My Experiments With Oracle

Soracle

All about the DUAL

Posted by sriram on May 4, 2011


The Magic table DUAL is owned by User SYS as its an dictionary object.

ind> select object_name,owner,object_type
2 from dba_objects
3 where object_name like ‘%DUAL%’;

OBJECT_NAME OWNER OBJECT_TYPE
———— ———————- ——————-
DUAL SYS TABLE
DUAL PUBLIC SYNONYM

2 rows selected.

‘Dual’ is a special table with 1X1 (1 row and 1 column) owned by sys which can be used to know the result I can say :).

ind> desc dual;
Name Null? Type
—————————————– ——– ————-
DUMMY VARCHAR2(1)

ind> desc x$dual
Name Null? Type
—————————————– ——– ———————-
ADDR RAW(4)
INDX NUMBER
INST_ID NUMBER
DUMMY VARCHAR2(1)

In specific cases like whenever Database is not fully up(recovery process),at that situation oracle reads the table dual from a fixed table/view

ind> select * from dual;

D

X

1 row selected.

ind> select * from x$dual;

ADDR INDX INST_ID D
——– ———- ———- –
0366CD54 0 1 X

1 row selected.

Never ever try to play with dual/dictionary objects which will ruin you DB.

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