The conclusion is a pretty good summary, if you don't want to read the whole lot. A big thing that's missing from Parallel DBMSs is fault tolerance, which the

The conclusion is a pretty good summary, if you don't want to read the whole lot. A big thing that's missing from Parallel DBMSs is fault tolerance, which the paper cops to (but also diminishes): "MR is able to recover from faults in the middle of query execution in a way most parallel database systems cannot. ... If a MR system needs 1,000 nodes to match the performance of a 100 node parallel database system, it is ten times more likely that a node will fail while a query is executing. That said, better tolerance to failures is a capability that any database user would appreciate." Also: "Although we were not surprised by the relative performance advantages provided by the two parallel database systems, we were impressed by how easy Hadoop was to set up and use in comparison to the databases."