Question

I have Oracle Database 11g Enterprise Edition Release 11.2.0.1.0.
I have parent table t1 and t2 with foreign key which references t1(col1).
What I’m wondering is why locking is there?
Please check what I’ve done…

session 1

SQL> create table t1(col1 char(1), primary key(col1));
Table created.

SQL> insert into t1 values('1');
1 row created.
SQL> insert into t1 values('2');
1 row created.
SQL> insert into t1 values('3');
1 row created.
SQL> insert into t1 values('4');
1 row created.
SQL> insert into t1 values('5');
1 row created.

SQL> commit;
Commit complete.


SQL> create table t2(col1 char(1), col2 char(2), foreign key(col1) references t1(col1));
Table created.

SQL> insert into t2 values('1','0');
1 row created.
SQL> commit;
Commit complete.

SQL> update t2 set col2='9';   --not committed yet!
1 row updated.

session 2

SQL> delete from t1;    -- Lock happens here!!!

session 1

SQL> commit;
Commit complete.        

session 2

delete from t1          -- The error occurs after I commit updating query in session 1.
*
ERROR at line 1:
ORA-02292: integrity constraint (KMS_USER.SYS_C0013643) violated - child record found

Could anyone explain me why this happens?

Answer 1

delete from t1; tries to lock the child table, T2. If the session is waiting on the entire table lock it can’t even try to delete anything yet.

This unusual locking behavior occurs because you have an unindexed foreign key.

If you create an index, create index t2_idx on t2(col1);, you will get the ORA-02292 error instead of the lock.