Horizontal partitioning depends on your data. Do you have lots of joins and interdependent data? Partitioning will be tricky. Do you just need to store a lump o

Horizontal partitioning depends on your data. Do you have lots of joins and interdependent data? Partitioning will be tricky. Do you just need to store a lump of data relevant to one customer, many times over? Horizontal partitioning is easy. Relational databases are really cool. They're not the right technology for all apps though. The big wave-makers right now are things like Google's BigTable and Amazon's SimpleDB. They're implicitly horizontal but with reduced querying abilities compared to traditional SQL. It's a hard problem to scale a traditional relational DB across multiple independent commodity boxes like we see at Google or with AWS. The "stuff everything in a giant hashtable" approach scales nicely across a bunch of cheap boxes.