Does the primary key serve any purposes apart from uniquely identifying a specific row? For example, a table with an autoincremented primary INT key could greatly aid in search due to the option of perhaps a binary search; but does MySQL actually take advantage of this option?
If so, is the primary key required to be in order? And while we're at this question, if a table is in order of a particular column--whether primary or not--would that aid in the SELECT process if that particular column was specified in the WHERE clause?
If not, what are some of the precautions I could take in building a speed-efficient infrastructure for my database? For example, I have a table of articles where the first two columns are: an 'ID' as the primary key, and an alphanumeric ID for the URL. To show related articles, I have a table with two columns for relating article IDs. I have noticed that multi-table SELECT query is much more faster when the "related" table logs the primary key ('ID') rather than the alphanumeric ID (which is not primary nor numeric).