Interesting relational integrity bug

Over in the OTN forums, Wilhelm demonstrates a method for disabling foreign key constraints in such a way that we can delete the parent data and then re-enable the foreign key without throwing an ORA-02298 exception.

The proper syntax for disabling a constraint is this:

SQL> ALTER TABLE temp_child DISABLE CONSTRAINT what_the_fk;

Table altered.

SQL>  delete from temp_parent;

2 rows deleted.

SQL>  ALTER TABLE temp_child ENABLE VALIDATE CONSTRAINT what_the_fk;
 ALTER TABLE temp_child ENABLE VALIDATE CONSTRAINT what_the_fk
                                                   *
ERROR at line 1:
ORA-02298: cannot validate (APC.WHAT_THE_FK) - parent keys not found


SQL>

But if we include the mystical keyword VALIDATE in the disabling command we can corrupt our relational integrity quite nicely:

SQL> INSERT INTO temp_parent VALUES(1);

1 row created.

SQL> INSERT INTO temp_parent VALUES(2);

1 row created.

SQL> ALTER TABLE temp_child ENABLE CONSTRAINT what_the_fk;

Table altered.

SQL> ALTER TABLE temp_child DISABLE VALIDATE CONSTRAINT what_the_fk;

Table altered.

SQL> delete from temp_parent;

2 rows deleted.

SQL> ALTER TABLE temp_child ENABLE CONSTRAINT what_the_fk;

Table altered.

SQL> select * from temp_parent;

no rows selected

SQL>