Detecting a Deadlock in a Stored Procedure - oracle

Question

I am trying to write a deadlock proof stored procedure. It is based on the following concept.
I have been trying to write a stored procedure which is based on the following concept. The procedure will try to drop a constraint on a table and if in case it detects a deadlock situation, it wait for some time before trying again. The important thing is it should only retry in case of a Deadlock or a NOWAIT error, all other errors are to be handled via exceptions.
Procedure test
is
BEGIN
<<label>>
DROP constraint on a table
if (deadlock(ORA-00060)/Nowait Error (ORA-0054)) detected
then
sleep for 60 seconds
Goto label
exception
when others.
It would be great if any of the experts please help me with this. A similar example would be highly helpful. Thank you for your help.

Answer 1

While some people harbour an irrational aversion to goto it remains true that usually we can implement the same logic without using the construct. That is true here: a simple while loop is all that is necessary:
create or replace procedure drop_constraint_for_sure
( p_table_name in varchar2
, p_constraint_name in varchar2
)
is
x_deadlock exception;
pragma exception_init(x_deadlock, -60);
x_nowait exception;
pragma exception_init(x_nowait, -54);
begin
loop
begin
execute immediate 'alter table '|| p_table_name
|| ' drop constraint ' || p_constraint_name
|| ' cascade' ;
exit;
exception
when x_deadlock then null;
when x_nowait then null;
end;
dbms_lock.sleep(60);
end loop;
end;
/
Note that the sleep function requires the execute privilege on SYS.DBMS_LOCK. This is not granted by default, so if you don't have it you'll need to ask your friendly DBA to grant it.
Also note that this implementation doesn't have any form of abort. So it will loop eternally, until the constraint is dropped or some other exception occurs. In real life you should include a loop count with an additional exit test on a threshold for the count. Although in real life I probably wouldn't want a stored procedure like this anyway: I prefer knowing as soon as possible when someone is using a table I'm trying to alter.

Answer 2

Hope below snippet gives you can an idea to achieve your requirement.
CREATE OR REPLACE
PROCEDURE DRP_CONST_DEALDOCK
AS
DEADLOCK_EX EXCEPTION;
NO_WAIT_EX EXCEPTION;
PRAGMA EXCEPTION_INIT(DEADLOCK_EX,-60);
PRAGMA EXCEPTION_INIT(NO_WAIT_EX, -54);
lv_cnt PLS_INTEGER;
BEGIN
BEGIN
EXECUTE IMMEDIATE 'ALTER TABLE STACK_OF_TEST DROP CONSTRAINT SYS_C00375020';
EXCEPTION
WHEN DEADLOCK_EX THEN
dbms_output.put_line('nowait exception');
DBMS_LOCK.SLEEP(10);
SELECT COUNT(1)
INTO lv_cnt
FROM v$locked_object
WHERE object_id IN
(SELECT OBJECT_ID FROM DBA_OBJECTS WHERE OBJECT_NAME = 'STACK_OF_TEST'
);
WHILE lv_cnt > 0
LOOP
dbms_lock.sleep(10);
SELECT COUNT(1)
INTO lv_cnt
FROM v$locked_object
WHERE object_id IN
(SELECT OBJECT_ID FROM DBA_OBJECTS WHERE OBJECT_NAME = 'STACK_OF_TEST'
);
END LOOP;
WHEN NO_WAIT_EX THEN
dbms_output.put_line('nowait exception');
DBMS_LOCK.SLEEP(10);
SELECT COUNT(1)
INTO lv_cnt
FROM v$locked_object
WHERE object_id IN
(SELECT OBJECT_ID FROM DBA_OBJECTS WHERE OBJECT_NAME = 'STACK_OF_TEST'
);
WHILE lv_cnt > 0
LOOP
dbms_lock.sleep(10);
SELECT COUNT(1)
INTO lv_cnt
FROM v$locked_object
WHERE object_id IN
(SELECT OBJECT_ID FROM DBA_OBJECTS WHERE OBJECT_NAME = 'STACK_OF_TEST'
);
END LOOP;
WHEN OTHERS THEN
dbms_output.put_line(SQLCODE||'-->'||SQLERRM);
END;
END;