ORA-04091 Error on "after update" trigger, how to get around? - sql

Question

I have a trigger set to fire after an update of a specific column. I want to update the value of a specific column after a specific value from another column changes to another specific value.
I'm getting the errors:
ORA-04091: table tableName is mutating, trigger/function may not see it
ORA-06512: at "triggerName", line 14
ORA-04088: error during execution of trigger 'triggerName'
I've rewritten this as a before update and after update, as well as tried storing the logic in a function, and using "pragma autonomous_transaction", but the error still gets thrown.
BEFORE update of columnName1 ON tableName
FOR EACH ROW
BEGIN
if :new.columnName1 = 3 AND :old.columnName1 = 1 then
update tablename
set columnName2= MOD(sequenceName.NextVal, 5) + 1
where tableName.columnName2 = :old.columnName2;
end if;
END;
/
I don't understand why the entire table is labelled as "mutating" when I am not updating the column affected by the update the trigger is responding to. Surely, you should be able to update the value of a column in a table if another value in the table changes, or am I crazy here?
Note: Only one entry would be affected at a time. In my application logic, I update the status of some person in the database. I want the database to do some logic only on a specific status change, and I want to avoid using API calls for the logic here, as you can see, it is simply one line of logic in PLSQL.
Thanks

Answer 1

Perhaps you don't really want to update the table but just change a value in the specific row being updated.
If so, eschew the update and just set the value:
BEFORE update of columnName1 ON tableName
FOR EACH ROW
BEGIN
if :new.columnName1 = 3 AND :old.columnName1 = 1 then
:new.columnName2 := MOD(sequenceName.NextVal, 5) + 1 ;
end if;
END;