Oracle中独一无二的Cache对象

那么为什么Oracle10g能够作出这样的变更呢？

如果注意一下我们可以发现，cache对象实际上是为了兼容性目的所保留的，其中的主要信息如下：

*** SESSION ID:(9.149) 2007-04-18 21:32:17.682
Start dump data blocks tsn: 0 file#: 1 minblk 417 maxblk 419
buffer tsn: 0 rdba: 0x004001a1 (1/417)
scn: 0x0004.6c47f014 seq: 0x01 flg: 0x04 tail: 0xf0140d01
frmt: 0x02 chkval: 0xa59e type: 0x0d=Compatibility segment
Header:  size 12  next rdba 0x0  entries 25  offset 536f
Compatibility entry for 'COMPATSG':
  Size: 24  Release 0x134217728  By 0x153092096
Dump of memory from 0x0AA84E34 to 0x0AA84E38
AA84E30          00000000                        [....]       
Compatibility entry for 'BOOTSTRP':
  Size: 24  Release 0x134217728  By 0x153092096
Dump of memory from 0x0AA84E4C to 0x0AA84E50
AA84E40                            00400179              [y.@.]
Compatibility entry for 'UNDODATA':
  Size: 36  Release 0x153092096  By 0x153092096
Dump of memory from 0x0AA84E64 to 0x0AA84E74
AA84E60          00000042 5221003E 6C47F00D      [B...>.!R..Gl]
AA84E70 00000004                            [....]           
Compatibility entry for 'LMST':
  Size: 20  Release 0x153092096  By 0x153092096
Compatibility entry for 'PDMLITLS':
  Size: 20  Release 0x150994944  By 0x153092096
Compatibility entry for 'LOB_RET':
  Size: 20  Release 0x153092096  By 0x153092096
Compatibility entry for 'NWIMGFMT':
  Size: 20  Release 0x135266304  By 0x153092096
Compatibility entry for 'FASTDROP':
  Size: 20  Release 0x135290880  By 0x153092096
Compatibility entry for 'LOCMGDTS':
  Size: 20  Release 0x135266304  By 0x153092096
Compatibility entry for 'UNDOTBSP':
  Size: 20  Release 0x150994944  By 0x153092096
Compatibility entry for 'TEMPTAB':
  Size: 20  Release 0x135266304  By 0x153092096
Compatibility entry for 'PGTMGDTS':
  Size: 20  Release 0x150999808  By 0x153092096
Compatibility entry for 'DATETIME':
  Size: 20  Release 0x135266304  By 0x153092096
Compatibility entry for 'OPERATOR':
  Size: 20  Release 0x135266304  By 0x153092096
Compatibility entry for 'VYCLOB':
  Size: 20  Release 0x135266304  By 0x153092096
Compatibility entry for 'SEVTRIG':
  Size: 20  Release 0x135266304  By 0x153092096
Compatibility entry for 'RLENG':
  Size: 20  Release 0x153092096  By 0x153092096
Compatibility entry for 'APPCTX':
  Size: 20  Release 0x135266304  By 0x153092096
Compatibility entry for 'HSC':
  Size: 20  Release 0x153092096  By 0x153092096
Compatibility entry for 'BMINVPRT':
  Size: 20  Release 0x135266304  By 0x153092096
Compatibility entry for 'EDTRIG':
  Size: 20  Release 0x135290880  By 0x153092096
Compatibility entry for 'EXTTAB':
  Size: 20  Release 0x150994944  By 0x153092096
Compatibility entry for 'JAVA':
  Size: 20  Release 0x135266304  By 0x153092096
Compatibility entry for 'WRDIR':
  Size: 20  Release 0x150994944  By 0x153092096
Compatibility entry for 'PLUGTS':
  Size: 20  Release 0x135266304  By 0x153092096
buffer tsn: 0 rdba: 0x004001a2 (1/418)

这个信息和v$compatseg的信息完全相同：

SQL> select * from V$COMPATSEG;

TYPE_ID  RELEASE              UPDATED
-------- -------------------- --------------------
COMPATSG 8.0.0.0.0            9.2.0.0.0
BOOTSTRP 8.0.0.0.0            9.2.0.0.0
UNDODATA 9.2.0.0.0            9.2.0.0.0
LMST    9.2.0.0.0            9.2.0.0.0
PDMLITLS 9.0.0.0.0            9.2.0.0.0
LOB_RET  9.2.0.0.0            9.2.0.0.0
NWIMGFMT 8.1.0.0.0            9.2.0.0.0
FASTDROP 8.1.6.0.0            9.2.0.0.0
LOCMGDTS 8.1.0.0.0            9.2.0.0.0
UNDOTBSP 9.0.0.0.0            9.2.0.0.0
TEMPTAB  8.1.0.0.0            9.2.0.0.0
PGTMGDTS 9.0.1.3.0            9.2.0.0.0
DATETIME 8.1.0.0.0            9.2.0.0.0
OPERATOR 8.1.0.0.0            9.2.0.0.0
VYCLOB  8.1.0.0.0            9.2.0.0.0
SEVTRIG  8.1.0.0.0            9.2.0.0.0
RLENG    9.2.0.0.0            9.2.0.0.0
APPCTX  8.1.0.0.0            9.2.0.0.0
HSC      9.2.0.0.0            9.2.0.0.0
BMINVPRT 8.1.0.0.0            9.2.0.0.0

TYPE_ID  RELEASE              UPDATED
-------- -------------------- --------------------
EDTRIG  8.1.6.0.0            9.2.0.0.0
EXTTAB  9.0.0.0.0            9.2.0.0.0
JAVA    8.1.0.0.0            9.2.0.0.0
WRDIR    9.0.0.0.0            9.2.0.0.0
PLUGTS  8.1.0.0.0            9.2.0.0.0

25 rows selected

所以在启动数据库时要读取这个信息是因为在每个数据文件头都记录有兼容性信息，如果兼容性设置改变，在数据库启动之前，Oracle需要去更新每个文件头，所以这个信息必须被优先读取。

在Oracle10g中，由于兼容性特典的变更，'V$COMPATSEG'已经不再存在，所以

SQL> select count(*) from dba_objects where object_name='V$COMPATSEG';

  COUNT(*)
----------
        0

由于这个变化，CACHE对象不再需要，root dba的指向也由此改变。

-The End-

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