对数据库有一定了解的人都知道，数据库启动是通过bootstarp$表来实现启动引导的。那这里有两个疑问：
1.引导表bootstarp$名字是否是唯一？
2.引导表的位置是不是在数据文件固定？比如11g/12c在file 1 block 520，10g在file 1 block 377？
跟踪数据库启动
[oracle@rac2 ~]$ sqlplus / as sysdba
SQL*Plus: Release 12.1.0.1.0 Production on Sun Dec 15 14:16:58 2013
Copyright (c) 1982, 2013, Oracle. All rights reserved.
Connected to:
Oracle Database 12c Enterprise Edition Release 12.1.0.1.0 - 64bit Production
With the Partitioning, Real Application Clusters, OLAP, Advanced Analytics
and Real Application Testing options
SQL> startup mount;
ORACLE instance started.
Total System Global Area 400846848 bytes
Fixed Size 2288872 bytes
Variable Size 310379288 bytes
Database Buffers 79691776 bytes
Redo Buffers 8486912 bytes
Database mounted.
SQL> oradebug setmypid
Statement processed.
SQL> alter session set db_file_multiblocK_read_count=1;
Statement processed.
SQL> oradebug EVENT 10046 TRACE NAME CONTEXT FOREVER, LEVEL 12
Session altered.
SQL> oradebug TRACEFILE_NAME
/u01/app/oracle/diag/rdbms/test/TEST/trace/TEST_ora_31364.trc
SQL> alter database Open;
Database altered.
SQL> oradebug EVENT 10046 trace name context off
Statement processed.
SQL> select HEADER_FILE,HEADER_BLOCK from dba_segments where segment_name='BOOTSTRAP$';
HEADER_FILE HEADER_BLOCK
----------- ------------
1 520
SQL> select object_id from dba_objects where object_name='BOOTSTRAP$';
OBJECT_ID
----------
59
分析trace文件
WAIT #140077386411120: nam='db file sequential read' ela= 56 file#=1 block#=520 blocks=1 obj#=-1 tim=1719385755334 ===================== PARSING IN CURSOR #140077386402760 len=188 dep=1 uid=0 oct=1 lid=0 tim=1719385757322 hv=4006182593 ad='6645d370' sqlid='32r4f1brckzq1' create table bootstrap$ ( END OF STMT PARSE #140077386402760:c=2000,e=1711,p=0,cr=0,cu=0,mis=1,r=0,dep=1,og=4,plh=0,tim=1719385757319 EXEC #140077386402760:c=0,e=430,p=0,cr=0,cu=0,mis=0,r=0,dep=1,og=4,plh=0,tim=1719385757909 CLOSE #140077386402760:c=0,e=9,dep=1,type=0,tim=1719385758105 ===================== PARSING IN CURSOR #140077386402760 len=55 dep=1 uid=0 oct=3 lid=0 tim=1719385759507 hv=2111436465 ad='6645bc80' sqlid='6apq2rjyxmxpj' select line#, sql_text from bootstrap$ where obj# != :1 END OF STMT PARSE #140077386402760:c=1000,e=1365,p=0,cr=0,cu=0,mis=1,r=0,dep=1,og=4,plh=0,tim=1719385759505 BINDS #140077386402760: Bind#0 oacdty=02 mxl=22(22) mxlc=00 mal=00 scl=00 pre=00 oacflg=08 fl2=1000001 frm=00 csi=00 siz=24 off=0 kxsbbbfp=7f664edb8780 bln=22 avl=02 flg=05 value=59 EXEC #140077386402760:c=3000,e=8859,p=0,cr=0,cu=0,mis=1,r=0,dep=1,og=4,plh=867914364,tim=1719385768574 WAIT #140077386402760: nam='db file sequential read' ela= 27 file#=1 block#=520 blocks=1 obj#=59 tim=1719385768753 WAIT #140077386402760: nam='db file sequential read' ela= 23 file#=1 block#=521 blocks=1 obj#=59 tim=1719385769575
这里我们可以发现，数据库启动的时候是读file 1 block 520，object_id为59，为bootstarp$对象
bbed查看root rdba地址
ub4 kcvfhrdb @96 0x00400208
SQL> select to_number('208','xxx') from dual;
TO_NUMBER('208','XXX')
----------------------
520
这里可以看出来，数据库启动的file header rdba地址和trace文件中的一致，也是指定到bootstarp$;
替换bootstrap$表为xifenfei
SQL> create table xifenfei as select * from bootstrap$;
Table created.
SQL> desc xifenfei
Name Null? Type
----------------------------------------- -------- ----------------------------
LINE# NOT NULL NUMBER
OBJ# NOT NULL NUMBER
SQL_TEXT NOT NULL VARCHAR2(4000)
SQL> select sql_text from xifenfei where line#=59;
SQL_TEXT
--------------------------------------------------------------------------------
CREATE TABLE BOOTSTRAP$("LINE#" NUMBER NOT NULL,"OBJ#" NUMBER NOT NULL,"SQL_TEXT
" VARCHAR2(4000) NOT NULL) PCTFREE 10 PCTUSED 40 INITRANS 1 MAXTRANS 255 STORAGE
( INITIAL 56K NEXT 1024K MINEXTENTS 1 MAXEXTENTS 2147483645 PCTINCREASE 0 OBJN
O 59 EXTENTS (FILE 1 BLOCK 520))
--清除bootstarp$记录
SQL> delete from xifenfei where line#=59;
1 row deleted.
SQL> commit;
Commit complete.
SQL> select objECT_ID from dba_objects where object_name='XIFENFEI';
OBJECT_ID
----------
20314
SQL> select HEADER_FILE,HEADER_BLOCK from dba_segments where segment_name='XIFENFEI';
HEADER_FILE HEADER_BLOCK
----------- ------------
1 45712
SQL> shutdown immediate;
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup upgrade;
ORACLE instance started.
Total System Global Area 400846848 bytes
Fixed Size 2288872 bytes
Variable Size 310379288 bytes
Database Buffers 79691776 bytes
Redo Buffers 8486912 bytes
Database mounted.
Database opened.
--删除bootstarp$表
SQL> drop table bootstrap$;
Table dropped.
SQL> exec DBMS_DDL_INTERNAL.SWAP_BOOTSTRAP('XIFENFEI');
PL/SQL procedure successfully completed.
SQL> shutdown immediate;
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> EXIT
Disconnected from Oracle Database 12c Enterprise Edition Release 12.1.0.1.0 - 64bit Production
With the Partitioning, Real Application Clusters, OLAP, Advanced Analytics
and Real Application Testing options
[oracle@rac2 ~]$ sqlplus / as sysdba
SQL*Plus: Release 12.1.0.1.0 Production on Sun Dec 15 14:29:54 2013
Copyright (c) 1982, 2013, Oracle. All rights reserved.
Connected to an idle instance.
SQL> STARTUP MOUNT
ORACLE instance started.
Total System Global Area 400846848 bytes
Fixed Size 2288872 bytes
Variable Size 310379288 bytes
Database Buffers 79691776 bytes
Redo Buffers 8486912 bytes
Database mounted.
SQL> oradebug setmypid
Statement processed.
SQL> oradebug EVENT 10046 TRACE NAME CONTEXT FOREVER, LEVEL 12
Statement processed.
SQL> alter session set db_file_multiblocK_read_count=1;
Session altered.
SQL> oradebug TRACEFILE_NAME
/u01/app/oracle/diag/rdbms/test/TEST/trace/TEST_ora_32727.trc
SQL> alter database Open;
Database altered.
SQL> SELECT OBJECT_ID FROM DBA_OBJECTS WHERE OBJECT_NAME='BOOTSTRAP$';
no rows selected
SQL> SELECT OBJECT_ID FROM DBA_OBJECTS WHERE OBJECT_NAME='XIFENFEI';
OBJECT_ID
----------
20314
分析trace文件
WAIT #139841534602352: nam='db file sequential read' ela= 23 file#=1 block#=45712 blocks=1 obj#=-1 tim=1720082484775 ===================== PARSING IN CURSOR #139841534593992 len=193 dep=1 uid=0 oct=1 lid=0 tim=1720082488552 hv=2096904950 ad='6645d650' sqlid='bs6v55xygsfrq' create table bootstrap$ ( END OF STMT PARSE #139841534593992:c=2000,e=2925,p=0,cr=0,cu=0,mis=1,r=0,dep=1,og=4,plh=0,tim=1720082488550 EXEC #139841534593992:c=0,e=466,p=0,cr=0,cu=0,mis=0,r=0,dep=1,og=4,plh=0,tim=1720082489124 CLOSE #139841534593992:c=0,e=8,dep=1,type=0,tim=1720082489266 ===================== PARSING IN CURSOR #139841534593992 len=55 dep=1 uid=0 oct=3 lid=0 tim=1720082490510 hv=2111436465 ad='6645c050' sqlid='6apq2rjyxmxpj' select line#, sql_text from bootstrap$ where obj# != :1 END OF STMT PARSE #139841534593992:c=1999,e=1211,p=0,cr=0,cu=0,mis=1,r=0,dep=1,og=4,plh=0,tim=1720082490509 BINDS #139841534593992: Bind#0 oacdty=02 mxl=22(22) mxlc=00 mal=00 scl=00 pre=00 oacflg=08 fl2=1000001 frm=00 csi=00 siz=24 off=0 kxsbbbfp=7f2f64fe8780 bln=22 avl=04 flg=05 value=20314 EXEC #139841534593992:c=1000,e=1789,p=0,cr=0,cu=0,mis=1,r=0,dep=1,og=4,plh=867914364,tim=1720082492533 WAIT #139841534593992: nam='db file sequential read' ela= 20 file#=1 block#=45712 blocks=1 obj#=20314 tim=1720082492685 WAIT #139841534593992: nam='db file sequential read' ela= 25 file#=1 block#=45713 blocks=1 obj#=20314 tim=1720082492986
这里可以看出来，数据库在启动的时候是读取file 1 block 45712，object_id为20314
bbed查看root rdba地址
ub4 kcvfhrdb @96 0x0040b290
SQL> select to_number('b290','xxxxxxx') from dual;
TO_NUMBER('B290','XXXXXXX')
---------------------------
45712
证明现在的文件头里面的file header rdba 已经修改为file 1 block 45712和trace里面看到的一致,都是XIFENFEI这个表
测试结论
1. 通过使用SWAP_BOOTSTRAP可以置换掉数据库启动开始表bootstrap$变为另外一个表,而且该过程直接修改文件头的kcvfhrdb值
2. 通过试验证明,oracle启动的时候不是程序里面写死的去读file 1的某个block,而是通过读取kcvfhrdb然后启动数据库
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