MMJ Spots

I plan on becoming a cop and I have a question. I’ve seen on different cop shows like Cops where a cop will choose to not arrest someone for marijuana possession. Is that a common thing? Personally I believe that marijuana should be decriminalized. I would like to ability to use discretion when it comes to arresting people for marijuana. Is that possible at all? Of course if its not up to discretion I would follow the law to the letter.