All, Thank you for your excellent suggestions. - No stats on any of the tables involved in this query. - Timestamps on the index objects for this table are really old. - No bind variables - No partitioning. It is a mystery, but at least I can assure my people that the leading Oracle experts on the planet have examined the situation. Thanks again, Dennis Williams On Fri, Nov 21, 2008 at 2:41 AM, Niall Litchfield < niall.litchfield@xxxxxxxxx> wrote: > Is partitioning or another feature that implies the cost based > optimiser in use? > > On 11/21/08, Dennis Williams <oracledba.williams@xxxxxxxxx> wrote: > > List, > > > > The situation: > > > > Oracle 8.1.7.4 database on Solaris 8 (soon to be upgraded to 10g) > > Rule-based Optimizer > > > > A query which has run for years using an indexed access to a very large > > table (maybe 100 million rows) > > suddenly decides to use a full-table scan, shutting down a critical > business > > process. > > > > Then after several hours, it switches back to using the index. > > All concerned claim that nothing changed before or after. > > > > Needless to say the business users are nervous. They think maybe the > > database grew beyond some limit. > > > > Can anyone think of an explanation? > > > > Thanks, > > Dennis Williams > > > > -- > Sent from Google Mail for mobile | mobile.google.com > > Niall Litchfield > Oracle DBA > http://www.orawin.info >