在Oracle10g之前，數據庫中存在一個獨一無二的Cache對象，這個對象就是： SQL> select segment_name,segment_type,header_file,header_block 2 from dba_segments where segment_type='CACHE';SEGMENT_NAME SEGMENT_TYPE HEADER_FILE HEADER_BLOCK------------- -------------- ------- --------1.417 CACHE 1 417
這個對象的名稱來自于文件號和數據塊號，1.417正好就是文件1的第417個數據塊。 這個Cache對象在Oracle數據庫中的含義非同一般，在bootstrap$的過程中，這個對象之前的所有對象都需要用來bootstrap。 我們看一下1.417之前的所有對象： SQL> select b.object_id,a.segment_name,a.segment_type,a.header_block from dba_segments a,dba_objects b 2 where a.segment_name=b.object_name(+) and a.header_file=1 and a.header_block <= 417 3 order by a.header_block 4 /OBJECT_ID SEGMENT_NAME SEGMENT_TYPE HEADER_BLOCK---------- -------------------- ------------------ ------------ SYSTEM ROLLBACK 92 C_OBJ# CLUSTER 253 I_OBJ# INDEX 496 C_TS# CLUSTER 577 I_TS# INDEX 658 C_FILE#_BLOCK# CLUSTER 739 I_FILE#_BLOCK# INDEX 8110 C_USER# CLUSTER 8911 I_USER# INDEX 9715 UNDO$ TABLE 10517 FILE$ TABLE 11318 OBJ$ TABLE 12123 PROXY_DATA$ TABLE 12924 I_PROXY_DATA$ INDEX 13725 PROXY_ROLE_DATA$ TABLE 14526 I_PROXY_ROLE_DATA$_1 INDEX 15327 I_PROXY_ROLE_DATA$_2 INDEX 16128 CON$ TABLE 16929 C_COBJ# CLUSTER 17730 I_COBJ# INDEX 185OBJECT_ID SEGMENT_NAME SEGMENT_TYPE HEADER_BLOCK---------- -------------------- ------------------ ------------33 I_TAB1 INDEX 19334 I_UNDO1 INDEX 20135 I_UNDO2 INDEX 20936 I_OBJ1 INDEX 21737 I_OBJ2 INDEX 22538 I_OBJ3 INDEX 23339 I_IND1 INDEX 24140 I_ICOL1 INDEX 24941 I_FILE1 INDEX 25742 I_FILE2 INDEX 26543 I_TS1 INDEX 27344 I_USER1 INDEX 28145 I_COL1 INDEX 28946 I_COL2 INDEX 29747 I_COL3 INDEX 30548 I_CON1 INDEX 31349 I_CON2 INDEX 32150 I_CDEF1 INDEX 32951 I_CDEF2 INDEX 33752 I_CDEF3 INDEX 34553 I_CDEF4 INDEX 353OBJECT_ID SEGMENT_NAME SEGMENT_TYPE HEADER_BLOCK---------- -------------------- ------------------ ------------54 I_CCOL1 INDEX 36155 I_CCOL2 INDEX 36956 BOOTSTRAP$ TABLE 377 1.417 CACHE 41745 rows selected
一共有44個對象。我們再來看一下初始化過程中bootstrap$中的信息： SQL> select * from bootstrap$ order by obj#; LINE# OBJ# SQL_TEXT ------ -------- ----------------------- -1 -1 8.0.0.0.0 0 0 CREATE ROLLBACK SEGMENT SYSTEM STORAGE (INITIAL 112K NEXT 1024K MINEXTENTS 1M 2 2 CREATE CLUSTER C_OBJ#("OBJ#" NUMBER) PCTFREE 5 PCTUSED 40 INITRANS 2 MAXTRANS 25 3 3 CREATE INDEX I_OBJ# ON CLUSTER C_OBJ# PCTFREE 10 INITRANS 2MAXTRANS 255 STORAGE 4 4 CREATE TABLE TAB$("OBJ#" NUMBER NOT NULL, "DATAOBJ#" NUMBER,"TS#" NUMBER NOT NULL 5 5 CREATE TABLE CLU$("OBJ#" NUMBER NOT NULL, "DATAOBJ#" NUMBER,"TS#" NUMBER NOT NULL 6 6 CREATE CLUSTER C_TS#("TS#" NUMBER) PCTFREE 10 PCTUSED 40 INITRANS 2 MAXTRANS 255 7 7 CREATE INDEX I_TS# ON CLUSTER C_TS# PCTFREE 10 INITRANS 2 MAXTRANS 255 STORAGE ( 8 8 CREATE CLUSTER C_FILE#_BLOCK#("TS#" NUMBER,"SEGFILE#" NUMBER,"SEGBLOCK#" NUMBER) 9 9 CREATE INDEX I_FILE#_BLOCK# ON CLUSTER C_FILE#_BLOCK# PCTFREE 10 INITRANS 2 MAXT 10 10 CREATE CLUSTER C_USER#("USER#" NUMBER) PCTFREE 10 PCTUSED 40 INITRANS 2 MAXTRANS 11 11 CREATE INDEX I_USER# ON CLUSTER C_USER# PCTFREE 10 INITRANS 2 MAXTRANS 255 STORA 12 12 CREATE TABLE FET$("TS#" NUMBER NOT NULL, "FILE#" NUMBER NOT NULL,"BLOCK#" NUMBER 13 13 CREATE TABLE UET$("SEGFILE#" NUMBER NOT NULL,"SEGBLOCK#" NUMBER NOT NULL,"EXT#" 14 14 CREATE TABLE SEG$("FILE#" NUMBER NOT NULL ,"BLOCK#" NUMBER NOT NULL,"TYPE#" NUMBE 15 15 CREATE TABLE UNDO$("US#" NUMBER NOT NULL, "NAME" VARCHAR2(30) NOT NULL,"USER#" NU 16 16 CREATE TABLE TS$("TS#" NUMBER NOT NULL, "NAME" VARCHAR2(30) NOT NULL,"OWNER#" NUM 17 17 CREATE TABLE FILE$("FILE#" NUMBER NOT NULL, "STATUS$" NUMBER NOT NULL,"BLOCKS" NU 18 18 CREATE TABLE OBJ$("OBJ#" NUMBER NOT NULL, "DATAOBJ#" NUMBER,"OWNER#" NUMBER NOT N 19 19 CREATE TABLE IND$("OBJ#" NUMBER NOT NULL, "DATAOBJ#" NUMBER,"TS#" NUMBER NOT NULL LINE# OBJ# SQL_TEXT---------- ---------- ---------------------------------------- 20 20 CREATE TABLE ICOL$("OBJ#" NUMBER NOT NULL, "BO#" NUMBER NOT NULL,"COL#" NUMBER NO 21 21 CREATE TABLE COL$("OBJ#" NUMBER NOT NULL, "COL#" NUMBER NOT NULL,"SEGCOL#" NUMBER 22 22 CREATE TABLE USER$("USER#" NUMBER NOT NULL, "NAME" VARCHAR2(30) NOT NULL,"TYPE#" 23 23 CREATE TABLE PROXY_DATA$("CLIENT#" NUMBER NOT NULL, "PROXY#" NUMBER NOT NULL,"CRE 24 24 CREATE UNIQUE INDEX I_PROXY_DATA$ ON PROXY_DATA$(CLIENT#,PROXY#) PCTFREE 10 INIT 25 25 CREATE TABLE PROXY_ROLE_DATA$("CLIENT#" NUMBER NOT NULL,"PROXY#" NUMBER NOT NULL 26 26 CREATE INDEX I_PROXY_ROLE_DATA$_1 ON PROXY_ROLE_DATA$(CLIENT#,PROXY#) PCTFREE 10 27 27 CREATE UNIQUE INDEX I_PROXY_ROLE_ DATA$_2ONPROXY_ROLE_DATA$(CLIENT#, PROXY#,ROLE 28 28 CREATE TABLE CON$("OWNER#" NUMBER NOT NULL, "NAME" VARCHAR2(30) NOT NULL,"CON#" N 29 29 CREATE CLUSTER C_COBJ#("OBJ#" NUMBER) PCTFREE 0 PCTUSED 50 INITRANS 2 MAXTRANS 2 30 30 CREATE INDEX I_COBJ# ON CLUSTER C_COBJ# PCTFREE 10 INITRANS 2 MAXTRANS 255 STORA 31 31 CREATE TABLE CDEF$("CON#" NUMBER NOT NULL, "OBJ#" NUMBER NOT NULL,"COLS" NUMBER," 32 32 CREATE TABLE CCOL$("CON#" NUMBER NOT NULL, "OBJ#" NUMBER NOT NULL,"COL#" NUMBER N 33 33 CREATE INDEX I_TAB1 ON TAB$(BOBJ#) PCTFREE 10 INITRANS 2 MAXTRANS 255 STORAGE ( 34 34 CREATE UNIQUE INDEX I_UNDO1 ON UNDO$(US#) PCTFREE 10 INITRANS 2 MAXTRANS 255 STO 35 35 CREATE INDEX I_UNDO2 ON UNDO$(NAME) PCTFREE 10 INITRANS 2 MAXTRANS 255 STORAGE ( 36 36 CREATE UNIQUE INDEX I_OBJ1 ON OBJ$(OBJ#) PCTFREE 10 INITRANS 2 MAXTRANS 255 STOR 37 37 CREATE UNIQUE INDEXI_OBJ2ONOBJ$(OWNER#, NAME,NAMESPACE,REMOTEOWNER,LINKNAME,SU 38 38 CREATE INDEX I_OBJ3 ON OBJ$(OID$) PCTFREE 10 INITRANS 2 MAXTRANS 255 STORAGE ( 39 39 CREATE UNIQUE INDEX I_IND1 ON IND$(OBJ#) PCTFREE 10 INITRANS 2 MAXTRANS 255 STOR 40 40 CREATE INDEX I_ICOL1 ON ICOL$(OBJ#) PCTFREE 10 INITRANS 2 MAXTRANS 255 STORAGE ( LINE# OBJ# SQL_TEXT---------- ------- -------------------------------------------- 41 41 CREATE UNIQUE INDEX I_FILE1 ON FILE$(FILE#) PCTFREE 10 INITRANS 2 MAXTRANS 255 S 42 42 CREATE UNIQUE INDEX I_FILE2 ON FILE$(TS#,RELFILE#) PCTFREE 10 INITRANS 2 MAXTRAN 43 43 CREATE UNIQUE INDEX I_TS1 ON TS$(NAME) PCTFREE 10 INITRANS 2 MAXTRANS 255 STORAG 44 44 CREATE UNIQUE INDEX I_USER1 ON USER$(NAME) PCTFREE 10 INITRANS 2 MAXTRANS 255 ST 45 45 CREATE UNIQUE INDEX I_COL1 ON COL$(OBJ#,NAME) PCTFREE 10 INITRANS 2 MAXTRANS 255 46 46 CREATE INDEX I_COL2 ON COL$(OBJ#,COL#) PCTFREE 10 INITRANS 2 MAXTRANS 255 STORAG 47 47 CREATE UNIQUE INDEX I_COL3 ON COL$(OBJ#, INTCOL#) PCTFREE 10 INITRANS 2 MAXTRANS 48 48 CREATE UNIQUE INDEX I_CON1 ON CON$(OWNER#, NAME) PCTFREE 10 INITRANS 2 MAXTRANS 2 49 49 CREATE UNIQUE INDEX I_CON2 ON CON$(CON#) PCTFREE 10 INITRANS 2 MAXTRANS 255 STOR 50 50 CREATE UNIQUE INDEX I_CDEF1 ON CDEF$(CON#) PCTFREE 10 INITRANS 2 MAXTRANS 255 ST 51 51 CREATE INDEX I_CDEF2 ON CDEF$(OBJ#) PCTFREE 10 INITRANS 2 MAXTRANS 255 STORAGE ( 52 52 CREATE INDEX I_CDEF3 ON CDEF$(ROBJ#) PCTFREE 10 INITRANS 2 MAXTRANS 255 STORAGE 53 53 CREATE INDEX I_CDEF4 ON CDEF$(ENABLED) PCTFREE 10 INITRANS 2 MAXTRANS 255 STORAG 54 54 CREATE INDEX I_CCOL1 ON CCOL$(CON#,COL#) PCTFREE 10 INITRANS 2 MAXTRANS 255 STOR 55 55 CREATE UNIQUE INDEX I_CCOL2 ON CCOL$(CON#, INTCOL#) PCTFREE 10 INITRANS 2 MAXTRAN 56 56 CREATE TABLE BOOTSTRAP$("LINE#" NUMBER NOT NULL,"OBJ#" NUMBER NOT NULL,"SQL_TEXT57 rows selected
bootstarp的對象都是在1.417之前的，當然有部分對象是基于Cluster創建的。那么這個對象在啟動之前有什么作用呢? itpub上的jametong同學給出了一個重要的信息，在SYSTEM文件頭，Oracle存儲了一個root dba:Root dba: This field only occurs in data file #1, and is the location of blocks required during bootstrapping the data dictionary (bootstrap$)。 這個root dba指向了1.417對象，而1.417對象的上一個對象正是bootstrap$，Oracle通過1.417找到了bootstrap$對象就可以啟動了數據庫。 我們可以從文件頭的轉儲中找到這個root dba，以下是一個Oracle9i的數據文件頭信息： DATA FILE #1: (name #9) /opt/oracle/oradata/eygle/system01.dbfcreation size=0 block size=8192 status=0xe head=9 tail=9 dup=1tablespace 0, index=1 krfil=1 prev_file=0unrecoverable scn: 0x0004.6c4b5cba 04/10/2007 13:34:26Checkpoint cnt:6933 scn: 0x0004.6c4dfc0c 04/16/2007 14:38:05Stop scn: 0xffff.ffffffff 04/07/2007 21:03:02Creation Checkpointed at scn: 0x0000.00000007 04/24/2006 11:34:39thread:0 rba:(0x0.0.0)enabled threads: 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000Offline scn: 0x0004.6c432ebf prev_range: 0Online Checkpointed at scn: 0x0004.6c432ec0 03/28/2007 11:22:37thread:1 rba:(0x1.2.0)enabled threads: 01000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000Hot Backup end marker scn: 0x0000.00000000aux_file is NOT DEFINED FILE HEADER: Software vsn=153092096=0x9200000, Compatibility Vsn=134217728=0x8000000 Db ID=1407686520=0x53e79778, Db Name='EYGLE' Activation ID=0=0x0 Control Seq=1299557=0x13d465, File size=27017=0x6989 File Number=1, Blksiz=8192, File Type=3 DATATablespace #0 - SYSTEM rel_fn:1 Creation at scn: 0x0000.00000007 04/24/2006 11:34:39Backup taken at scn: 0x0004.6c2d657e 02/12/2007 15:54:52 thread:1reset logs count:0x24dc1f7d scn: 0x0004.6c432ec0 recovered at 04/07/2007 21:04:11status:0x4 root dba:0x004001a1 chkpt cnt: 6933 ctl cnt:6932begin-hot-backup file size: 32000Checkpointed at scn: 0x0004.6c4dfc0c 04/16/2007 14:38:05thread:1 rba:(0x17.2.10)enabled threads: 01000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000Backup Checkpointed at scn: 0x0004.6c2d657e 02/12/2007 15:54:52thread:1 rba:(0x18e1.30.10)enabled threads: 01000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000External cache id: 0x0 0x0 0x0 0x0Absolute fuzzy scn: 0x0000.00000000Recovery fuzzy scn: 0x0000.00000000 01/01/1988 00:00:00Terminal Recovery Stamp scn: 0x0000.00000000 01/01/1988 00:00:00
這個root dba：0x004001a1 正好指向的是file 1 block 417而我們dump file 1 block 417就可以找到bootstrap$的地址： Start dump data blocks tsn: 0 file#: 1 minblk 417 maxblk 419buffer tsn: 0 rdba: 0x004001a1 (1/417)scn: 0x0004.6c47f014 seq: 0x01 flg: 0x04 tail: 0xf0140d01frmt: 0x02 chkval: 0xa59e type: 0x0d=Compatibility segmentHeader: size 12 next rdba 0x0 entries 25 offset 536fCompatibility entry for 'COMPATSG': Size: 24 Release 0x134217728 By 0x153092096Dump of memory from 0x0AA84E34 to 0x0AA84E38AA84E30 00000000 [....] Compatibility entry for 'BOOTSTRP': Size: 24 Release 0x134217728 By 0x153092096Dump of memory from 0x0AA84E4C to 0x0AA84E50AA84E40 00400179 [y.@.]Compatibility entry for 'UNDODATA': Size: 36 Release 0x153092096 By 0x153092096Dump of memory from 0x0AA84E64 to 0x0AA84E74AA84E60 00000042 5221003E 6C47F00D [B...>.!R..Gl]AA84E70 00000004 [....]
dba號00400179 正好指向的file 1 block 377。 在Oracle10g中，Oracle將這個root dba更改得更為直接： DATA FILE #1: (name #7) +ORADG/danaly/datafile/system.264.600173859creation size=38400 block size=8192 status=0xe head=7 tail=7 dup=1tablespace 0, index=1 krfil=1 prev_file=0unrecoverable scn: 0x0000.00000000 01/01/1988 00:00:00Checkpoint cnt:45507 scn: 0x081a.88397cdd 04/19/2007 10:36:17Stop scn: 0xffff.ffffffff 01/27/2007 17:56:34Creation Checkpointed at scn: 0x0000.00000007 09/03/2006 10:57:47thread:1 rba:(0x1.3.10)enabled threads: 01000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000Offline scn: 0x0000.00000000 prev_range: 0Online Checkpointed at scn: 0x0000.00000000 thread:0 rba:(0x0.0.0)enabled threads: 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000Hot Backup end marker scn: 0x0000.00000000aux_file is NOT DEFINED V10 STYLE FILE HEADER: Compatibility Vsn = 169869568=0xa200100 Db ID=3965153484=0xec5770cc, Db Name='DANALY' Activation ID=0=0x0 Control Seq=2912565=0x2c7135, File size=84180=0x148d4 File Number=1, Blksiz=8192, File Type=3 DATATablespace #0 - SYSTEM rel_fn:1 Creation at scn: 0x0000.00000007 09/03/2006 10:57:47Backup taken at scn: 0x0000.00000000 01/01/1988 00:00:00 thread:0reset logs count:0x23c5ed0c scn: 0x0000.00000001 reset logs terminal rcv data:0x0 scn: 0x0000.00000000prev reset logs count:0x0 scn: 0x0000.00000000 prev reset logs terminal rcv data:0x0 scn: 0x0000.00000000recovered at 01/28/2007 21:29:12status:0x2004 root dba:0x00400179 chkpt cnt: 45507 ctl cnt:45506begin-hot-backup file size: 0
這里的root dba直接指向了0x00400179 ，也就是file 1 block 377。隨之，這個唯一的Cache的對象在Oracle10g中不復存在。 SQL> select COUNT(*) from dba_segments where segment_type='CACHE'; COUNT(*)---------- 0
那么為什么Oracle10g能夠作出這樣的變更呢？ 假如注重一下我們可以發現，cache對象實際上是為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 21:32:17.682Start dump data blocks tsn: 0 file#: 1 minblk 417 maxblk 419buffer tsn: 0 rdba: 0x004001a1 (1/417)scn: 0x0004.6c47f014 seq: 0x01 flg: 0x04 tail: 0xf0140d01frmt: 0x02 chkval: 0xa59e type: 0x0d=Compatibility segmentHeader: size 12 next rdba 0x0 entries 25 offset 536fCompatibility entry for 'COMPATSG': Size: 24 Release 0x134217728 By 0x153092096Dump of memory from 0x0AA84E34 to 0x0AA84E38AA84E30 00000000 [....] Compatibility entry for 'BOOTSTRP': Size: 24 Release 0x134217728 By 0x153092096Dump of memory from 0x0AA84E4C to 0x0AA84E50AA84E40 00400179 [y.@.]Compatibility entry for 'UNDODATA': Size: 36 Release 0x153092096 By 0x153092096Dump of memory from 0x0AA84E64 to 0x0AA84E74AA84E60 00000042 5221003E 6C47F00D [B...>.!R..Gl]AA84E70 00000004 [....] Compatibility entry for 'LMST': Size: 20 Release 0x153092096 By 0x153092096Compatibility entry for 'PDMLITLS': Size: 20 Release 0x150994944 By 0x153092096Compatibility entry for 'LOB_RET': Size: 20 Release 0x153092096 By 0x153092096Compatibility entry for 'NWIMGFMT': Size: 20 Release 0x135266304 By 0x153092096Compatibility entry for 'FASTDROP': Size: 20 Release 0x135290880 By 0x153092096Compatibility entry for 'LOCMGDTS': Size: 20 Release 0x135266304 By 0x153092096Compatibility entry for 'UNDOTBSP': Size: 20 Release 0x150994944 By 0x153092096Compatibility entry for 'TEMPTAB': Size: 20 Release 0x135266304 By 0x153092096Compatibility entry for 'PGTMGDTS': Size: 20 Release 0x150999808 By 0x153092096Compatibility entry for 'DATETIME': Size: 20 Release 0x135266304 By 0x153092096Compatibility entry for 'OperaTOR': Size: 20 Release 0x135266304 By 0x153092096Compatibility entry for 'VYCLOB': Size: 20 Release 0x135266304 By 0x153092096Compatibility entry for 'SEVTRIG': Size: 20 Release 0x135266304 By 0x153092096Compatibility entry for 'RLENG': Size: 20 Release 0x153092096 By 0x153092096Compatibility entry for 'APPCTX': Size: 20 Release 0x135266304 By 0x153092096Compatibility entry for 'HSC': Size: 20 Release 0x153092096 By 0x153092096Compatibility entry for 'BMINVPRT': Size: 20 Release 0x135266304 By 0x153092096Compatibility entry for 'EDTRIG': Size: 20 Release 0x135290880 By 0x153092096Compatibility entry for 'EXTTAB': Size: 20 Release 0x150994944 By 0x153092096Compatibility entry for 'java': Size: 20 Release 0x135266304 By 0x153092096Compatibility entry for 'WRDIR': Size: 20 Release 0x150994944 By 0x153092096Compatibility entry for 'PLUGTS': Size: 20 Release 0x135266304 By 0x153092096buffer tsn: 0 rdba: 0x004001a2 (1/418)
這個信息和v$compatseg的信息完全相同： SQL> select * from V$COMPATSEG;TYPE_ID RELEASE UPDATED-------- -------------------- --------------------COMPATSG 8.0.0.0.0 9.2.0.0.0BOOTSTRP 8.0.0.0.0 9.2.0.0.0UNDODATA 9.2.0.0.0 9.2.0.0.0LMST 9.2.0.0.0 9.2.0.0.0PDMLITLS 9.0.0.0.0 9.2.0.0.0LOB_RET 9.2.0.0.0 9.2.0.0.0NWIMGFMT 8.1.0.0.0 9.2.0.0.0FASTDROP 8.1.6.0.0 9.2.0.0.0LOCMGDTS 8.1.0.0.0 9.2.0.0.0UNDOTBSP 9.0.0.0.0 9.2.0.0.0TEMPTAB 8.1.0.0.0 9.2.0.0.0PGTMGDTS 9.0.1.3.0 9.2.0.0.0DATETIME 8.1.0.0.0 9.2.0.0.0OPERATOR 8.1.0.0.0 9.2.0.0.0VYCLOB 8.1.0.0.0 9.2.0.0.0SEVTRIG 8.1.0.0.0 9.2.0.0.0RLENG 9.2.0.0.0 9.2.0.0.0APPCTX 8.1.0.0.0 9.2.0.0.0HSC 9.2.0.0.0 9.2.0.0.0BMINVPRT 8.1.0.0.0 9.2.0.0.0TYPE_ID RELEASE UPDATED-------- -------------------- --------------------EDTRIG 8.1.6.0.0 9.2.0.0.0EXTTAB 9.0.0.0.0 9.2.0.0.0JAVA 8.1.0.0.0 9.2.0.0.0WRDIR 9.0.0.0.0 9.2.0.0.0PLUGTS 8.1.0.0.0 9.2.0.0.025 rows selected
因為在啟動數據庫時要讀取這個信息是因為在每個數據文件頭都記錄有兼容性信息，假如兼容性設置改變，在數據庫啟動之前，Oracle需要去更新每個文件頭，所以這個信息必須被優先讀取。 在Oracle10g中，由于兼容性特典的變更，‘V$COMPATSEG’已經不再存在，所以: SQL> select count(*) from dba_objects where object_name='V$COMPATSEG'; COUNT(*)---------- 0
由于這個變化，CACHE對象不再需要，root dba的指向也由此改變。
