|Subject||Re: [firebird-support] Optimizer request|
I'm continuing in old conversation because I have similar question.
How to avoid using IN(subselect) in UPDATE?
update orders o
set o.something = 'something other'
where o.delivery_date is null
and o.partid in (select p.partid from partners p where p.country='Spain')
I'm using FB 2.5.x
Right now I don't have big real data for testing.
I'm just wondering because from what I understand from here then for every row of orders firebird would make that constant subselect again and again.
Am I right?
Would using EXISTS() help?
---------- Původní zpráva ----------
Od: Svein Erling Tysvær setysvar@... [firebird-support] <firstname.lastname@example.org>
Datum: 9. 9. 2016 17:10:21
Předmět: Re: [firebird-support] Optimizer request
=Never use IN (subselect). Change toselect * from orders where exists( select * from partners where partners.partid = orders.partid and partners.country = ‘Spain’)2016-09-09 14:30 GMT+02:00 'Djordje Radovanovic' softsistem@... [firebird-support] <email@example.com>:
I tried query with subquery in where clause and found big issue for this type of subquery.for example:select * from orders where orders.partid in (select partners.partid from partners where partners.country = ‘Spain’)Perfomance Analysis returns me thispartners 687660 non index readsorders 28657 index readsIf you analyze this result you’ll find that there is 687659 unnecessary non index reads. If developer of optimizer accept that all queries on the left side of where clouse has priority (and there is no way to be opposite) than we have big improvement in optimization.Best regards,Djordje Radovanovic