Re: Add a test case related to the error "cannot fetch toast data without an active snapshot"

> if a procedure fetches a toasted value into a local variable, commits,
> and then tries to detoast the value.

I spent some time and tried to reproduce this error by using [1]
queries. But the error did not occur. Not sure whether I followed what
is mentioned in the above comment. Please correct me if I am wrong.

[1]:
CREATE TABLE toasted(id serial primary key, data text);
INSERT INTO toasted(data) VALUES((SELECT string_agg(random()::text,
':') FROM generate_series(1, 1000)));
INSERT INTO toasted(data) VALUES((SELECT string_agg(random()::text,
':') FROM generate_series(1, 1000)));

DO $$
DECLARE v_r record;
DECLARE vref_cursor REFCURSOR;
BEGIN
OPEN vref_cursor FOR SELECT data FROM toasted;
LOOP
fetch vref_cursor into v_r;
INSERT INTO toasted(data) VALUES(v_r.data);
COMMIT;
END LOOP;
END;$$;

Thanks & Regards,
Nitin Jadhav

On Fri, Jan 27, 2023 at 6:26 PM Nitin Jadhav
<nitinjadhavpostgres(at)gmail(dot)com> wrote:
>
> Hi,
>
> I was going through the comments [1] mentioned in
> init_toast_snapshot() and based on the comments understood that the
> error "cannot fetch toast data without an active snapshot" will occur
> if a procedure fetches a toasted value into a local variable, commits,
> and then tries to detoast the value. I would like to know the sample
> query which causes such behaviour. I checked the test cases. Looks
> like such a case is not present in the regression suit. It is better
> to add one.
>
>
> [1]:
> /*
> * GetOldestSnapshot returns NULL if the session has no active snapshots.
> * We can get that if, for example, a procedure fetches a toasted value
> * into a local variable, commits, and then tries to detoast the value.
> * Such coding is unsafe, because once we commit there is nothing to
> * prevent the toast data from being deleted. Detoasting *must* happen in
> * the same transaction that originally fetched the toast pointer. Hence,
> * rather than trying to band-aid over the problem, throw an error. (This
> * is not very much protection, because in many scenarios the procedure
> * would have already created a new transaction snapshot, preventing us
> * from detecting the problem. But it's better than nothing, and for sure
> * we shouldn't expend code on masking the problem more.)
> */
>
> Thanks & Regards,
> Nitin Jadhav