found=1

Dump of memory from 0x0D50A600 to 0x0D50C600

UNDO BLK:

xid: 0x0006.028.000001ab seq: 0x156 cnt: 0x7 irb: 0x7 icl: 0x0 flg: 0x0000

Rec Offset Rec Offset Rec Offset Rec Offset Rec Offset

---------------------------------------------------------------------------

0x01 0x1ecc 0x02 0x1e00 0x03 0x1d1c 0x04 0x1c80 0x05 0x1c04

0x06 0x1ad0 0x07 0x19c4

*-----------------------------

* Rec #0x1 slt: 0x06 objn: 49948(0x0000c31c) objd: 49948 tblspc: 2(0x00000002)

* Layer: 11 (Row) opc: 1 rci 0x00

Undo type: Regular undo Last buffer split: No

Temp Object: No

Tablespace Undo: No

rdba: 0x00800062

*-----------------------------

KDO undo record:

KTB Redo

op: 0x02 ver: 0x01

op: C uba: 0x00800062.0156.1e

KDO Op code: URP row dependencies Disabled

xtype: XA flags: 0x00000000 bdba: 0x00c05be8 hdba: 0x00c05be3

itli: 2 ispac: 0 maxfr: 4858

tabn: 0 slot: 7(0x7) flag: 0x2c lock: 0 ckix: 0

ncol: 12 nnew: 2 size: 0

col 9: [ 1] 80

col 10: [ 1] 80

*-----------------------------

* Rec #0x2 slt: 0x06 objn: 49871(0x0000c2cf) objd: 49871 tblspc: 2(0x00000002)

* Layer: 10 (Index) opc: 22 rci 0x01

Undo type: Regular undo Last buffer split: No

Temp Object: No

Tablespace Undo: No

rdba: 0x00000000

*-----------------------------

index undo for leaf key operations

KTB Redo

op: 0x04 ver: 0x01

op: L itl: xid: 0x0003.02a.000001b9 uba: 0x008000d7.0174.23

flg: C--- lkc: 0 scn: 0x0000.000f5659

Dump kdilk : itl=2, kdxlkflg=0xc1 sdc=0 indexid=0xc059ab block=0x00c059b0

*-----------------------------

* Rec #0x6 slt: 0x28 objn: 53018(0x0000cf1a) objd: 53018 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: 0x00000000

*-----------------------------

uba: 0x00800063.0156.04 ctl max scn: 0x0000.000f5354 prv tx scn: 0x0000.000f535c

txn start scn: scn: 0x0000.000f59ac logon user: 64

prev brb: 8388871 prev bcl: 0

KDO undo record:

KTB Redo

op: 0x03 ver: 0x01

op: Z

KDO Op code: URP row dependencies Disabled

xtype: XA flags: 0x00000000 bdba: 0x0200001c hdba: 0x0200001b

itli: 2 ispac: 0 maxfr: 4858

tabn: 0 slot: 1(0x1) flag: 0x2c lock: 0 ckix: 183

ncol: 12 nnew: 1 size: 2

col 2: [ 4] c3 06 1d 2d

*-----------------------------

* Rec #0x7 slt: 0x28 objn: 53018(0x0000cf1a) objd: 53018 tblspc: 9(0x00000009)

* Layer: 11 (Row) opc: 1 rci 0x06

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

op: C uba: 0x00800063.0156.06

KDO Op code: URP row dependencies Disabled

xtype: XA flags: 0x00000000 bdba: 0x0200001c hdba: 0x0200001b

itli: 2 ispac: 0 maxfr: 4858

tabn: 0 slot: 2(0x2) flag: 0x2c lock: 0 ckix: 0

ncol: 12 nnew: 1 size: 2

col 2: [ 4] c3 06 1f 13

由兩次的dump 對比可以得出：塊延遲清除只是更改了 undo segment header 的事務資訊狀態，資料塊與 undo 塊資訊均保持不變

--

SQL> set echo off

SQL> set autotrace on

SQL> select * from gabriel.test_delayed; ---將資料塊再次快取在 cache buffer

Note

-----

- dynamic sampling used for this statement

Statistics

----------------------------------------------------------

178 recursive calls

0 db block gets

27 consistent gets

7 physical reads

72 redo size

1364 bytes sent via SQL*Net to client

385 bytes received via SQL*Net from client

2 SQL*Net roundtrips to/from client

4 sorts (memory)

0 sorts (disk)

3 rows processed

再次轉存資料塊資訊

SQL> alter system dump datafile 8 block 28;

System altered.

SQL> @gettrname.sql

TRACE_FILE_NAME

--------------------------------------------------------------------------------

/u01/app/oracle/admin/gabriel/udump/gabriel_ora_5017.trc

再次觀察資料塊的dump資訊

*** 2011-08-21 00:50:23.744

*** SERVICE NAME:(SYS$USERS) 2011-08-21 00:50:23.743

*** SESSION ID:(152.73) 2011-08-21 00:50:23.743

Start dump data blocks tsn: 9 file#: 8 minblk 28 maxblk 28

buffer tsn: 9 rdba: 0x0200001c (8/28)

Block header dump: 0x0200001c

Object id on Block? Y

seg/obj: 0xcf1a csc: 0x00.f6e54 itc: 3 flg: E typ: 1 - DATA

brn: 0 bdba: 0x2000019 ver: 0x01 opc: 0

inc: 0 exflg: 0

Itl Xid Uba Flag Lck Scn/Fsc

0x01 0xffff.000.00000000 0x00000000.0000.00 C--- 0 scn 0x0000.000f4707

0x02 0x0006.028.000001ab 0x00800063.0156.07 C--- 0 scn 0x0000.000f6a64

0x03 0x0000.000.00000000 0x00000000.0000.00 ---- 0 fsc 0x0000.00000000

---事務資訊清除

block_row_dump:

tab 0, row 0, @0x1f1d

tl: 99 fb: --H-FL-- lb: 0x0 cc: 12

col 0: [30]

42 49 4e 24 70 6c 6f 37 43 4f 51 50 35 34 54 67 51 4b 6a 41 62 67 41 4d 4a

51 3d 3d 24 30

col 1: *NULL*

col 2: [ 4] c3 06 1d 19

col 3: [ 4] c3 06 1d 19

col 4: [ 5] 54 41 42 4c 45

col 5: [ 7] 78 6f 04 1a 05 1c 21

col 6: [ 7] 78 6f 06 17 0d 08 3c

col 7: [19] 32 30 31 31 2d 30 36 2d 32 33 3a 31 32 3a 30 37 3a 35 39

col 8: [ 5] 56 41 4c 49 44

col 9: [ 1] 4e

col 10: [ 1] 4e

col 11: [ 1] 4e

tab 0, row 1, @0x1cf8

tl: 76 fb: --H-FL-- lb: 0x0 cc: 12

col 0: [ 9] 54 45 53 54 5f 52 45 44 4f

col 1: *NULL*

col 2: [ 2] c1 02

col 3: [ 4] c3 06 1d 2d

col 4: [ 5] 54 41 42 4c 45

col 5: [ 7] 78 6f 06 17 0d 0c 23

col 6: [ 7] 78 6f 06 17 0d 0c 23

col 7: [19] 32 30 31 31 2d 30 36 2d 32 33 3a 31 32 3a 31 31 3a 33 34

col 8: [ 5] 56 41 4c 49 44

col 9: [ 1] 4e

col 10: [ 1] 4e

col 11: [ 1] 4e

tab 0, row 2, @0x1ca9

tl: 79 fb: --H-FL-- lb: 0x0 cc: 12

col 0: [12] 54 45 53 54 5f 44 45 4c 41 59 45 44

col 1: *NULL*

col 2: [ 2] c1 03

col 3: [ 4] c3 06 1f 13

col 4: [ 5] 54 41 42 4c 45

col 5: [ 7] 78 6f 08 14 16 02 35

col 6: [ 7] 78 6f 08 14 16 02 35

---鎖資訊已經清除

總結整個block delaye cleanout 過程

(1) 本次事務相關的資料塊已經被刷出了 buffer cache，當本次事務提交後，事務相關的data block ,undo block 上的事務資訊，鎖資訊不會被清除。

(2) 當data block 再次進入buffer cache， oracle在讀取次資料塊時作事務資訊鎖資訊的清除處理

oracle 塊延遲清除(delayed block cleanout) 理解

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