[development] Doubt on Index and Primary key

HI All In .install file, in hook_schema unction, we are mentioning index field and primary key field. I am wondering, how to choose which one should be index and which one should be primary key. Does that affect the performance (in search, add, delete of records). What I am thinking at this point of time, my database will contain job seekers name, mail-id, contact number, skill set (as comma separated values) and resume. For index purpose, I am thinking I will use '$user->uid' as Index and and 'name' as primary key. I am bit confused how it it is going to affect search/add/delete time (assuming huge number of records in database) and also database management. Please guide me. Regards Austin