关于dual表的破坏性测试，既然是破坏性测试，就需要确定这个测试仅限于测试或者个人学习所用，可能有些sql看似极为简单，但是一旦运行就会导致整个业务系统崩溃。
比如说我们拿dual表开刀，这个表是一个dummy表，里面的内容没有特定的意义，就是为了存在而存在。但是一旦这个表出现问题，所有相关的基础操作都会受到影响，后果不敢想象。
来简单模拟一下，在个人的机器上开始做下面的尝试，drop 表dual

SQL> show user
USER is "SYS"
SQL>
SQL> show parameter insta
NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
active_instance_count                integer
cluster_database_instances           integer     1
instance_groups                      string
instance_name                        string      TEST01
instance_number                      integer     0
instance_type                        string      RDBMS
open_links_per_instance              integer     4
parallel_instance_group              string
parallel_server_instances            integer     1
SQL> drop table dual;  --运行这个命令也没有出现任何的问题
Table dropped.

SQL> select count(*)from dual;  --但是一旦开始尝试查询操作就会报出1775的错误。
select count(*)from dual
                    *
ERROR at line 1:
ORA-01775: looping chain of synonyms

SQL> select *from dual;               --再次尝试，问题依旧，可以想象在线业务系统出现这个问题是致命的影响。
select *from dual
             *
ERROR at line 1:
ORA-01775: looping chain of synonyms
--如果没有尝试重启数据库的情况下，完全可以采用如下的方式来解决，我们可以完全重建表dual,插入dummy的数据

SQL> CREATE TABLE "SYS"."DUAL" 
           (       "DUMMY" VARCHAR2(1)
           ) TABLESPACE "SYSTEM" ;
Table created.

SQL> GRANT SELECT ON "SYS"."DUAL" TO PUBLIC WITH GRANT OPTION;
Grant succeeded.

SQL> select *from dual;
no rows selected

SQL> insert into dual values('X');
1 row created.

SQL> commit;
Commit complete.

SQL> select *from dual; --简单的验证，会发现问题似乎解决了。
D
-
X

SQL> select sysdate from dual;  --尝试使用dual来查询时间，也没有发现问题。

SYSDATE
------------------
20-NOV-14
这个时候可以查看相关的sys下的失效对象，会发现受到影响的还不少，这个时候可以重新编译。

SQL> select object_name,owner ,object_type from dba_objects where status='INVALID'
OBJECT_NAME                    OWNER                          OBJECT_TYPE
------------------------------ ------------------------------ -------------------
AQ$_ALERT_QT_V                 SYS                            EVALUATION CONTEXT
ALERT_QUE_R                    SYS                            RULE SET
ALERT_QUE_N                    SYS                            RULE SET
DBMS_RCVMAN                    SYS                            PACKAGE BODY
DBMS_BACKUP_RESTORE            SYS                            PACKAGE BODY
DBMS_CDC_UTILITY               SYS                            PACKAGE BODY
DBMS_CDC_ISUBSCRIBE            SYS                            PACKAGE BODY
AQ$_AQ$_MEM_MC_V               SYS                            EVALUATION CONTEXT
AQ$_AQ_PROP_TABLE_V            SYS                            EVALUATION CONTEXT
AQ_PROP_NOTIFY_R               SYS                            RULE SET
AQ_PROP_NOTIFY_N               SYS                            RULE SET

OBJECT_NAME                    OWNER                          OBJECT_TYPE
------------------------------ ------------------------------ -------------------
ALERT_QUE$1                    SYS                            RULE SET
ALERT_QUE$1                    SYS                            RULE
AQ$_KUPC$DATAPUMP_QUETAB_V     SYS                            EVALUATION CONTEXT
DBMS_AW_EXP                    SYS                            PACKAGE BODY
DBMS_AW_STATS                  SYS                            PACKAGE BODY
STREAMS$_EVALUATION_CONTEXT    SYS                            EVALUATION CONTEXT
AQ$_SYS$SERVICE_METRICS_TAB_V  SYS                            EVALUATION CONTEXT
SYS$SERVICE_METRICS_R          SYS                            RULE SET
SYS$SERVICE_METRICS_N          SYS                            RULE SET
MGMT_RESPONSE                  DBSNMP                         PACKAGE BODY
DBMS_JDM_INTERNAL              SYS                            PACKAGE BODY

OBJECT_NAME                    OWNER                          OBJECT_TYPE
------------------------------ ------------------------------ -------------------
AQ$_KUPC$DATAPUMP_QUETAB_1_V   SYS                            EVALUATION CONTEXT
SQL>@?/rdbms/admin/utlrp.sql

如果不幸的是我们在出现问题的时候尝试重启数据库，就会发现数据库就起不来了。
SQL> drop table dual;
Table dropped.

SQL> select sysdate from dual;
select sysdate from dual
                    *
ERROR at line 1:
ORA-01775: looping chain of synonyms

SQL> shutdown immediate
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup
ORACLE instance started.

Total System Global Area  313159680 bytes
Fixed Size                  2227944 bytes
Variable Size             255852824 bytes
Database Buffers           50331648 bytes
Redo Buffers                4747264 bytes
Database mounted.
ORA-01092: ORACLE instance terminated. Disconnection forced
ORA-01775: looping chain of synonyms
Process ID: 434
Session ID: 237 Serial number: 5

alert日志中的相关内容如下：
Undo initialization finished serial:0 start:236214754 end:236215144 diff:390 (3 seconds)
Verifying file header compatibility for 11g tablespace encryption..
Verifying 11g file header compatibility for tablespace encryption completed
SMON: enabling tx recovery
Database Characterset is AL32UTF8
No Resource Manager plan active
Errors in file /u03/ora11g/diag/rdbms/test01/TEST01/trace/TEST01_ora_434.trc:
ORA-01775: looping chain of synonyms
Errors in file /u03/ora11g/diag/rdbms/test01/TEST01/trace/TEST01_ora_434.trc:
ORA-01775: looping chain of synonyms
Error 1775 happened during db open, shutting down database
USER (ospid: 434): terminating the instance due to error 1775
Instance terminated by USER, pid = 434
ORA-1092 signalled during: ALTER DATABASE OPEN...
opiodr aborting process unknown ospid (434) as a result of ORA-1092
Thu Nov 20 06:31:13 2014
ORA-1092 : opitsk aborting process
Thu Nov 20 06:32:02 2014

日志中提到的trace 文件的内容如下：

*** 2014-11-20 06:31:11.920
*** SESSION ID:(237.5) 2014-11-20 06:31:11.920
*** CLIENT ID:() 2014-11-20 06:31:11.920
*** SERVICE NAME:(SYS$USERS) 2014-11-20 06:31:11.920
*** MODULE NAME:(sqlplus@rac1 (TNS V1-V3)) 2014-11-20 06:31:11.920
*** ACTION NAME:() 2014-11-20 06:31:11.920
 
ORA-01775: looping chain of synonyms
ORA-01775: looping chain of synonyms

*** 2014-11-20 06:31:11.947
USER (ospid: 434): terminating the instance due to error 1775

如果确实知道问题的原因就轻车熟路的解决了，要不还需要费一番周折，开启一些更为详尽的trace来排查。
这个错误和数据库参数replication_dependency_tracking有关,默认是TRUE,我们需要暂时绕过这个校验，先把库启动起来，然后重建表dual就可以了。在修复以后，重启数据库恢复replication_dependency_tracking的默认值TRUE

SQL> show parameter track

NAME                                 TYPE
------------------------------------ ---------------------------------
VALUE
------------------------------
db_unrecoverable_scn_tracking        boolean
TRUE
replication_dependency_tracking      boolean
TRUE
SQL> alter system set replication_dependency_tracking=false;
alter system set replication_dependency_tracking=false
                 *
ERROR at line 1:
ORA-02095: specified initialization parameter cannot be modified

SQL>  alter system set replication_dependency_tracking=false scope=spfile;
System altered.

SQL> shutdown immediate
ORA-01507: database not mounted

ORACLE instance shut down.
SQL> startup
ORACLE instance started.

Total System Global Area  313159680 bytes
Fixed Size                  2227944 bytes
Variable Size             255852824 bytes
Database Buffers           50331648 bytes
Redo Buffers                4747264 bytes
Database mounted.
Database opened.
SQL>

SQL> select sysdate from dual;  --启动起来之后尝试，会发现问题没有修复，我们需要重建表dual
select sysdate from dual
                    *
ERROR at line 1:
ORA-01775: looping chain of synonyms

SQL>  CREATE TABLE "SYS"."DUAL"
  2             (       "DUMMY" VARCHAR2(1)
  3             ) TABLESPACE "SYSTEM" ;
Table created.

SQL> GRANT SELECT ON "SYS"."DUAL" TO PUBLIC WITH GRANT OPTION;
Grant succeeded.

SQL> insert into dual values('X');
1 row created.

SQL> commit;
Commit complete.

SQL> select sysdate from dual;  --修复以后问题似乎就解决了。
SYSDATE
------------------
20-NOV-14

SQL> show parameter track
NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
db_unrecoverable_scn_tracking        boolean     TRUE
replication_dependency_tracking      boolean     FALSE

SQL> alter system set replication_dependency_tracking=TRUE scope=spfile;
System altered.

SQL> shutdown immediate
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup
ORACLE instance started.

Total System Global Area  313159680 bytes
Fixed Size                  2227944 bytes
Variable Size             255852824 bytes
Database Buffers           50331648 bytes
Redo Buffers                4747264 bytes
Database mounted.
Database opened.
SQL> select sysdate from dual;
SYSDATE
------------------
20-NOV-14
然后可以重新编译失效对象，这个问题的解决就告一段落了。