title: IQueryable<T> first free spot
body: <p>What's the most efficient way to get a <strong>first</strong> free spot from sorted query returning <strong>int</strong> collection ?</p>
  
  <p>eg.: {1,2,3,4,6} | result.: 5</p>
  
  <p>At the moment I am using foreach and counter that compares current value from sorted quety.ToList()
  which takes about 600ms on 100 000 records.</p>