On 07/19/2018 09:43 AM, Torsten Förtsch wrote:
On Thu, Jul 19, 2018 at 6:35 PM Adrian Klaver <adrian.klaver@xxxxxxxxxxx
<mailto:adrian.klaver@xxxxxxxxxxx>> wrote:
On 07/19/2018 07:15 AM, Torsten Förtsch wrote:
> Hi,
>
> assuming
>
> SELECT nextval('s'), currval('s');
>
> or
>
> SELECT * FROM (VALUES (nextval('s'), currval('s'))) t;
>
> is there any guarantee that the 2 output values are the same?
Assuming you are only working in single session:
https://www.postgresql.org/docs/10/static/functions-sequence.html
"currval
Return the value most recently obtained by nextval for this
sequence in the current session. (An error is reported if nextval has
never been called for this sequence in this session.) Because this is
returning a session-local value, it gives a predictable answer whether
or not other sessions have executed nextval since the current
session did."
I know that. My question was about the execution order of f1 and f2 in
"SELECT f1(), f2()". In theory they can be executed in any order. But
since the side effect in nextval determines the result of currval, I am
asking if that order is well-defined or considered an implementation
detail like in C.
To eliminate plan caching:
DO
$$
DECLARE
rs record;
BEGIN
FOR i IN 1..1000 LOOP
EXECUTE 'SELECT nextval($1), currval($1)' INTO rs USING
'order_test';
RAISE NOTICE 'Currval is %', rs.currval;
END LOOP;
END$$;
It still works over multiple runs, even when bumping LOOP counter to
100,000.
--
Adrian Klaver
adrian.klaver@xxxxxxxxxxx
