In "Why the Clock is Ticking for MongoDB," Robert Haas examines a recent ZDNet interview with MongoDB CEO Max Schireson in which Schireson argues that relational databases are not long for this world. According to Haas, though, the opposite is true: MongoDB won't last.
Haas says that there are times when MongoDB's schemaless design is ideal for the job at hand, but his central point seems to be that document stores are not quite the exciting innovation they purport themselves to be. More importantly, there are basic advantages of relational databases that NoSQL solutions have not yet captured:
Relational databases allow complex transactions that affect multiple records, synchronous commit so that each transaction is guaranteed to be durable on disk before the client is notified that the commit has succeeded, support not only for JSON but also for other complex datatypes such as XML and geospatial data types, mature query optimizers that not only support combining data from multiple indexes (which Schireson mentions as a forthcoming feature; PostgreSQL added that capability in 2005) but also the ability to combine data for multiple tables via joins.
The problem, Haas suggests, is that MongoDB is hampered by narrowness. It innovates in some areas and excels at solving some very specific problems, but doesn't yet compare to relational databases on the basics. And these basics are features that MongoDB users will at some point require, if they don't already.
That is why Haas believes that MongoDB cannot spell the end of relational databases:
Rather, I think it's likely that PostgreSQL and other database engines will continue to innovate, providing many of the features that have caught the imagination of developers who are now choosing NoSQL engines; and that NoSQL systems will struggle to add features which relational databases have had for years.