They scale pretty well, but when you get to gigantic data sets, they don't do so well and things like map reduce do a better job. I don't think the evidence su

They scale pretty well, but when you get to gigantic data sets, they don't do so well and things like map reduce do a better job. I don't think the evidence supports that. See the recent MR vs. parallel DB comparison, for example: http://database.cs.brown.edu/sigmod09/ http://database.cs.brown.edu/sigmod09/ There isn't a good massively parallel open source SQL implementation right now, but that is just a matter of engineering -- SQL databases have been shown to scale to massive data volumes (e.g. Fox Interactive have ~200 TB of user data in Greenplum, which is a parallel DB based on Postgres).