title: Details of impact for indexes, primary keys, unique keys
body: <p>I like to think I know enought theory, but I have little experience optimizing DB in real world.
  I would like to know points of view, thoughts or experiences.</p>
  
  <p>Let's imagine a scenario like:</p>
  
  <p>Table A
  Key: c1, c2, c3, c4
  Index: c7, c3, c2</p>
  
  <p>Table B
  Key: c1, c2, c3, c4
  Index: c1, c5</p>
  
  <p>All are non-clustered.
  The tables have 40+ fields.
  They are feeded daily by night and have some updates during the day.</p>
  
  <p>Table A, if more queries benefit from Key than Index, could the index impact negatively?
  Because insert/delete has to update 2 indexes instead of 1.</p>
  
  <p>Table B, has an extra field at the index, not present in key.</p>
  
  <p>Could a query using c1, c5</p>
  
  <p>Benefit from this Key?:
  Key: c1, c2, c3, c4, c5</p>
  
  <p>So that Index could be droped.</p>
  
  <p>What impact does the order of the fields has?
  Key: c1, c2, c3
  Key: c3, c1, c2</p>
  
  <p>A typical scenario for me is process_date, client_number, operation.
  And it feeds with a bunch of data each day (process_date).</p>