What PRAGMA statements would be used inside an EXCEPTION block? - oracle

Question

I was playing around with Oracle exceptions and tried to do some preprocessing between EXCEPTION and the WHERE statements. That is,
EXCEPTION
some_operation_here();
WHEN yaddayadda THEN
...
PL/SQL Developer said that that wasn't kosher – oh well – but its error message intrigued me: it was expecting either WHEN or PRAGMA. I am not thoroughly familiar with all the PRAGMA directives, but it doesn't seem like any of them are applicable in an exception block unless for some reason you waited till this point to bind an error code to an exception you had declared earlier. Why would you ever need to use a PRAGMA directive here?

Answer 1

A little experimentation tells me that you can, in fact put a PRAGMA in the exception block, but I can't see much use to it. The following executes successfully, but the raised error triggers the OTHER section, not the section for the newly defined exception (e.g. it returns "Old exception"). It would appear that this is an undocumented feature.
DECLARE
v NUMBER;
new_divide_zero EXCEPTION;
BEGIN
v := 1 / 0;
EXCEPTION
PRAGMA EXCEPTION_INIT (new_divide_zero, -1476);
WHEN new_divide_zero THEN
DBMS_OUTPUT.put_line ('New exception');
WHEN OTHERS THEN
DBMS_OUTPUT.put_line ('Old exception');
END;
Similarly, I tried putting a AUTONOMOUS_TRANSACTION in the exception block, but it too appears to have no effect (in this case, both rows are inserted).
CREATE TABLE test_results (result VARCHAR2 (2000));
BEGIN
DECLARE
v NUMBER;
new_divide_zero EXCEPTION;
BEGIN
insert into test_results values ('Test Value');
v := 1 / 0;
EXCEPTION
PRAGMA AUTONOMOUS_TRANSACTION;
WHEN OTHERS THEN
INSERT INTO test_results
VALUES ('Old exception');
Commit;
END;
ROLLBACK;
END;
The Oracle documentation (12g, which is the version i tested this on) does not mention using PRAGMA in the exception block, so it's definitely undocumented. On the other hand, it doesn't seem to be much of a feature, as it doesn't appear to actually do anything...
WHEN { exception_name [ OR exception_name ]... | OTHERS } THEN
statement [ statement ]...

Answer 2

It is syntax error. To catch exception you must use following syntax:
declare
yaddayadda exception;
begin
...
exception
WHEN yaddayadda THEN
some_operation_here();
end;
You need to use pragma exception_Init to assign you custom exception with sql error code.
For example:
declre
l_id number;
e_resource_busy EXCEPTION;
PRAGMA EXCEPTION_INIT(e_resource_busy, -54);
begin
select id
into l_id
from job_table
where status = 'RUNNING'
for update nowait;
exception
WHEN e_resource_busy THEN
-- row is locked
some_operation_here();
end;