How to recreate DUAL table in Oracle after deleting it ?

We should never drop a dual table, it will have serious impact on the database functionality .If we have dropped the dual table from sys user and startup the database next time it will be not open. To open the database we have follow the below steps .Therefore it is highly recommended not to delete the dual table .

SQL> conn /as sysdba
SQL> drop table dual;
 Table dropped.

SQL> desc dba_objects

Name                                 Null?                Type

---------------------        --------          ---------------

OWNER                                                 VARCHAR2(30)

OBJECT_NAME                                        VARCHAR2(128)

SUBOBJECT_NAME                                   VARCHAR2(30)

OBJECT_ID                                             NUMBER

DATA_OBJECT_ID                                    NUMBER

OBJECT_TYPE                                         VARCHAR2(19)

CREATED                                                DATE

LAST_DDL_TIME                                      DATE

TIMESTAMP                                            VARCHAR2(19)

STATUS                                                 VARCHAR2(7)

TEMPORARY                                            VARCHAR2(1)

GENERATED                                            VARCHAR2(1)

SECONDARY                                            VARCHAR2(1)

NAMESPACE                                            NUMBER

EDITION_NAME                                        VARCHAR2(30)

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

Check the alertlog file

Errors in file d:\oracle\diag\rdbms\god\god\trace\god_j000_26455.trc:
ORA-00604: error occurred at recursive SQL level 1
ORA-01775: looping chain of synonyms
ORA-12012: error on auto execute of job 21
ORA-01775: looping chain of synonyms
ORA-4063  :encountered when generating server alert SMG-3503 

SQL> shut immediate;                               
SQL> startup;

ORACLE instance started.
Total System Global Area 167772160 bytes
Fixed Size 1266392 bytes
Variable Size 113249576 bytes
Database Buffers 50331648 bytes
Redo Buffers 2924544 bytes
Database mounted.

ORA-01092: ORACLE instance terminated. Disconnection forced

Checking the alert logfile

 Errors in file d:\oracle\diag\rdbms\god\god\trace\god_ora_26538.trc:
ORA-01775: looping chain of synonyms
Error 1775 happened during db open, shutting down database
USER: terminating instance due to error 1775
Instance terminated by USER, pid = 26538

ORA-1092 signalled during: ALTER DATABASE OPEN...

Now we will create pfile  and add the below parameter and startup database.Create a pfile from spfile and add the below line in the pfile .

replication_dependency_tracking= false 

Now start the database using pfile;
 SQL> startup pfile='c:\initgod.ora'; 

Now database is open but it throws the error while creating the dummt table .

SQL>create table "SYS"."DUAL" ("DUMMY"  VARCHAR2(1)) ;
 CREATE TABLE "SYS"."DUAL" ( "DUMMY" VARCHAR2(1))
*
ERROR at line 1:
ORA-00604: error occurred at recursive SQL level 1
ORA-01775: looping chain of synonyms

Again Check the Alertlog file

 Errors in file d:\oracle\diag\rdbms\god\god\trace\god_j000_26583.trc:
ORA-12012: error on auto execute of job 21
ORA-01775: looping chain of synonyms

 SQL> shut immediate
 Now open the database using upgrade mode

SQL> startup upgrade pfile='c:\initgod.ora';
 ORACLE instance started.

Total System Global Area 167772160 bytes
Fixed Size 1266392 bytes
Variable Size 113249576 bytes
Database Buffers 50331648 bytes
Redo Buffers 2924544 bytes
Database mounted.
Database opened.

SQL> create table  "SYS"."DUAL" ("DUMMY"    VARCHAR2(1)) ;
Table created.

SQL> Insert Into Dual Values ('X') ;
 1 row created.

SQL> commit;
 Commit complete.

SQL> Grant Select On Dual To Public;
 Grant succeeded.

SQL> column OBJECT_NAME format a20;
SQL> select OWNER,OBJECT_NAME,OBJECT_TYPE,CREATED from dba_objects where object_name='DUAL';

OWNER         OBJECT_NAME     OBJECT_TYPE       CREATED     

---------    ---------------    ---------------    ------------  

SYS                 DUAL               TABLE                02-APR-10

PUBLIC              DUAL               SYNONYM          02-APR-10

Hence, Dual table is recreated.

Enjoy          J J J

What is Dual Table ?

Dual is a table which is created by oracle along with the data dictionary. It consists of exactly one column whose name is dummy and one record. The value of that record is X.  The reason to use the DUAL table isn't just portability, but optimization.  The owner of dual is SYS but dual can be accessed by every user.Dual is useful because it always exists, and has a single row, which is handy for select statements with constant expressions.
Example:


SQL> select 1+1  from dual ;
     1+1
----------
       2
SQL> select sysdate from dual;

SYSDATE
---------
3-MAR-11


DUAL was originally a table and the database engine would perform disk IO on the table when selecting from DUAL. This disk IO was usually logical IO (not involving physical disk access) as the disk blocks were usually already cached in memory. This resulted in a large amount of logical IO against the DUAL table.Later versions of the Oracle database have been optimized and the database no longer performs physical or logical IO on the DUAL table even though the DUAL table still actually exists.

If we drop a dual table then it will have serious impact on the database functionality. so we should never drop dual table. TOM KYTE explained about this in more details .Click the below link 

http://asktom.oracle.com/pls/asktom/f?p=100:11:0::::P11_QUESTION_ID:1562813956388

Also Check this link for more about dual table :
http://radiofreetooting.blogspot.com/2006/12/why-dual.html

Enjoy          J J J

Displaying Oracle background processes on Windows

We knows how to use the "ps -ef" command in UNIX to see Oracle background processes but we are not able to see the background processes in Windows ? When we view Oracle processes on Windows, all we see is one background process called oracle.exe.?

This is because in Windows, the "thread" model is used, and Oracle dispatches his own background tasks within the domain of the single process, oracle.exe. Hence, we cannot see any background processes from the Windows OS (but we can see listener process and parallel query slaves).To see details about the background processes in Windows,we need to run a dictionary query against the v$bgprocess view to see what the background processes are doing in Window .

The below sql statement is useful to view the background process in windows .

SQL> select a.sid,a.serial#, a.program, p.pid, p.spid, a.osuser, b.name, b.DESCRIPTION, p.PGA_USED_MEM   from v$session a,v$process p, v$bgprocess b where a.paddr=b.paddr 
      and    a.paddr=p.addr and p.background=1;
Output  : 

Enjoy          J J J