title: how clustered and non clustered index on primary key on same column can improve performance
body: <p>I have a non clustered index on column 'a' that is primary key. then if I create another clustered index on the same column. it improve performance with huge margin like from 16 seconds to 2 seconds? can anyone explain how this thing is happening ?</p>
  
  <p>if I don't add new index and just convert the non clustered index that we have to clustered index then there is no improvement. how this is going on?</p>
  
  <p>can't show code sorry!!!</p>