undo transaction slot被覆盖引起ORA(13)

##从上面的输出可以看到8/16 Rec #0x19，8/17 Rec#0x1先后被覆盖，前者包含数据修改前的拷贝，后者包含了事务槽回滚所需的uba信息，这时必然导致下面session 2遇到ORA-01555错误

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//session 2:

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##久违的ORA-01555错误终于来了

select * from scott.tabnow1

*

ERROR at line 1:

ORA-01555: snapshot too old: rollback segment number 2 with name "_SYSSMU2$"

too small

alter system dump datafile 8 block min 9 block max 31;

##把所有undo block dump到文件stest2_ora_29753578.trc

alter system dump datafile 8 block min 9 block max 31;

##在trace文件中未能查到02000010.187e.19，意味着事务表不能回溯到足够旧的版本

cat stest2_ora_29753578.trc | grep 02000010.187e.19 | wc –l

0

##在8/16 #0x19中的记录也已经被覆盖掉了，意味着即使事务表即使能够顺利回滚，但也找不到修改前的数据拷贝了

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

* Rec #0x19  slt: 0x10  objn: 89731(0x00015e83)  objd: 89731  tblspc: 8(0x00000008)

*      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: 0x02000010.1881.18 ctl max scn: 0x0000.00ae9c40 prv tx scn: 0x0000.00ae9c43

txn start scn: scn: 0x0000.00ae9ca6 logon user: 0

prev brb: 33554463 prev bcl: 0

KDO undo record:

KTB Redo

op: 0x04  ver: 0x01

compat bit: 4 (post-11) padding: 1

op: L  itl: xid:  0x0002.006.000011e8 uba: 0x02000010.1881.17

flg: C---    lkc:  0    scn: 0x0000.00ae9ca3

Array Update of 2 rows:

tabn: 0 slot: 0(0x0) flag: 0x2c lock: 0 ckix: 1

ncol: 1 nnew: 1 size: 0

KDO Op code:  21 row dependencies Disabled

xtype: XAxtype KDO_KDOM2 flags: 0x00000080  bdba: 0x010005be  hdba: 0x010005ba

itli: 2  ispac: 0  maxfr: 4858

vect = 0

col  0: [ 2]  c1 3b

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

ncol: 1 nnew: 1 size: 0

KDO Op code:  21 row dependencies Disabled

xtype: XAxtype KDO_KDOM2 flags: 0x00000080  bdba: 0x010005be  hdba: 0x010005ba

itli: 2  ispac: 0  maxfr: 4858

vect = 0

col  0: [ 2]  c1 3b

附proc1.sql：

///Proc1.sql///

set serveroutput on

declare 

v_xidusn number;

v_xidslot number;

v_xidsqn number;

v_ubafil number;

v_ubablk number;

v_ubasqn number;

v_ubarec number;

v_cnt number:=0;

v_rownum number;

v_startscn varchar2(20);

v_rowid rowid;

begin 

select to_char(timestamp_to_scn(sysdate),'xxxxxxxx') into v_startscn from dual;

dbms_output.put_line('START SCN:'||v_startscn);

for i in 1..340 loop 

update scott.t1 set id = i; 

select XIDUSN,XIDSLOT,XIDSQN,UBAFIL,UBABLK,UBAREC,UBASQN into v_xidusn,v_xidslot,v_xidsqn,v_ubafil,v_ubablk,v_ubarec,v_ubasqn from v$transaction;

v_cnt:=v_cnt+1;