title: sql primary key and index
body: <p>Say I have an ID row (int) in a database set as the primary key. If I query off the ID often do I also need to index it? Or does it being a primary key mean it's already indexed?</p>
  
  <p>Reason I ask is because in MS SQL Server I can create an index on this ID, which as I stated is my primary key.</p>
  
  <p>Edit: an additional question - will it do any harm to additionally index the primary key?</p>