Is it possible someone gathered stats on the table, so the query went into CBO mode, and came up with a bad plan? Other than that, as I recall about the only thing that can go wrong with RBO is a bad index. But I would have expected that to throw an error message. On Thu, Nov 20, 2008 at 9:05 PM, Dennis Williams < oracledba.williams@xxxxxxxxx> wrote: > List, > > The situation: > > Oracle 126.96.36.199 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 > -- Andrew W. Kerber 'If at first you dont succeed, dont take up skydiving.'