Re: is this trigger safe and efective? - locking (caching via triiggers) - Mailing list pgsql-general

On Aug 15, 2007, at 4:57 AM, Pavel Stehule wrote:

> I write sample about triggers and i have question. is my solution
> correct and exists better solution?
>
> Regards
> Pavel Stehule
>
> DROP SCHEMA safecache CASCADE;
>
> CREATE SCHEMA safecache;
>
> CREATE TABLE safecache.source_tbl(category int, int_value int);
>
> CREATE TABLE safecache.cache(category int, sum_val int);
>
> CREATE OR REPLACE FUNCTION safecache.source_tbl_trg_fce()
> RETURNS trigger AS
> $$
> BEGIN
>   IF TG_OP = 'INSERT' THEN
>     -- row cannot exists in cache -- complication
>     -- I would to finish these transaction without conflict
>     IF NOT EXISTS(SELECT category
>                      FROM safecache.cache
>                     WHERE category = NEW.category) THEN
>       LOCK TABLE safecache.cache IN SHARE ROW EXCLUSIVE MODE;
>       -- I have to repeat test
>       IF NOT EXISTS(SELECT category
>                        FROM safecache.cache
>                       WHERE category = NEW.category) THEN
>         INSERT INTO safecache.cache
>            VALUES(NEW.category, NEW.int_value);
>       END IF;
>     ELSE
>       -- simple
>       UPDATE safecache.cache
>          SET sum_val = sum_val + NEW.int_value
>         WHERE category = NEW.category;
>     END IF;
>   ELSEIF TG_OP = 'UPDATE' THEN
>     -- if category is without change simple
>     IF NEW.category = OLD.category THEN
>       UPDATE safecache.cache
>          SET sum_val = sum_val + (NEW.int_value - OLD.int_value)
>         WHERE category = OLD.category;
>     ELSE
>       -- old category has to exists
>       UPDATE safecache.cache
>          SET sum_val = sum_val - OLD.int_value
>         WHERE category = OLD.category;
>       -- new category is maybe problem
>       IF NOT EXISTS(SELECT category
>                        FROM safecache.cache
>                       WHERE category = NEW.category) THEN
>         LOCK TABLE safecache.cache IN SHARE ROW EXCLUSIVE MODE;
>         -- I have to repeat test
>         IF NOT EXISTS(SELECT category
>                          FROM safecache.cache
>                         WHERE category = NEW.category) THEN
>           INSERT INTO safecache.cache
>              VALUES(NEW.category, NEW.int_value);
>         END IF;
>       ELSE
>         -- simple, new category exists
>         UPDATE safecache.cache
>            SET sum_val = sum_val + OLD.int_value
>           WHERE category = NEW.category;
>       END IF;
>     END IF;
>   ELSE -- DELETE
>     -- value have to exist in cache, simple
>     UPDATE safecache.cache
>        SET sum_val = sum_val - OLD.int_value
>       WHERE category = OLD.category;
>   END IF;
>   RETURN NEW;
> END
> $$ LANGUAGE plpgsql;
>
> CREATE TRIGGER actualise_cache
>    AFTER INSERT OR UPDATE OR DELETE
>    ON safecache.source_tbl
>    FOR EACH ROW EXECUTE PROCEDURE safecache.source_tbl_trg_fce();

 From what I can tell from your example it's "correct" and should
work under light loads.  However, if that trigger will fire a lot,
you might see those updates "stacking" due to the necessary locking
(both your explicit locks and those take out by the updates).  What
I've done in that case (this is actually a pretty standard setup), is
to have the trigger just make inserts into another table of the
category that needs to be updated and by how much.  The you have some
other (probably user-land) process periodically sweep that table,
aggregate the updates to the cache table, then delete the interim
entries just processed.  Oh yeah, you could simplify that function a
lot by simply initializing your cache table with a row for each
category with sum_val = 0.  Then it's all updates and you don't need
those locks to determine if the category exists there.

Erik Jones

Software Developer | Emma®
erik@myemma.com
800.595.4401 or 615.292.5888
615.292.0777 (fax)

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