title: Database query time complexity
body: <p>I'm pretty new to databases, so forgive me if this is a silly question.</p>
  
  <p>In modern databases, if I use an index to access a row, I believe this will be O(1) complexity. But if I do a query to select another column, will it be O(1) or O(n)? Does the database have to iterate through all the rows, or does it build a sorted list for each column?</p>