Mysql optimization

[CarmeloLabadie]

I have this mysql table called comments which looks like this:

commentID parentID type userID date comment
The commentID is set as Primary key, but most of the time I fetch the data using the parentID. How should I set my indexes?

Should I just add an index on parentID and let commentID be the primary key?

Edit: Would it be bad to have 3 indexes? For example on a forum table which has a categoryID boardID and topicID?

[aimeos]

Should I just add an index on parentID and let commentID be the primary key?

Yes, this should be the first step.

Edit: Would it be bad to have 3 indexes? For example on a forum table which has a categoryID boardID and topicID?

That depend on what queries you perform. If they contain categoryID, boardID and/or topicID in the WHERE clause, then it makes sense to add a multi-column index for the used columns too.