On Fri, Feb 27, 2004 at 12:05:48PM -0500, Mike Mascari wrote:
> Michael Chaney wrote:
>
> >Please, before continuing this thread, read my post below. What you're
> >all getting around to, albeit painfully, is that this subquery is
> >worthless as-is. This is the mysql way of finding rows in one table
> >with no match in another without the convenience of the "in" or "exists"
> >constructs.
> >
> >Because we're using Postgres and have those constructs, the original
> >query can be rewritten simply with either:
> >
> >delete from LogEvent where EventType != 'i' and ItemID not in
> > (select ItemID from Item)
> >
> >That's it. That's the whole query. It does what he wants.
>
> One more minor point. :-)
>
> If you are using 7.3 or earlier, PostgreSQL will sequentially scan
> the IN subquery result, which executes quite slowly and therefore
> the EXISTS method Stephan stated should be used:
>
> DELETE FROM LogEvent
> WHERE EventType != 'i' AND NOT EXISTS (
> SELECT 1
> FROM Item
> WHERE Item.ItemID = LogEvent.ItemID
> );
>
> If you are using >= 7.4, then your query above is optimal:
Not necessarily. I had a query just last week that still wouldn't
optimize with the "in" notation, but did optimize with "exists"
notation. My other post about this showed both queries for that reason,
but I still feel that, for academic purposes, the "in" clause is far
more readable.
Anyway, good point.
Michael
--
Michael Darrin Chaney
mdchaney@michaelchaney.com
http://www.michaelchaney.com/
