The best answer: RTFM!
I found this example in the docs, although it should really exit the
Loop after 10 failed attempts instead of going indefinitely:
CREATE FUNCTION merge_db(key INT, data TEXT) RETURNS VOID AS
$$
BEGIN
LOOP
UPDATE db SET b = data WHERE a = key;
IF found THEN
RETURN;
END IF;
BEGIN
INSERT INTO db(a,b) VALUES (key, data);
RETURN;
EXCEPTION WHEN unique_violation THEN
-- do nothing
END;
END LOOP;
END;
$$
LANGUAGE plpgsql;
On 3/4/07, Berend Tober <btober@xxxxxxxxxxxxxxxx> wrote:
Postgres User wrote:
> I'm using this code to increment a counter table:
>
> IF Exists (SELECT keyfld FROM counter_tbl WHERE key_fld = 'key_val') THEN
> UPDATE counter_tbl SET counter_fld = counter_fld + 1
> WHERE key_fld = 'key_val';
> ELSE
> INSERT INTO counter_tbl(key_fld, counter_fld) VALUES('key_val', 1);
> END IF;
>
> Now, I assume that it's possible for another session to INSERT a row
> in the microseconds that exist between the Select and Insert
> statements above.
>
> I also assume that I can wrap the above code in a transaction, and if
> the transaction fails (because another session's Insert causes my
> Insert to fail), then I simply need to re-execute it once. (Updates
> should never fail.)
>
> Does anyone have a simple example of the best way to code this type of
> transaction- and the best way to re-execute the same code on failure?
> I could use a loop but I'm not sure if that's the best solution.
I think I would try the INSERT first. If it fails, then trap the
exception and do the UPDATE
