Here is a thought. May be some implicit data conversion is happening? For example, assuming the query involves bind values, passing a number type instead of character type can lead to "unexpected" behavior. The table has possibly grown large enough that you are able to feel the "pain" now. -Madhu Sreeram On Thu, Nov 20, 2008 at 9:05 PM, 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 >