Question about catching exception

Hello!

I have a question about catching exceptions.

If I write a plpgsql function like this

begin
do stuff;
exception
when X then
....
when Y then
...
end;

If the "do stuff" part can result in two different unique_violation
exception (having two unique constraints), how can I detect which one
was triggered?

Hello

you have to parse a sqlerrm variable

CREATE OR REPLACE FUNCTION public.test(a integer, b integer)
RETURNS void
LANGUAGE plpgsql
AS $function$
begin
insert into foo values(a,b);
exception when unique_violation then
raise notice '% %', sqlerrm, sqlstate;
end;
$function$

postgres=# select test(4,2);
NOTICE: duplicate key value violates unique constraint "foo_b_key" 23505
test
──────

(1 row)

Time: 9.801 ms
postgres=# select test(3,2);
NOTICE: duplicate key value violates unique constraint "foo_a_key" 23505
test
──────

(1 row)

Time: 17.167 ms

regards

Pavel Stehule

> If the "do stuff" part can result in two different unique_violation
> exception (having two unique constraints), how can I detect which one
> was triggered?
>
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> Hello
>
> you have to parse a sqlerrm variable

That's one way to do that. Another - more complex but more correct in many
cases is using two separate blocks.

BEGIN
... do stuff involving constraint A
EXCEPTION
WHEN unique_violation THEN ...
END;

BEGIN
... do stuff involving constraint B
EXCEPTION
WHEN unique_violation THEN ...
END;

But that's not possible if there are two unique constraints involved in a
single SQL statement (e.g. inserting into a table with two unique
constraints).

regards
Tomas