It is often repeated that the Big Data problem is that relational databases can not scale to process the massive volumes of data that are now being created.
But what are these scalability limitations that Big Data solutions like Hadoop are not bound to? Why can't Oracle RAC or MySQL sharding or MPP RDBMS like Teradata (etc.) achieve these feats?
I am interested in the technical limitations - I am aware that the financial costs of clustering RDBMS can be prohibitive.