from Hacker News

I'm trying to understand why anyone would use MongoDB. MongoDB sells itself as being faster than the competition by virtue of the denormalized data structures it encourages (no joins!).

Does an RDBMS not also provide this functionality? Why not just create a table in PostgreSQL with a single JSON field? This is essentially the same as MongoDB but with myriad additional benefits. You now have a "get out of jail free card" when you realize you actually needed relationships and ACID. With a "one field to rule them all" RDBMS structure you can start pulling out fields from your JSON goop into meaningful structured relations when you outgrow the document-oriented nature of MongoDB.

What is the appeal of MongoDB when an RDBMS can do everything better and more reliably?