If a defense attorney subpoena's a police officer will/can they dig in the police officers past?

Will/Can a Defense/Prosecution Attorney/Lawyer court order (or dig them up themselves) previous past/old records of a active/retired police officer if he/she is subpoenaed for a court case? If the allegations were many, many years ago before the police officer was commissioned or even on the force? Yes, or No. What is the purpose? Does it hold any weight in the court?