There could be many reasons why good running sql starts running slow.
- Underlying objects/views may have maodified and oracle cannot use previous plan.
- For every sql execution, if the sql is not parsed and plan is not availabale in memory. (The sql is parsed again and new execution plans are generated around 100s of them and if may chooses different plan (if plan is not in baseline or sql profile is not created for the sqlid)
- The other reason sql may run slow even it uses same plan, there may be load on that time for cpu or i/o and sql may slow.
orasolution 