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，
Dropdual
。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 abortORACLE instance shut down.
重启数据库：
SYS@orcl11g>startup mountORACLE instance started.Total System Global Area 417546240 bytesFixed Size 2213936 bytesVariable Size 327157712 bytesDatabase Buffers 83886080 bytesRedo Buffers 4288512 bytesDatabase 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 WRITESYS@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 mountORACLE instance started.Total System Global Area 417546240 bytesFixed Size 2213936 bytesVariable Size 327157712 bytesDatabase Buffers 83886080 bytesRedo Buffers 4288512 bytesDatabase mounted.SYS@orcl11g>show parameter _sysNAME TYPE VALUE------------------------------------ ---------------------- ------------------------------_system_trig_enabled boolean FALSEaudit_sys_operations boolean FALSEaudit_syslog_level stringfilesystemio_options string noneldap_directory_sysauth string noSYS@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.oraORACLE instance started.Total System Global Area 417546240 bytesFixed Size 2213936 bytesVariable Size 327157712 bytesDatabase Buffers 83886080 bytesRedo Buffers 4288512 bytesSYS@orcl11g>show parameter repNAME TYPE VALUE------------------------------------ ---------------------- ------------------------------replication_dependency_tracking boolean TRUESYS@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 forcedORA-01775: looping chain of synonymsProcess ID: 2485Session 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，
电脑资料
《Dropdual》(https://www.unjs.com)。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 mountORACLE instance started.Total System Global Area 417546240 bytesFixed Size 2213936 bytesVariable Size 327157712 bytesDatabase Buffers 83886080 bytesRedo Buffers 4288512 bytesDatabase mounted.SYS@orcl11g>show parameter repNAME TYPE VALUE------------------------------------ ---------------------- ------------------------------replication_dependency_tracking boolean FALSESYS@orcl11g>ak^HSP2-0042: unknown command "a" - rest of line ignored.SYS@orcl11g>a;ter^HSYS@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 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已经恢复完成
SYS@orcl11g>select 'www.zbdba.com' from dual;'WWW.ZBDBA.COM'--------------------------www.zbdba.com