|
>From: Vernon Hamberg [mailto:vhamberg@attbi.com] >I believe (may be wrong) that this subselect is run repeatedly... >Select * from item_master >where vendor in (select vnd > from vndmst > where crt_date = current_date) You're 1/2 right and 1/2 wrong. Correlated subqueries are rerun for each row, but they have to be. A correlated subquery is one where the records selected in the subquery depend on a value in the row being evaluated in the main query. However, non-correlated sub queries should only be run once for the entire statement. If this is not the case it's a bug in the SQL optimizer and should be reported to IBM. In the case above the 'in' predicate select will only be evaluated once since the results don't depend on any values in the main select. Had the select been Select * from item_master where vendor in (select vnd from vnditm where vnditm.itm = item_master.itm) Where the idea was to select all the rows in item_master where the vendor on the item was listed as a vendor that sold that item in the vnditm file then that subselect would be reevaluated each time. (Granted this would be a stupid way to get this result set) -Walden ------------ Walden H Leverich III President Tech Software (516) 627-3800 x11 (208) 692-3308 eFax WaldenL@TechSoftInc.com http://www.TechSoftInc.com
As an Amazon Associate we earn from qualifying purchases.
This mailing list archive is Copyright 1997-2024 by midrange.com and David Gibbs as a compilation work. Use of the archive is restricted to research of a business or technical nature. Any other uses are prohibited. Full details are available on our policy page. If you have questions about this, please contact [javascript protected email address].
Operating expenses for this site are earned using the Amazon Associate program and Google Adsense.