[jsr]

It's useful to differentiate between "SQL Databases do not scale" and "SQL Databases do not _cost effectively_ scale". The second argument is more accurate.

Vertical scaling of a DB is definitely an option for many people and has been used to scale many applications. However, the cost curve associated with buying bigger and bigger hardware is super-linear; doubling CPU & Memory in a single system leads to more than doubling the hardware cost. This can be problematic for many businesses whose revenue growth is exceeded by cost growth of the database.

Sharding is also an option for scaling, leveraged to great success by Facebook, Yahoo, and many others. However as the article points out, sharding prevents the developer from using many of the features that make a relational database a productive development environment. There are lots of foot guns that emerge in a sharded SQL environment and if you have not set up your development constraints appropriately, you can slow the pace of development considerably. This again leads to a cost problem because the incremental costs of adding features grows as you add more things like sharding around your database.

SQL is not useless and not hopeless. In a large number of cases, SQL is the right solution. However the techniques used to scale SQL tend to be options only to very large budget organisations. NoSQL solutions tend to be more cost effective in their scaling approach (scale out vs. scale up) without crippling the developers productivity. For these reasons, NoSQL solutions tend to be the better choice for the cost-conscious.