Subject Re: [firebird-support] Optimizer request
Author Djordje Radovanovic
Post date 2016-09-09T16:06:06Z
Same result. No changes. Perfomance analyzer gives same result.
 
Djordje
 
Sent: Friday, September 09, 2016 5:10 PM
Subject: Re: [firebird-support] Optimizer request
 
 

Never use IN (subselect). Change to
 
select * 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] <firebird-support@yahoogroups.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 this
 
partners     687660 non index reads
orders          28657 index reads
 
If 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