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();
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