Re: How to keep at-most N rows per group? periodic DELETEs or constraints or..?

Hi codeWarrior,

codeWarrior wrote:
> > For user_ids that have more than 50 rows, I want to keep the most
> > recent 50 and delete the rest.
> How about using a trigger to call a stored procedure ? [ON INSERT to
> user_item_history DO ...]

[snip]

Thanks for your input! I've implemented this but I'm concerned about
performance. As I mentioned it's frequently being added to and this
function will be called maybe a couple of times a second. In my brief
initial tests it seems like this is running quite slowly...

Just to make sure I haven't done anything obviously wrong, I've
included my implementation below.. if you could look over it I'd
really appreciate it.

CREATE OR REPLACE FUNCTION user_item_history_limit() RETURNS TRIGGER
AS $_$
DECLARE
threshold integer = 50;
numrows integer;
BEGIN
SELECT INTO numrows count(*) FROM user_item_history WHERE user_id
= new.user_id;
IF numrows > threshold THEN
DELETE FROM user_item_history WHERE user_id = new.user_id AND
id NOT IN (
SELECT id FROM user_item_history
WHERE user_id = new.user_id
ORDER BY timestamp DESC LIMIT threshold);
RAISE NOTICE '% rows exceeds threshold of % for user_id %;
trimming..', numrows, threshold, new.user_id;
END IF;
RETURN new;
END;
$_$
LANGUAGE plpgsql;

CREATE TRIGGER user_item_history_limit AFTER INSERT ON user_item_history
FOR EACH ROW EXECUTE PROCEDURE user_item_history_limit();

Any suggestions greatly appreciated!

Thanks again,
Jamie