Utilising DB Indexes on table without defining in ABAP Dictionary

Hi Experts,

My question is, will DB(Oracle or DB2) be able to detect and utilize an index which has been created at database level and has not been defined in ABAP Dictionary.

If yes, then how ? How is it related to the concept of DB optimizer& access path of data. If there is a relevant reference document, then please share it here.

Thanks

Abhijeet