So, I'm hearing again about the latest generation of "in-memory databases". Apparently Gartner even has a category for them now. Let me define an in-memory database for you:
An in-memory database is one which lacks the capability of spilling to disk.
As far as I know in my industry literature reading, nobody has demonstrated any useful way in which data should be stored differently if it never spills to disk. While the talented engineers of several database products have focused on other performance optimizations to the exclusion of making disk access work, that's not an optimization of the database; it's an optimization of engineer time. The exact same database, with disk access capabilities, would be automatically superior to its predecessor, because users would now have more options.
PostgreSQL can be an "in-memory" database too, if you simply turn all of the disk storage features off. This is known as "running with scissors" mode, and people do it for useful effect on public clouds with disposable replicas.
So an "in-memory" database is a database with a major limitation. It's not a feature, any more than an incapability of supporting SQL access is a feature. Let's define databases by their useful features, not by what they lack, please.
Besides which, with the new types of persistent memory and fast random access storage coming down the pipe in a couple years, there soon won't be any difference between disk and memory anyway.