10g Upgrade

When you upgrade to 10g from 9i you may find lots of execution plans suddenly “go wrong”. There are three main reasons for this. The first is that 10g automatically enables CPU costing, and if you weren’t using it in 9i this is likely to make a difference – often an improvement but you may be unlucky. The second is that there are numerous new optimisation strategies in 10g, and some of them may not be good for your particular data patterns, or your particular types of query.