一些互联网用户删除dual表还有一个问题：
删除dual时间表hang直播，然后直接shutdown abort。话又说回来，当您启动数据库。发现open时间已经hang直播。但该数据库是真正开放的另一个窗口已经打开。
下面的模拟：
SYS@orcl11g>drop table dual;
一直hang在这不动
在另外一个窗体：
SYS@orcl11g>shutdown abort
ORACLE instance shut down.
重新启动数据库：
SYS@orcl11g>startup mount
ORACLE instance started.
Total System Global Area 417546240 bytes
Fixed Size 2213936 bytes
Variable Size 327157712 bytes
Database Buffers 83886080 bytes
Redo Buffers 4288512 bytes
Database mounted.
SYS@orcl11g>alter database open;
一直hang这个不动。打开另外一个窗体：
SYS@orcl11g>select open_mode from v$database;
OPEN_MODE
----------------------------------------
READ WRITE
SYS@orcl11g>
发现数据库已经打开
这是由于系统触发器造成，在初始化參数中增加：
_system_trig_enabled=flase
再次启动数据库：
SYS@orcl11g>startup mount
ORACLE instance started.
Total System Global Area 417546240 bytes
Fixed Size 2213936 bytes
Variable Size 327157712 bytes
Database Buffers 83886080 bytes
Redo Buffers 4288512 bytes
Database mounted.
SYS@orcl11g>show parameter _sys
NAME TYPE VALUE
------------------------------------ ---------------------- ------------------------------
_system_trig_enabled boolean FALSE
audit_sys_operations boolean FALSE
audit_syslog_level string
filesystemio_options string none
ldap_directory_sysauth string no
SYS@orcl11g>alter database open;
Database altered.
最開始删除dual表的时候也是由于系统触发器的存在。
如今我们禁用了系统触发器将dual删除看是什么效果：
SYS@orcl11g>drop table dual;
Table dropped.
重新启动数据库：
SYS@orcl11g>startup nomount pfile=initorcl11g.ora
ORACLE instance started.
Total System Global Area 417546240 bytes
Fixed Size 2213936 bytes
Variable Size 327157712 bytes
Database Buffers 83886080 bytes
Redo Buffers 4288512 bytes
SYS@orcl11g>show parameter rep
NAME TYPE VALUE
------------------------------------ ---------------------- ------------------------------
replication_dependency_tracking boolean TRUE
SYS@orcl11g>
SYS@orcl11g>
SYS@orcl11g>
SYS@orcl11g>
SYS@orcl11g>alter database mount;
Database altered.
SYS@orcl11g>alter database open;
alter database open
*
ERROR at line 1:
ORA-01092: ORACLE instance terminated. Disconnection forced
ORA-01775: looping chain of synonyms
Process ID: 2485
Session ID: 1 Serial number: 3
这里我们不用trace就知道是由于dual表的缘故，所以dual也是非常重要的。不要乱玩
这时候我们须要将REPLICATION_DEPENDENCY_TRACKING參数设置为False才干打开数据库：
官方文档给出的解释：
REPLICATION_DEPENDENCY_TRACKING enables or disables dependency tracking for
read/write operations to the database. Dependency tracking is essential for
propagating changes in a replicated environment in parallel
增加參数打开数据库：
SYS@orcl11g>startup mount
ORACLE instance started.
Total System Global Area 417546240 bytes
Fixed Size 2213936 bytes
Variable Size 327157712 bytes
Database Buffers 83886080 bytes
Redo Buffers 4288512 bytes
Database mounted.
SYS@orcl11g>show parameter rep
NAME TYPE VALUE
------------------------------------ ---------------------- ------------------------------
replication_dependency_tracking boolean FALSE
SYS@orcl11g>ak^H
SP2-0042: unknown command "a" - rest of line ignored.
SYS@orcl11g>a;ter^H
SYS@orcl11g>alter database open;
Database altered.
SYS@orcl11g>select * from dual;
select * from dual
*
ERROR at line 1:
ORA-01775: looping chain of synonyms
这时候我们呢须要重建dual表：
SYS@orcl11g>CREATE TABLE "SYS"."DUAL"
2 ( "DUMMY" VARCHAR2(1)
3 ) PCTFREE 10 PCTUSED 40 INITRANS 1 MAXTRANS 255
4 NOCOMPRESS LOGGING
5 STORAGE(INITIAL 16384 NEXT 1048576 MINEXTENTS 1 MAXEXTENTS 2147483645
6 PCTINCREASE 0 FREELISTS 1 FREELIST GROUPS 1
7 BUFFER_POOL DEFAULT FLASH_CACHE DEFAULT CELL_FLASH_CACHE DEFAULT)
8 TABLESPACE "SYSTEM" ;
Table created.
SYS@orcl11g>SYS@orcl11g>GRANT SELECT ON "SYS"."DUAL" TO PUBLIC WITH GRANT OPTION;
Grant succeeded.
SYS@orcl11g>SYS@orcl11g>@?/rdbms/admin/utlrp.sql
至此我们dual已经恢复完毕
SYS@orcl11g>select 'www.zbdba.com' from dual;
'WWW.ZBDBA.COM'
--------------------------
www.zbdba.com
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