ORA-04091: table is mutating, trigger/function may not see

Question

Hello All,
I am not an expert in oracle and I have been facing a problem where I get a message as follows :
ORA-04091: table <table> is mutating, trigger/function may not see it
I have a table tab1 which has ID, name, and some other columns. the rows in this table form a parent-child type relationship based on some columns within that table. I have created another table called relationship table. The relationship table has 2 columns ID and parentID. I keep this table up to date using triggers. For example if a new record is inserted or upadted in tab1 which affects the parent - child relationship, i update the relationship table accordingly using an insert / update trigger and it works fine.
I created another trigger as follows
create or replace trigger MY_DELETE_TRIGGER
after delete on tab1
for each row
BEGIN
update relationships a set a.parentID = null where a.parentID = :OLD.ID;
delete from relationships a where a.id = :OLD.ID;
END MY_DELETE_TRIGGER;
Basically what I am doing is, the record once deleted, if it was parent of anybody else, i am setting the parent as null and the record itself, if it was in the relationships as a child, I am removing that row.
This is when I get the table is mutating error, and I don't know how to fix it.
Can anybody please help me,
Thanks in advance

Answer 1

Could you provide more details?
SQL> select * from tab1;
        ID NAME
         1 test1
         2 test2
SQL> select * from relationships;
        ID   PARENTID
         1          1
         2          1
SQL> create or replace trigger MY_DELETE_TRIGGER
  2  after delete on tab1
  3  for each row
  4  BEGIN
  5  update relationships a set a.parentID = null where a.parentID = :OLD.ID;
  6  delete from relationships a where a.id = :OLD.ID;
  7  END MY_DELETE_TRIGGER;
  8  /
Trigger created.
SQL> delete from tab1
  2  where id = 1;
1 row deleted.
SQL> select * from relationships;
        ID   PARENTID
         2
SQL> rollback;
Rollback complete.
SQL> insert into relationships values(1,1);
1 row created.
SQL> select * from relationships;
        ID   PARENTID
         1          1
         2          1
         1          1
SQL> delete from tab1
  2  where id = 1;
1 row deleted.
SQL> select * from relationships;
        ID   PARENTID
         2
SQL> disconnect
Disconnected from Oracle9i Enterprise Edition Release 9.2.0.3.0 - 64bit Production
With the Partitioning option
JServer Release 9.2.0.3.0 - Production