[OT] Re: installing Oracle on Debian AMD64
On Mon, 31 Oct 2005, in a new attempt to enlighten us, Adam Skutt wrote:
AS> Sven Mueller wrote:
Leaving the all-times classical "My RDBMS is better than yours"
war aside (hey!, I have my preferences too ;)...
Correct me if I'm wrong, but...
AS> Consider the simple case of a forum, and a column that indicates posts per
AS> thread. That must be updated on every post, and the code might look
AS> something like this (psuedo code):
AS> VAR pc;
AS> INSERT INTO posts VALUES (22, 'my post', 'my name', 'some post text');
AS> SELECT INTO pc post_count FROM threads WHERE tid = 22;
AS> UPDATE threads SET post_count = pc+1 WHERE tid=22;
AS> But they say nothing about other transactions. As such, it's possible for
AS> two transactions to run at the same time, see the same value of pc, and
AS> update it to the same value. That's a bug.
If you need to update a value in threads.post_count, that NEEDS to
take in consideration what OTHER transactions do on table posts, then that
code shouldn't be inside the transaction. I think that's wrong, because
transactions are there to provide, among other things, "a snapshot" of
the DB to an atomic operation. And as such, you are not supposed to know
what other transactions are doing at the same time that yours.
If threads.post_count needs to be updated CONSIDERING what ALL the
transactions on table posts are doing, I think you should be using a POST
INSERT TRIGGER on posts, that takes care of the issue of updating
Assuming the first insert column of posts is named tid (you didn't
gave the column names), something like should work
CREATE TRIGGER updateThreads
AFTER INSERT ON posts
FOR EACH ROW
EXECUTE PROCEDURE updateThreads();
CREATE FUNCTION updateThreads() RETURNS "trigger"
SELECT count(*) +1 FROM threads
WHERE tid = NEW.tid
UPDATE threads SET post_count = pc WHERE tid=NEW.tid;
' LANGUAGE plpgsql;
Then, you don't even need a transaction ;)
INSERT INTO posts VALUES (22, 'my post', 'my name', 'some post text');
will fire the post insert trigger if the insert operation on post
was succesful, and threads.post_count will be updated to the correct
As I said, correct me if I'm wrong. Just don't shout me :))))
"It is only as we develop others that we permanently succeed."
(Harvey S. Firestone)