Cannot drop nonexistent constraint and cannot create it either While testing some migration scripts with a copy of production data (scripts run fine with development data) I found a curious situation. A CONSTRAINT has changed so I'm issuing DROP + ADD commands:

ALTER TABLE A_DUP_CALLE
DROP CONSTRAINT A_DUP_CALLE_UK1;

ALTER TABLE A_DUP_CALLE
ADD CONSTRAINT A_DUP_CALLE_UK1 UNIQUE (
    CONTROL_ID,
    CALLE_AYTO_DUPL
)
ENABLE;


The DROP command worked fine but the ADD one failed. Now, I'm into a vicious circle. I cannot drop the constraint because it doesn't exist (initial drop worked as expected):


  ORA-02443: Cannot drop constraint  - nonexistent constraint


And I cannot create it because the name already exists:


  ORA-00955: name is already used by an existing object


I type A_DUP_CALLE_UK1 into SQL Developer's Search box and... there it is! Owner, table name, tablescape... everything matches: it isn't a different object with the same name, it is my original constraint. The table appears in the constraint details but the constraint does not appear in the table's details.

My questions:


What's the explanation for this?
How can I ensure it won't happen when I make the real upgrade in live server?


(Server is 10g XE, I don't have enough reputation to create the tag.)
At a guess I'd say Marian is right and this is caused by a unique index and constraint having the same name, eg:

create table t( k1 integer, k2 integer, 
                constraint u1 unique(k1,k2) using index(create unique index u1 on t(k1,k2)),
                constraint u2 unique(k2,k1) using index u1);

select count(*) from user_indexes where index_name='U1';

COUNT(*)               
---------------------- 
1  

alter table t drop constraint u1;

select count(*) from user_indexes where index_name='U1';

COUNT(*)               
---------------------- 
1  


Normally when you add a unique constraint, a unique index with the same name is created - but the index and constraint are not the same thing. Have a look at all_indexes to see if there is an index called A_DUP_CALLE_UK1 and try and figure out if it is used by something else before you drop it!