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

I don't like the locking... take a look at Ex 37-1 at the end of
http://lnk.nu/postgresql.org/fhe.html for a better way (though, the
comment below about going into an infinite loop is a good observantion,
but I think perhaps after some number of fast tries it should start
putting a sleep in the loop, rather than just arbitrarily bombing after
10 tries.

Also, I remember discussion on -performance about this from folks using
it in the real world... the problem they ran into is that doing the
updates in the cache/mview table directly bloated it too much... they
found it was better to just insert changes into an interim table, and
then periodically batch-process that table.

On Wed, Aug 15, 2007 at 08:01:24AM +0200, Pavel Stehule wrote:
> Hello
>
> 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();
>
> ---------------------------(end of broadcast)---------------------------
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--
Decibel!, aka Jim Nasby decibel(at)decibel(dot)org
EnterpriseDB http://enterprisedb.com 512.569.9461 (cell)