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

Question

I need to modify an existing PK. Therefore I drop an recreate it.

ALTER TABLE B DROP CONSTRAINT PK_B;
ALTER TABLE B ADD CONSTRAINT PK_B PRIMARY KEY ("TYP", "NR", "HH", "QUART");

Unfortunately the last Statement will give me an error ORA-00955

If I create the PK constraint like it was defined originally with:

ALTER TABLE B ADD CONSTRAINT PK_B PRIMARY KEY ("TYP", "NR", "HH");

everything works fine.

Answer 1

Perhaps there is an INDEX associated with the PRIMARY KEY CONSTRAINT, and it is also named as PK_B.

You can check it as :

SELECT * FROM USER_INDEXES WHERE TABLE_NAME='<table_name>';

If that's true, then do :

ALTER INDEX "PK_B" RENAME TO "PK_XYZ";

Update : Regarding ALTER INDEX statement, few important points as mentioned by Justin in the comments

Oracle implicitly creates an UNIQUE index to support the PRIMARY KEY CONSTRAINT. Since, the index is of the same name that of the primary key, and now that the primary key is being modified, it is better to drop and re-create the index again as per the definition of the old primary key.

My conclusion :

• The primary key constraint is enforced through a unique index.
• If Oracle already finds an index – unique or non-unique – it uses it for the primary key.
• If the index was initially created as non-unique, it will continue to show as non-unique, however it will actually be a unique index.

A good demonstration and quite detailed on other aspects too, by Arup : Primary Keys Guarantee Uniqueness? Think Again.