Why are joins bad when considering scalability?

Question

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

Answer 1

Also from the article you cited:

Many mega-scale websites with billions of records, petabytes of data, many thousands of simultaneous users, and millions of queries a day are doing is using a sharding scheme and some are even advocating denormalization as the best strategy for architecting the data tier.

and

And unless you are a really large website you probably don't need to worry about this level of complexity.

and

It's more error prone than having the database do all this work, but you are able to do scale past what even the highest end databases can handle.

The article is discussing mega-sites like Ebay. At that level of usage you are likely going to have to consider something other than plain vanilla relational database management. But in the "normal" course of business (applications with thousands of users and millions of records) those more expensive, more error prone approaches are overkill.