Tens of millions of rows with up to hundreds of columns per row is pretty solidly in the relational database range. MySQL, PostgreSQL or even H2 will handle th

Tens of millions of rows with up to hundreds of columns per row is pretty solidly in the relational database range. MySQL, PostgreSQL or even H2 will handle this pretty well on a single machine. However if you are doing any expensive per-row calculation (like natural language processing) or any sort of all-pairs work (like nearest neighbor) then you will see benefits from multiple machines and something like Hadooop to distribute and manage the work.