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.
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