title: Why are joins bad when considering scalability?
body: <p>Why are joins bad or 'slow'. I know i heard this more then once. I found this quote</p>
  
  <blockquote>
    <p>The problem is joins are relatively
    slow, especially over very large data
    sets, and if they are slow your
    website is slow. It takes a long time
    to get all those separate bits of
    information off disk and put them all
    together again.</p>
  </blockquote>
  
  <p><a href="http://highscalability.com/scaling-secret-2-denormalizing-your-way-speed-and-profit" rel="noreferrer">source</a></p>
  
  <p>I always thought they were fast especially when looking up a PK. Why are they 'slow'?</p>