How to solve ORA-04091

Question

hi,
i made a trigger on a table fire on after insertion but i m having these error so plz try to solve it.
CREATE OR REPLACE TRIGGER TG1
AFTER INSERT
ON MASTER
REFERENCING NEW AS NEW OLD AS OLD
FOR EACH ROW
DECLARE
COUNT NUMBER;
BEGIN
SELECT COUNT(*) INTO COUNT FROM MASTER
WHERE P_ID =:NEW.P_ID ;
BEGIN
IF COUNT=1 THEN
UPDATE MASTER SET PCODE =:COUNT;
END IF;
IF COUNT=2 THEN
UPDATE MASTER SET PCODE =:COUNT+1;
END IF;
END;
END;
ORA-04091: table master is mutating, trigger/function may
not see it
ORA-06512: at "TG1", line 5
ORA-04088: error during execution of trigger 'TG1'

Answer 1

You don't understand mutating tables and how to avoid them. Nor do you understand how multi-user systems work. Finally you appear not to understand how to use the documentation Given all that, your soubriquet is overambitious. You might want to consider a more modest handle.
As for the specifics, how to avoid the ORA-4091 error, the root cause is that we cannot select on the table in question in a FOR EACH ROW trigger. You seem to be after an incremental count, presumably as unique identifier. This is the wrong approach: use a sequence and stop worrying about gaps.
Cheers, APC
blog: http://radiofreetooting.blogspot.com