有网友删除dual表出现了问题：
删除dual表的时候hang住，然后直接shutdown abort。再重新启动数据库的时候，发现open的时候一直hang住，但是打开另外一个窗口数据库确实已经打开的。
下面进行模拟：
SYS@orcl11g>drop table dual;
一直hang在这不动
在另外一个窗口：
SYS@orcl11g>shutdown abort
ORACLE instance shut down.
重启数据库：
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，打开另外一个窗口：
SYS@orcl11g>select open_mode from v$database;
OPEN_MODE
----------------------------------------
READ WRITE
SYS@orcl11g>
发现数据库已经打开
这是因为系统触发器造成，在初始化参数中加入：
_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.
最开始删除dual表的时候也是因为系统触发器的存在。
现在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.
重启数据库：
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就知道是因为dual表的缘故，所以dual也是很重要的。不要乱玩
这时候我们需要将REPLICATION_DEPENDENCY_TRACKING参数设置为False才能打开数据库：
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
加入参数打开数据库：
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这时候我们呢需要重建dual表：
SYS@orcl11g>CREATE TABLE "SYS"."DUAL"
2 ( "DUMMY" VARCHAR2(1)
3 ) PCTFREE 10 PCTUSED 4