Outside of Postgres, none of the truly open-source databases scale well with MPPs. You're still stuck with mostly single-core processing.
So you can pay good $$$$$ to the commercial database providers to provide you with a good database that may scale to your needs.
Or use Redshift for a great analytics DB at a reasonable price. (Note: we heavily use HBase where I am at, but that's comparing apples to oranges.)
* Disclosure: I'm an Apache HBase committer, I've written parts of Impala's HBase integration, and I work at Cloudera.
On the other hand, Impala, RedShift, Oracle Exadata, etc. let you ask the database to do work at a much higher level, which allows for much better performance because the data storage and computation layers can work in tandem (so you can prune down to only the data your query needs at each storage node before hitting the network, for example), and the database does the work of optimizing for multiple cores and nodes, not the writer of the analysis routine.