ray
Update
of 18
rows:
tabn: 0 slot: 80(0x50) flag: 0x2c
lock: 0 ckix: 157
ncol: 2 nnew: 1
size: 0
KDO Op code: 21
row dependencies Disabled
xtype: XAxtype KDO_KDOM2 flags: 0x00000080 bdba: 0x010002ab hdba: 0x010002aa
itli: 2 ispac: 0 maxfr: 4858
vect = 3
.....................省略这一undo record中的一部分
col 1: [12] 44 44 44 44 44 44 44 44 44 44 44 44
tabn: 0 slot: 97(0x61) flag: 0x2c
lock: 0 ckix: 157
ncol: 2 nnew: 1
size: 0
KDO Op code: 21
row dependencies Disabled
xtype: XAxtype KDO_KDOM2 flags: 0x00000080 bdba: 0x010002ab hdba: 0x010002aa
itli: 2 ispac: 0 maxfr: 4858
vect = 3
col 1: [12] 44 44 44 44 44 44 44 44 44 44 44 44
End dump
data blocks tsn: 7 file#: 6 minblk 180 maxblk 180
可以看到，这个undo block中只有两个undo record。无法再找到延迟事务对应的undo record 即1b。 （10）根据步骤（9）中undo header trace中的uba信息尝试进行回滚 dump uba ：uba: 0x018000bc.07a9.23 alter system dump datafile 6 block 188; UNDO BLK:
xid: 0x0016.01f.0000014b seq: 0x7a9 cnt: 0x2b irb: 0x2b icl: 0x0 flg: 0x0000
Rec
Offset Rec
Offset Rec
Offset Rec
Offset Rec
Offset
---------------------------------------------------------------------------
0x01 0x1fa4 0x02 0x1f60 0x03 0x1f1c 0x04 0x1ed8 0x05 0x1e94
0x06 0x1e50 0x07 0x1e0c 0x08 0x1dc8 0x09 0x1d84 0x0a 0x1d40
0x0b 0x1cfc 0x0c 0x1cb8 0x0d 0x1c74 0x0e 0x1c30 0x0f 0x1bec
0x10 0x1ba8 0x11 0x1b64 0x12 0x1b20 0x13 0x1adc 0x14 0x1a98
0x15 0x1a54 0x16 0x1a10 0x17 0x19cc 0x18 0x1988 0x19 0x1944
0x1a 0x1900 0x1b 0x18bc 0x1c 0x1878 0x1d 0x1834 0x1e 0x13ec
0x1f 0x0fa4 0x20 0x0b5c 0x21 0x0714 0x22 0x0330 0x23 0x02a8
0x24 0x0264 0x25 0x0220 0x26 0x01dc 0x27 0x0198 0x28 0x0154
0x29 0x0110 0x2a 0x00cc 0x2b 0x0088
*-----------------------------
* Rec #0x1 slt: 0x1e objn: 73429(0x00011ed5) objd: 73429 tblspc: 4(0x00000004)
* Layer: 11 (
Row) opc: 1 rci 0x00
Undo
type: Regular undo
Last buffer split:
No
Temp
Object:
No
Tablespace Undo:
No
rdba: 0x018000bb
* Rec #0x23 slt: 0x1f objn: 73429(0x00011ed5) objd: 73429 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: 0x018000bb.07a9.04 ctl
max scn: 0x0000.00150d48 prv tx scn: 0x0000.00150d58
txn
start scn: scn: 0x0000.00151164 logon
user: 85
prev brb: 25166010 prev bcl: 0
KDO undo record:
KTB Redo
op: 0x04 ver: 0x01
compat
bit: 4 (post-11) padding: 1
op: L itl: xid: 0x0019.01f.0000014b uba: 0x018000eb.07ab.04 ---又可以根据这个uba尝试回滚，一步一步查看。可以发现无法回滚到我们游标查询的SCN，因此可以预见游标查询会由于延迟块清除而抛出ORA-01555错误
flg:
C--- lkc: 0 scn: 0x0000.00151159
KDO Op code: LKR
row dependencies Disabled
xtype: XA flags: 0x00000000 bdba: 0x010002ab hdba: 0x010002aa
itli: 3 ispac: 0 maxfr: 4858
tabn: 0 slot: 0
to: 0 （11）SESSION B发起游标查询，与我们预期一致，抛出ORA-01555错误！
SQL> print :cur1
ERROR:
ORA-01555: snapshot too
old:
rollback segment
number 22
with
name "_SYSSMU22$"
too small
备注：此处游标查询时可以设置逻辑读跟踪，或者10046+set db_file_multiblock_read_count=1来跟踪延迟块清除时具体访问的undo块，但是由于实验时出现了些许问题，没有及时跟踪下来。
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