Before delete trigger and foreign key relationship

Question

Hi,
I am analysing one database for migration. On one parent table there is before delete trigger , to delete records from child. Also there is foreign key relationship on child table for this parent table.
When I am deleting a row from parent, message gets displayed as "there are child records found."
I would like to know, if there is foreign key relatioship then delete trigger on parent does't work, what is exactly happening?

Answer 1

Could you post that trigger code and the Oracle version as well?
With basic assumptions, I can't reproduce what you have stated here.
Connected to:
Oracle Database 10g Enterprise Edition Release 10.2.0.4.0 - Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options
SQL> create table parent (id number primary key);
Table created.
SQL> create table child (id number);
Table created.
SQL> alter table child add constraint fk_parent foreign key (id) references parent;
Table altered.
SQL> create or replace trigger bdr_parent
  2  before delete on parent
  3  for each row
  4  begin
  5  delete from child where id = :old.id;
  6  end;
  7  /
Trigger created.
SQL> insert into parent (id) values (1);
1 row created.
SQL> insert into child (id) values (1);
1 row created.
SQL> commit;
Commit complete.
SQL> delete from parent where id = 1;
1 row deleted.
SQL> select * from parent;
no rows selected
SQL> select * from child;
no rows selected
SQL> rollback;
Rollback complete.
SQL> alter table child drop constraint fk_parent;
Table altered.
SQL> alter table child add constraint fk_parent foreign key (id) references parent on delete cascade;
Table altered.
SQL> delete from parent where id = 1;
delete from parent where id = 1
ERROR at line 1:
ORA-04091: table SCOTT.CHILD is mutating, trigger/function may not see it
ORA-06512: at "SCOTT.BDR_PARENT", line 2
ORA-04088: error during execution of trigger 'SCOTT.BDR_PARENT'
SQL>