There are very few RDBMS usecases where scale is a legitimate performance concern. Index scans scale to a very, very high cardinality. There are use cases that

There are very few RDBMS usecases where scale is a legitimate performance concern. Index scans scale to a very, very high cardinality. There are use cases that aren’t best suited to an RDBMS at any scale, like managing a collection of denormalized documents, or a graph. But in my experience, considering these factors isn’t what’s motivating adoption of Mongo, or Dynamo. Your key lookups will always be fast (until you get a hot shard), but when you cram a highly relational set into a document db, eventually you’re forced to realize that there’s a lot of mutations and access patterns that you simply can’t implement, because you chose the wrong tool for the job. Either that or you’ll decide that it’s a good idea to implement your own concurrency control mechanism on top of your document database. Which I’ve personally seen at least 3 people try and do.