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

On 1/8/08, codeWarrior <gpatnude(at)hotmail(dot)com> wrote:
> Jamie:
>
> I think you are probably having slowdown issues in your "DELETE FROM WHERE
> NOT IN SELECT ORDER BY DESCENDING" construct -- that seems a bit convoluted
> to me....

Hmm so rather than NOT IN ( .. LIMIT 50) would you suggest IN ( ...
OFFSET 50) like in Erik's example? Or something else entirely?

> ALSO: It looks to me like you have a column named "timestamp' ??? This is
> bad practice since "timestamp" is a reserved word... You really ought NOT to
> use reserved words for column names... different debate.

I do realize it would be better to use something else and thanks for
the tip :-) This is an established database and "timestamp" has been
used in other tables which is why I stuck to it here.. one day when
time permits maybe I'll rename them all!

> Why bother deleting records anyway ? Why not alter your query that tracks
> the 50 records to LIMIT 50 ???

The read query does LIMIT 50 and the reason for deleting the rest of
the records is because they're not needed by the application and
there's loads of them being created all the time (currently several
million unnecessary rows) -- I imagine eventually this will slow
things down?

Do you think a regular batch process to delete rows might be more
appropriate than a trigger in this scenario?

Thanks,
Jamie