独特的varchar字段与独特的bigint的MySQL性能

I'm working on an application that will be implementing a hex value as a business key (in addition to an auto increment field as primary key) similar to the URL id seen in Gmail. I will be adding a unique constraint to the column and was originally thinking of storing the value as a bigint to get away from searching a varchar field but was wondering if that's necessary if the field is unique.I'm working on an application that will be impl