Why are joins bad when considering scalability?

Posted by acidzombie24 on Stack Overflow See other posts from Stack Overflow or by acidzombie24
Published on 2010-04-12T17:02:43Z Indexed on 2010/04/12 17:12 UTC
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Why are joins bad or 'slow'. I know i heard this more then once. I found this quote

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.

I always thought they were fast especially when looking up a PK. Why are they 'slow'?

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