轉載: 原文地址:http://blog.csdn.net/oradh/article/details/25632529
- SQL語句執行時,需要讀取UNDO(前映像數據)來構造CR數據塊,完成一致性讀取。但是在讀取undo前映像數據時發現,undo信息已經被覆蓋(undo空間循環使用),不能構造一致性讀的CR塊,拋出ORA-01555錯誤
- SQL語句執行時,訪問到的數據塊,需要進行延遲塊清除,但是在進行延遲塊清除時,不能確定這個數據塊的事務提交時間與SQL執行開始時間的先後次序,從而拋出ORA-01555錯誤
- 增加UNDO空間,延緩UNDO信息被覆蓋,也可以理解爲增加undo空間循環使用一次的時間。
- 優化拋出錯誤的SQL語句,縮短SQL語句執行的時間,也可以避免語句需要訪問undo信息被覆蓋。
- 避免頻繁的提交也是一種可行方法,不過需要改動的量較前兩個都要大。提交頻率降低後,導致undo 信息被覆蓋的可能性也降低了(oracle數據庫中未提交的undo是不可能被覆蓋),或者undo 事務表被覆蓋的可能性也降低了。
Tablespace created.
SQL> ALTER SYSTEM SET undo_tablespace='UNDO1';
System altered.
Connected.
SQL> create table test as select object_id,object_name from dba_objects;
Table created.
SQL> SELECT
2 dbms_rowid.rowid_object(rowid) object_id,
3 dbms_rowid.rowid_relative_fno(rowid) REL_FNO,
4 dbms_rowid.rowid_block_number(rowid) BLOCKNO,
5 dbms_rowid.rowid_row_number(rowid) ROWNO,rowid,object_id,object_name
6 FROM test WHERE object_name='EMP';
OBJECT_ID REL_FNO BLOCKNO ROWNO ROWID OBJECT_ID
---------- ---------- ---------- ---------- ------------------ ----------
OBJECT_NAME
--------------------
73424 4 611 201 AAAR7QAAEAAAAJjADJ 73201
EMP
SQL> select * from test where rowid='AAAR7QAAEAAAAJjADJ';
OBJECT_ID OBJECT_NAME
---------- --------------------
73201 EMP
Block header dump: 0x01000263
Object id on Block? Y
seg/obj: 0x11ed0 csc: 0x00.f3559 itc: 3 flg: E typ: 1 - DATA
brn: 0 bdba: 0x1000201 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 0x0000.000f3559
0x02 0x0000.000.00000000 0x00000000.0000.00 ---- 0 fsc 0x0000.00000000
0x03 0x0000.000.00000000 0x00000000.0000.00 ---- 0 fsc 0x0000.00000000
SQL> begin
2 open :cur1 for select * from test where rowid='AAAR7QAAEAAAAJjADJ';
3 end;
4 /
PL/SQL procedure successfully completed.
1 row updated.
SQL> select * from test where rowid='AAAR7QAAEAAAAJjADJ';
OBJECT_ID OBJECT_NAME
---------- --------------------
73201 DH
SQL> commit;
Commit complete.
SQL> update test set object_name='DH' where rowid='AAAR7QAAEAAAAJjADJ'; ------第二次更新,實驗以此爲準
1 row updated.
Block header dump: 0x01000263
Object id on Block? Y
seg/obj: 0x11ed0 csc: 0x00.f3a21 itc: 3 flg: E typ: 1 - DATA
brn: 0 bdba: 0x1000201 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 0x0000.000f3559
0x02 0x0010.001.00000004 0x014000d4.0001.2b C--- 0 scn 0x0000.000f3a01
0x03 0x0012.003.00000005 0x014000f4.0001.01 ---- 1 fsc 0x0000.00000000
bdba: 0x01000263
A transaction identifier consists of:
–Undo segment number
–Transaction table slot number
–Sequence number or wrap#
XID = usn# . slot# . wrap#
A UBA consists of:
–Data block address (DBA) of the block
–The sequence number of the block
–The record number within the block
UBA = DBA. seq#. rec#
SQL> select dbms_utility.data_block_address_file(to_number(substr(uba,3,8),'xxxxxxxxxxx')) undo_file#,
2 dbms_utility.data_block_address_block(to_number(substr(uba,3,8),'xxxxxxxxxxx')) blockno#,
3 to_number(substr(uba,12,4),'xxxxxxxxx') undo_seq,
4 to_number(substr(uba,17,2),'xxxxxxxxx') undo_record
5 from (select '0x014000f4.0001.01' uba from dual);
UNDO_FILE# BLOCKNO# UNDO_SEQ UNDO_RECORD
---------- ---------- ---------- -----------
5 244 1 1
tab 0, row 201, @0x4e7
tl: 11 fb: --H-FL-- lb: 0x3 cc: 2 --lb:0x3 對應上面的第三個itl事務槽,表示這一被鎖定,即session 1中的更新操作導致
col 0: [ 4] c3 08 21 02
col 1: [ 2] 44 48
2 to_char(start_scnb,'xxxxxxxx') start_scnb, start_scnb+start_scnw*power(2,32) start_scn
3 from v$transaction;
XIDUSN XIDSLOT XIDSQN UBABLK UBAFIL UBAREC START_SCNW START_SCNB START_SCN
---------- - --------- ---------- ---------- - --------- ---------- ---------- ---------- ---------
18 3 5 244 5 1 0 f3a22 997922
可以看到xidusn對應xid中的0x0012(十六進制轉換爲十進制後爲18),xidslot對應xid中的003,xidsqn對應xid中的00000005
uba中的5、244、1分別對應UBAFIL、UBABLK、UBAREC
mgc: 0xb000 xts: 0x0068 flg: 0x0001 opt: 2147483646 (0x7ffffffe)
uba: 0x014000f4.0001.01 scn: 0x0000.000f36b8 -----0x014000f4.0001.01對應數據塊中ITL中的UBA
Version: 0x01
FREE BLOCK POOL::
uba: 0x00000000.0001.26 ext: 0x0 spc: 0xc6a
uba: 0x00000000.0000.00 ext: 0x0 spc: 0x0
uba: 0x00000000.0000.00 ext: 0x0 spc: 0x0
uba: 0x00000000.0000.00 ext: 0x0 spc: 0x0
uba: 0x00000000.0000.00 ext: 0x0 spc: 0x0
TRN TBL::
index state cflags wrap# uel scn dba parent-xid nub stmt_num cmt
------------------------------------------------------------------------------------------------
0x00 9 0x00 0x0005 0x0001 0x0000.000f39c4 0x014000f3 0x0000.000.00000000 0x00000001 0x00000000 1399878011
0x01 9 0x00 0x0005 0x0002 0x0000.000f39ce 0x014000f3 0x0000.000.00000000 0x00000001 0x00000000 1399878011
0x02 9 0x00 0x0005 0xffff 0x0000.000f39d7 0x00000000 0x0000.000.00000000 0x00000000 0x00000000 1399878011
0x03 10 0x80 0x0005 0x0000 0x0000.000f3a22 0x014000f4 0x0000.000.00000000 0x00000001 0x00000000 0
0x04 9 0x00 0x0004 0x0005 0x0000.000f3722 0x014000f2 0x0000.000.00000000 0x00000001 0x00000000 1399876988
0x05 9 0x00 0x0004 0x0006 0x0000.000f372d 0x014000f2 0x0000.000.00000000 0x00000001 0x00000000 1399876988
0x06 9 0x00 0x0004 0x0007 0x0000.000f3736 0x014000f2 0x0000.000.00000000 0x00000001 0x00000000 1399876988
....後面的省略.....
UNDO BLK:
xid: 0x0012.003.00000005 seq: 0x1 cnt: 0x1 irb: 0x1 icl: 0x0 flg: 0x0000
Rec Offset Rec Offset Rec Offset Rec Offset Rec Offset
---------------------------------------------------------------------------
0x01 0x1f64
*-----------------------------
* Rec #0x1 slt: 0x03 objn: 73424(0x00011ed0) objd: 73424 tblspc: 4(0x00000004)
* 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: 0x014000f3.0001.27 ctl max scn: 0x0000.000f35fb prv tx scn: 0x0000.000f36b8
txn start scn: scn: 0x0000.000f3a22 logon user: 85
prev brb: 20971762 prev bcl: 0
KDO undo record:
KTB Redo
op: 0x03 ver: 0x01
compat bit: 4 (post-11) padding: 1
op: Z
KDO Op code: URP row dependencies Disabled
xtype: XAxtype KDO_KDOM2 flags: 0x00000080 bdba: 0x01000263 hdba: 0x010000aa
itli: 3 ispac: 0 maxfr: 4858
tabn: 0 slot: 201(0xc9) flag: 0x2c lock: 0 ckix: 0
ncol: 2 nnew: 1 size: 0
Vector content:
col 1: [ 2] 44 48 ---根據下面的查詢,可以知道保存的前映像數據爲‘DH',與我們實驗相符
SQL> SELECT utl_raw.cast_to_varchar2('4448') from dual;
UTL_RAW.CAST_TO_VARCHAR2('4448')
--------------------------------------------------------------------------------
DH
Commit complete.
SQL> create table test1 as select object_id, object_type from dba_objects;
Table created.
SQL> update test1 set object_type='AAAAAAAAAA' where rownum<100;
99 rows updated.
SQL> /
99 rows updated.
SQL> /
99 rows updated.
SQL> /
99 rows updated.
SQL> /
99 rows updated.
SQL> /
99 rows updated.
SQL> /
99 rows updated.
SQL> /
update test1 set object_type='AAAAAAAAAA' where rownum<100
*
ERROR at line 1:
ORA-30036: unable to extend segment by 8 in undo tablespace 'UNDO1'
SQL> begin
2 for i in 1..10000 loop
3 update test1 set object_type='xxxxxx' where rownum<3;
4 commit;
5 end loop;
6 end;
7 /
Object id on Block? Y
seg/obj: 0x11ed0 csc: 0x00.f4247 itc: 3 flg: E typ: 1 - DATA
brn: 0 bdba: 0x1000201 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 0x0000.000f3559
0x02 0x0010.001.00000004 0x014000d4.0001.2b C--- 0 scn 0x0000.000f3a01
0x03 0x0012.003.00000005 0x014000f4.0001.01 C--- 0 scn 0x0000.000f407e
bdba: 0x01000263
xid: 0x0012.001.0000001f seq: 0x7 cnt: 0x3f irb: 0x3f icl: 0x0 flg: 0x0000
Rec Offset Rec Offset Rec Offset Rec Offset Rec Offset
---------------------------------------------------------------------------
0x01 0x1f60 0x02 0x1f1c 0x03 0x1e74 0x04 0x1dec 0x05 0x1da8
0x06 0x1d00 0x07 0x1c78 0x08 0x1c34 0x09 0x1b8c 0x0a 0x1b04
0x0b 0x1ac0 0x0c 0x1a18 0x0d 0x1990 0x0e 0x194c 0x0f 0x18a4
0x10 0x181c 0x11 0x17d8 0x12 0x1730 0x13 0x16a8 0x14 0x1664
0x15 0x15bc 0x16 0x1534 0x17 0x14f0 0x18 0x1448 0x19 0x13c0
0x1a 0x137c 0x1b 0x12d4 0x1c 0x124c 0x1d 0x1208 0x1e 0x1160
0x1f 0x10d8 0x20 0x1094 0x21 0x0fec 0x22 0x0f64 0x23 0x0f20
0x24 0x0e78 0x25 0x0df0 0x26 0x0dac 0x27 0x0d04 0x28 0x0c7c
0x29 0x0c38 0x2a 0x0b90 0x2b 0x0b08 0x2c 0x0ac4 0x2d 0x0a1c
0x2e 0x0994 0x2f 0x0950 0x30 0x08a8 0x31 0x0820 0x32 0x07dc
0x33 0x0734 0x34 0x06ac 0x35 0x0668 0x36 0x05c0 0x37 0x0538
0x38 0x04f4 0x39 0x044c 0x3a 0x03c4 0x3b 0x0380 0x3c 0x02d8
0x3d 0x0250 0x3e 0x020c 0x3f 0x0164
*-----------------------------
* Rec #0x1 slt: 0x0f objn: 73428(0x00011ed4) objd: 73428 tblspc: 4(0x00000004)
* 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: 0x014000f3.0007.31 ctl max scn: 0x0000.000f9d10 prv tx scn: 0x0000.000f9d31
txn start scn: scn: 0x0000.000f9f44 logon user: 85
prev brb: 20971762 prev bcl: 0
KDO undo record:
KTB Redo
op: 0x04 ver: 0x01
compat bit: 4 (post-11) padding: 1
op: L itl: xid: 0x0012.00e.0000001e uba: 0x014000f3.0007.33
flg: C--- lkc: 0 scn: 0x0000.000f9f3c
KDO Op code: LKR row dependencies Disabled
xtype: XA flags: 0x00000000 bdba: 0x0100012b hdba: 0x0100012a
itli: 2 ispac: 0 maxfr: 4858
tabn: 0 slot: 0 to: 0 ---已經不存在session 1中第一個事務的undo信息
ERROR:
ORA-01555: snapshot too old: rollback segment number 18 with name "_SYSSMU18$"
too small
alter system dump undo header '_SYSSMU18$';
alter system dump datafile 5 block 244; 或者 alter system dump undo block '_SYSSMU18$' xid 18 3 5;